f
THE ROLE OF ENV 1 RONMENTAL HETEROGENEITY IN THE EVOLUTION OF
J..IFE HISTORY STRATEGIES OF THE STRIPED GROUND CRICKET
Michael James Bradford
Department of Biology McGill University, Montréal
December 1991
A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfilment of the requirements for the degree of Doctor of Philosophy.
@ Michael J. Bradford il
ABSTRACT r cxamincd the E·ffect of heterogeneity in the thermal environment on the 11[e history o[ the cricket Allonemobius fasciatus. Variation in the lHe cycle was the result of a mixture of phenotypic plasticity and gcnet_ic differentlation in phenology-related traits along a latitudinal cline in glowing season. Females from a partially bivoltine population hllve a conditional life history because the y can adjust the proportion of diapause eggs in accordance with the likelihood that a second generation will grot" and reproduce before winter. The thermal environment is not variable enough to resul t in the evolution of a
mllrked bet hedging response, Ils is predicted by the ory . A quantitative
genetic analysis of the diapause reaction norm revealed significant heritabilities as weIl as correlations with other traits that could be related to common physiological mechanisms.
c iii
RESUME
J'ai examiné l'effect de l'hétérogénoité do l'Pllvlrollllcmcnl thermique sur la cyclo-biologie du grillon Allonemobius faseiatuR. Une variation du cycle vital issue d'un mélange de plasticHé phénol ypique et la différentiation génétique de traits phénologiqucs [ut rel i 0(' A Ull(' tendance nord-sud influençant la longeur de lél sa i son dt' crot ssnllCC. LN; femelles provenent d'une population partiellement hi volt i llC ont LIllC' cyclo-biologie candi tionelle puisque' elles peuvant ajusLer 1a propol't i 011 des oeufs en diapause suivant la probabilité qu'une scconc\(' !j0néri-ll ion se reprodiuse avec succès avant l'hiver. L'environnement thennic}llC' n'cHI pas assE"Z variable pour favoriser l'évolution d'une sLrnl-égic précisc dC' minimimisation des risques de mortalité, tel que prédit par ln I-hi·orip.
Une analyse génétique quantitative a démontré 1 ' exi sl-anc:e d'héritabilit~s significative et de corre]atLons entre dirf~rents trnils reliés à des méchanismes phsio10giques communs. (' iv PREFACE
The [aculty of Graduate Studies and Research requires the following statement to be roprinted:
"'rhe candidate has the option, subject to the approval of their Department, of including as part of the thesi s the text, or dupl icated published text, of an original paper or papers. Manuscript-style theses must still conform to aIl other requirements explained in the Guidelines Goncerning Thesis Preparation. Additional material (procedural and dosign data as weIl as descriptions of equipment) must be provided in
sufficient detail (e.g. 1 in appendices) to allow clear and precise judgcment to be made of the importance and original i ty of the research reported. The thesis should be more than a mere collection of
manuscripts published or to be published. 1t must include a generaI abstract, a full introduction and literature review and a final overall conclusion. Connecting texts which provide logical bridges between difEerent manuscripts are usually desirable in the interest of cohesion. "It is acceptable for theses to include, as chapters, authentic copies of papers already published, provided these are duplicated clearly and bound as an integral part of the thesis. ln such instances, connecting texts are mandatory a supplementary explanatory material is always necessary. Photographs or other materials which do not duplicate wall must be included in their original form. "While the inclusion of manuscripts co-authored by the candidate is acceptable. the candidate is required to make an explicit statement ln the thesis of who contributed to such work and to what extent, at
supervisors must attest to the accuracy of the c1aims at the Ph. D. Oral Defence. Since the task of the Examiners is made more difficult in these cases, it is in the candidate' s interest to make the responsibili ties of v authors perfectly clear,"
In accordance with the above. my thesis consi!lts of [0111' manuseripts intended for suhmissi on to scient i fi c joli\' na 1 s, i\ gpnC'I'nl summary has been added to provide an overview of Lhe r tC'l ci. 1I1ld t 0 indieate how the manuscripts an:! linked in wha\ l ùelicvo to hl' H logieal sequence of investigation, AlI chaptcrs deal wi th cl Lf! corollt aspects of a single theme. and this resul ts in some dupl tO.lt i on of t Il(' introduetory material. Nonetheless the introductions, in their C'ntlrcly. provide a thorough literature review. There art' slightly di/fol'C'nl styles used in the presentation of thC' text, in anticipatioll or tl\(' manuscripts being sent to different journals. At the lime of wr i t i 118
(September 1991), only Chapter 2 has been submitted, Lo the journrtl
Eeology,
AU four chapters are written in the plural voice. llIleI ni 1 will he' co-authored with my thesis supervisor. Dr. Derek Roff, I\llhough Dr, Hon has provided much advice in the design, execut ion and ana1ys i s of 1 hi!> research, his contribution falls wi thin the reillm of Il thes i Il supervisor; the thesis represents largcly my own work,
Contributions ta Original Knowledge
1) Although phenotypie plasticity can relieve the nced [or gerll'tic diversification in the adaptation to diverse habi Lat s, l WflS Ilbl e Lo demonstrate in the case of diapause ln Allonemobius EasciaLus, thllt bath plasticity and genetie differentiation are requircd,
2) A detailed investigation of the 1 He history of individuaJ [cmlJlNl
1ead to different conclusions about the role of envi ronmental heterogeneity than would have been made from overa] l popuLttLon vi ( averages. Recause sc] cc t j on ae ts on indi vidual s, this resul t underscorcs
the nood to study lifc history variation at the level of the individual.
3) This i s the first use of long-term meteorologicai records ta study
li le history evolution for a specifie organism. Al though insect
seasonal i ty has becn used as an example in a vadety of theoretical
mode} s of ovo] ution in a variable cnvironment. these moclels have rarely
been Lested.
'f) The final chapter is one of thE' first investigations of the
quanti tat! vc geneties of li c1early adaptive case of phenotypic
plasticity. 1 was able ta relate aspects of this plasticity ta different
types of selection imposed by the thermal regime, as indicated by my
modell1ng work. The patterns of trait correlations were aiso suggestive
of a common hormonal mechanism regulating the expression of these
plastic traits.
5) Fillally, from a technical perspective, this thesis ls one of the few
cxamples of the successful use, in laboratory experiments, of naturally
changing photoperiod and tempe rature cycles ta induce the patterns
phenotypic variation seen in the field. vi i • ACKNOWLEJ)GEMENTS First and foremost r must thank my thcsis supC'rvlsor. D"rpl, Rofr.
for his role in this research. He fj t"st suggcstcd t 11:11 i nSl'cl H 1111 ght hl'
a very profitable way of studying t11C' evolutlon of Hfl' hisloriC's. and
his guidance throughout has made my ontry into titis fll'ld v(,l"y
rewarding. l must also thank Tim Mousseau for inlroduc:illg 1IIl' 10
crickets, for tolerating many hours of quest 1011 1ng, for hl' i nE,
encollraging throughollt and providing the tempera turc dat <1 of Cillipler 3.
Members of my supervisory commi ttee. Don Kramcr and Gntll(llll He 11 .
provided usefui advLce, especiaIIy in the early going. Mnrk JOhllstoll mld
Robert McLaughlin commented on drafls of some of t lw chnpt ('l'S. Gi Il)('1'1
Cabana had something to say about near]y evcrythillg lIncl lrmlslaLpc! IIIl'
abstract. Rearing crickets takes III11ny hours of work, fllld 1 t hflllk l'mil
Guerette, Nathalie Roy. Jackie Farrell and Sharon David fOI" , hpl r III') p.
Special thanks to AnnR Chandler, who ran the expel'Î lOc>nl of Clwpll'r ï 011
her spare time. Finally, thanks to aIl the 6th floor people who mmll' my
four year stay at McGill fly by. Financial support was provl d('d by 1111
NSERG and Max Bell post-graduate scholarships, as wc 11 as NSlmC
operating grants ta D. Roff. This thesis ls dedicated to LOllisC' Fock1!'1-
and Anna Bradford. for encouragement. SUprot"t and love 1Il th<.' PlIS! ,
present and future .
.". viii
TABLE OF CONTENTS • ABSTRACT...... • ...... ii RESUME ...... " ...... i ii
PRgFACE ...... " ...... • ...... iv
ACKNOWLEOGEMENTS...... vii
1.1 ST OF TABLES...... x
LIST OF FIGURES ...... •...... xi
GENgRAL OVERVI EW...... 1
GI~PTER 1: Life history variation along a cline in voltinism in the cricket A11onemobius fasciatus.
Abstract...... 10 Introduction ...... Il Methods and Materlals...... 13
Results ...... 17
Discussion...... 31
Conclusions...... 36 References...... 37
CIIAPTER 2: Bet-hedging and phenotypic plasticity in the diapause
strategies of the cricket Al1onemobius fasciatus.
Abstract...... 41
Introduction...... 42
Methods...... 45 Results...... 47 Di scussÎon...... 51, Li teta ture Ci ted ...... 58 lX • CHAPTER 3: Seasonality. environmental lItlccrtajnty and ill!H'ct dOl'l11dIlCV: an empirical model of diapaust" strat0gi0s in Il,,, crickl't Allonemobius fasciatlls.
Abstract ...... id
Introduction ...... '" hl,
The Model...... ()ll Parameter Estimates ...... " ...... "1 Results ...... 11
DiscLlssion...... BB
Conclusions...... Il)
References...... 1/')
CHAPTER l,: The quantitative genetics of scnsonality-n'latNj lif('
history traits in thE.' cricket !\l1onemobius lafi!:i,tUJ!i
Abstracto ...... 0...... 0...... 101
Introduction ...... 0 •••••••••••••••• " •••••• 10)
Methods and Materials...... 1(lI,
Resul ts...... 1{)11
Discussion 0 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •• •••••• 1/1
l References...... 1ï } x
• LI ST DI<' TABLES
GhapLQLJ.:
Table 1: ANOVA results for development time and body size along
the latiLudinal cline ...... 20
Tnhle 2: Rep~ated measures ANOVA for diapause ...... 21
Table 'j: ANOVA results for dcvelopmcnt time and body size for
transi t ion zone groups ...... 22
ChaRter l, : Table l : Phenotypie and genetie correlation matrices ...... 112
Tllblc 2: ANOVA resu1ts for diapause ...... 113
'l'Able 3 : ANOVA resu1 ts for deve10pment time ...... 114
Table l, : fleritabilites of life history traits ...... 115
( xi
LIST OF }o'IGURgS
Chapter 1:
Figure 1: Map showing collection si tes ...... )1
Figure 2: Cumulative emergence CUfV0S for populatiolls ill till'ir
na tal envi ronments ...... )1,
Figure 3 : Mean development timcs for al1 popuL:ltions ...... }')
Figure l, : Mean femur lengths for Hll populations...... n
Figure 5 : Diapause egg proportions [or ail popul,llions ...... , ..... )/
Figure 6 : Mean development timcs for tral1s i t ion 2011(' eroups...... ;;tH
Figure 7 : Mean femur 1 engths for transi tian 201ll' groups...... )1}
Figure 8 : Diapause E'gg proportions for transi t iOI1 zOl1e eroups ...... 'Hl
Chapter 2:
Figure l: Temperatures and photoperiods Ilsed i Il Ll1C' exper i 11I('lIt .. . .. ')0
Figure 2: Experiment-wise average cliapausc ceg prodlle! i 011 •••.•••.•. ') 1
Figure 3: Diapause E'gg pl'oduction [or individuill fcmalN; witlt Il
histogram of median switching da tes...... '»)
Figure Il: Averaged diapause reaction norm for Imlividual I"c/O,tles .... ')'~
Chapter 3:
Figure 1: The phenology of transition zone 8. fasciatus ...... 7')
Figure 2: Mean and standard devlation of the growing senson
available for developrnent of a second genenl t ion ... , ..... HO
Figure 3: Distribution of development times of lab-reared femElles .. RI
Figure 4: Fecundityof lab-reared females ...... B?
Figure 5: Time series of optimal switching dates caleulnled from
historieal temperature records ...... 8'3
Figure 6: Model predictions of the optimal diapause reaction norms.R~
Figure 7: Comparison of model and experimental results ...... 81 xii • Figure 8: Relative fitness of hypothetical females differing in swi tchi ng da tes ...... 86 Figure 9: Relative fitness of strategies of random phenotypie variation in switehing date ...... 87
Chapter. l,: Figure 1: Phenotypie variabi1ity in diapause estimated from Chapter 2 data ...... 116
J~igure 2: Mean diapause proportion laid in the two environments .... 117 Figure 3: Mean diapause proportions for the two wing morphs ...... 118
Figure I~: Correlation of fami1y median dia pause across environments ...... 119 Figure 5: f1ypothetiea1 reaetion norms differing in the nature of genetie variability ...... 120
1 ! ,j 1
c 1
GENERAL OVERVIEW Life history theory is a branch of evo1utionary biology that attempts to exp1ain the diversity of 1ife styles observed Ln nature. Based on the premise that natural selection maximizes fitness. the the ory makes predictions about the optimal combinations of demographic traits such as growth. age at maturity and reproductive effort (Partridge and Harvey 1989). Fundamenta1 to the theory is the concept of trade-offs or constraints on certain trait combinations: an examp1e ls the observation that organisms cannot bath grow and reproduce at a high rate. and there will have to be sorne optimal balance between the two (Stearns 1976, Bell and Koufopanou 1986, Pease and Bull 1988). Whi1e the mathematical the ory of 1ife history evolution i5 weIl deve10ped it is probably fair ta suggest that there i5 a dearth of tests of this theory. This is particu1arly true for tests of the raIe of the external environment on the life history: this large1y stems from difficu1ty in isolating and quantifying the selective agents independent of the organism. For example. it has long been observed that there Is a positive correlation between morta1ity and growth in marine fish (Beverton and HaIt 1959). however without an independent assessment of the sources of mortality. it is impossible ta say whether mortality has selected for certain rates of growth, or if moctality is a consequence of growing at a certain rate. A test of this theory in the case of a guppy is provided by Reznick et al. (1990), who estimated mortality directly, and were able to relate this to observed 1ife history variation. Through manipulations of predator abundances they were a1so able ta document the evolution of the life history, in the direction indicated by theory. The incorporation of environmental heterogeneity in life history analysis is a recent development, and various models have suggested that optimal life history strategies may be different when variation is 2
inc1uded (Cohen 1966, Levins 1968, Gillespie 1977. Seger and Brockmann • 1987). Temporal variation can have bath a predictab1e and uncertain component that can result in the evolution of different strategies to maximize fitness. Predictab1e change (e.g .. seasonality, hast plant pheno1ogy) may result in the evolution of phenotypie plastieity where the organism uses cues in the environment to adjust its phenotype or the phenotype of its offspring (Bradspaw 1965). On the other hand random environmentaJ. fluctuations require either a risk-adverse or bet-hedging strategy to minimize variability in fitness caused by environmental variabi1ity (Stearns 1976. Frank and Slatkin 1990). Irrespective of the type of temporal variability, fitness is maximized with sorne form of phenotypic diversification that is developmental rather than genetic in origin. Recently, researchers have attempted to integrate genetic analysis with life history theory (Roff 1990). Life history the ory has
traditionally ignored the genetic bas~s of traits. and searched for optimal combinations under the assumption that there are no genetical constralnts to evolution. Quantitative geneticists. however. have suggested that traits should not be considered in isolation and have developed multivariate models that place life history traits in a variance-covariance matrix that may constrain evolution to any particular optimal combination (Lande 1982. Char1esworth 1991). The situation is further complicated by plastic traits, because their variance-covariance matrix may be environmentally dependent, making potential constraints to evo1ution transitory (Stearns et al. 1991). Although genetie covariances between traits have been documented, it i5 yet unclear whether they are strang enough ta retard or prevent evolution on the long term. Nonetheless. in discussing the evolution of life history traits it i5 important ta demanstrate that genetie variance does exist (so evo1ution can occur) and to consider the functional and 3 l genetic correlations that may exist among traits (Roff 1990). This is particularly relevant if trait combinations are being called "adaptive complexes" when an alternative hypothesis of trait integration imposed by physiological or developmental homologies may be more important. Recent calls for a greater understanding of the physiologica] nature of life history traits seems timely since an experimental program for elucidating physiological mechanisms might be more revea1ing and less extensive than the traditional "black box" approach of quantitative genetics (Clark 1987. Stearns et al. 1991). In this thesis l address sorne of these issues in the context of insect seasonality. In tempera te climates insects can grow and reproduce only during the surnrner months and they must either migrate or go into dormancy during the cold season. For non-rnigratory species dormancy is usua1ly in the form of diapause. which is normally specifie to a single stage in the life cycle (Tauber et al. 1986). This places sorne unusua] constraints on the evolution of the life cycle, which renders it amenable for ana1ysis. The insect must complete an integra1 number of life cycles before returning to the co1d-hardy stage before winter. Failure to complete the cycle will incur large fitness losses because it is likely (although not very weIl doeumented. Taylor 1986) that aIl individuals not in diapause will not survive winter (Roff 1980). Because insect deve10pment is temperature dependent, it is possible to analyze quantitative selection on life histories with long-term meterologiea1
records. Thus ~ priori predictions are possible about seasonality related traits from the consideration of selective agents external to the organism. The organism l have used is the striped ground cricket Allonemobius fasciatus, a species that is common throughout North America. Along the east coast of the continent there exists c1inal variation in the nurnber of generations produced per year (voltinism; 4 f Fulton 1938, MOUSSE-au and Roff 1989). This variation is suggestive of adaptation to the climate, as the number generations per year is correlated to the length of the growing season. My thesis consists of four chapters detailing a hierarchical investigation of lire history evolution in a seasonal environment. The thesis begins wi.th an interpopulational study of traits associated with seasonality, and then concentrates on a single population exhibiting phenotypic plasticity for voltinism. In the first chapter l use a common garden experiment to dissect genette and environmentally induced sources of variation along the cline of voltinism in A. fasciatus. Because many phenology-related traits are phenotypically plastic. it not clear whether the phenotypic variation ohserved in the field is the result of genetic differentiation resulting from selection pressures imposed by the climate, or to the effects of different environmental influences on the plasticity of a single genotype. In the second part of this study l focus on a population from the area where the region Ol the univoltine (one generation/year) and bivoltine (two generations/year) phenologies come in contact. l asked whether this transition zone is a hybrid zone of two genetically differentiated morphs, or whether voltinism in this region is a phenotypically plastic trait. My results suggested the latter alternative. The discovery that voltinism is a conditional strategy led to the
more detailed investigation described in remaining 3 chapters. Conditional strategies are interesting because the organism can make "ehoiees" about its life history to maximize fitness. and these choices are dependent on the state of the organism with respect to its environment (Lloyd 1984). Chapter 2 is an experimental study of the
diapause response of individual A. fasciatu~ females from the transition area. 1 found that there was a eharacteristic diapause function for 5
1 fema1es (a norm of reaction). but considerah1e wlrial ion E':dstecl élIlIonp, fema1es.
ln the third chapter 1 Rssess the ro1e of Lemporal environmenlal heterogeneity on optimal diapause strategies. As menLioned ;Ibove, a convenient aspect of insect seasona1ity ls thal long-term tE'llIperatUt-e records can be used to quant ify the selective reg,ime imposed by l h<:> thermal environment. 1 asked whether interannulll varültÎon in thE' climate wouln select for a bet-hedging strategy. as has been suggcsLed in recent reviews and models (Hairston and Muons 1984, Boyce and Perrins
1987, Seger and Brockmann 1987). For diapause this wO\lld E'ntai! li strategy of producing mixtures of diapause and direct-developing eggs.
found. however that the thermal environment was noL variabl e enough for. the evolution of such strategies; this resu1 t was supported by thE' agreement between model, field and experimental resul l s. ln the final chapter 1 used a quantitative genetie approaeh to examine the architecture of phenol ogy traits in fl. fasciaLus. 1
dissected the diapause reaction norm into two traits and related r.helll 1.0 different aspects of adaptation in a seasonal environment. Diapause,
deve10pment time and wingform were aIl correlated, which cou] cl be
exp1ained by our understanding of insect physio1ogy. Evolution to new habitats is not constrained by a lack of genetic variaLion or by stronB genetic correlations among the traits l investigated. The trait architecture may contribute to the maintenance of both genetlc and
deve10pmental variabi1ity that has been observed in the field. Finally, i t must be noted that this species 1s under LaxonolIIi c review (Howard and Furth 1986; Tanaka 1991) and the populations usec! in my thesis would be named A. socius under the revised nomenclature. 1 have retained the original species name, as have Mousseau and Roff (1989) and Mousseau (1991), until further evidence is availab1e thal suggests the cline in electrophoretic aIle1es is an indicator of r
6
specia tion.
(' 7
References Cited
Bell. G. and V. Koufoupanou. 1986. The cost of reproduction. Oxford Surv. Evol. Biol. 3:83-131. Boyce. M.S. and C.M. Perrins. lQ87. Optlmizing Great Til clutch size ln a f1uctuating environment. Ec010gy 71:624-634. Bradshaw. A.D. 1965. Evolutionary significance of phenotypic plaslicity in plants. Adv. Genet. 13:115-155. Charlesworth. B. 1991. Optimization model, quantitative genetics. ami mutation. Evolution 44:520-538. Clark, A.G. 1987. Genetic correlations: a quantitative genetics of evo1utionary constraints. p. 25-45. In V. Loeschcke (ed.) Genct 1c constraints on adaptive evo1uLion. Springer-Verlag, Berlin. Cohen, D. 1966. Optil11izing reproduction in a randolllly varying environment. J. theor. Biol. 12:119-129. Frank and M. Slatkin. 1990. Evolution in a variable envirotll11ent. Amer. Nat. 136:244-260. Fulton, 193]. A study of the gertus Nemobius (Orthoptera: Gryl1idae). Ann. Entom. Soc. Am. 24:205-237.
Gillespie. 1977. Natural selection for variances in offspring numbers: fi new evolutionary princip1e. Amer. Nat. 111:1010-1014. Hairston, N.G. and W.R. Munns. 1984. The timing of copepod dlapause as an evo1utionari1y stable strategy. Amer. Nat. 123:733-751. Lande. R. 1982. A quantitative genetie theory of 1He hisLory evo1uLion. Eeology 63:607-615. Levins, R. 1968. Evolution in changing environmenls. Princeton Univ. Press, Princeton, NJ. Lloyd, D.G. 1984. Variation strategies of plants in heterogenous environments. Biol. J. Linn. Soc. 21:357-385. ------~-
8
Mousseau. T .A. and D.A. Rofr. 1989. Adaptation ta seasonality in a • cricket: patterns of phenotypic and genetic variation in body size and diapause expression along a cline in season length. Evolution 43:1483-1496. Partridge. L. and P.H. Harvey. 1989. The ecologica1 context of 1iIe history evolution. Science 241:1449-1455. Pease, C.M. and J.J. Bull. 1988. A critique of methods for measuring lire history trade-offs. J. evol. Biol. 1:293-303. Reznick, D.A., H. Bryga, J.A. Endler. Experimentally induced life history ev01ution in a natural population. Nature 346:357-359. Roff, D.A. 1990. Understanding the evolution of insect life cycles: the role of genetical analysis, p. 5-27.1n F. Gilbert (ed.) 1nsect 1He cycles. Springer-Verlag, Berlin. Seger, J. and H.J. Brockmann. 1987. What is bet-hedging? Oxford Surv. Evo1. Biol. 4:182-211. Stearns. s.e. 1976. Life-history tactics: a review of the ideas. Q. Rev. Biol. 51:3-47. Stearns. s.e., G. de Jong and R. Newman. 1991. The effects of phenotypic plasticity on genetic correlations. Trends Evol. Eco1. 6:122-126. Tauber, M.A .. C.A. Tauber and S. Masaki. 1986. Seasonal adaptations of insects. Oxford Univ. Press, New York. Taylor, F. 1986. Toward a theory for the evolution of the timing of hibernal diapause, p. 236-257. In F. Taylor and R. Karban (eds.) The ev01ution of insect life cycles. Springer-Verlag, Berlin. Q 1
GHAPTER 1
Life History Variation Along 8 Cline in VoltinLsm ln lllD Grick~l Allonemobius fascia tus 10
f ABSTRACT
ClinaJ variation in ]ife histories might be genetically based. resulting from selection imposed by different environments. or it may due to the dlfferential expression of phenotypically plastic traits. We examined the cline in voltinism in the cricket 6llonemobius fasciatus. using a common garden experiment and populations spanning the switch from a univoltine to a bivoltine phenology. There were only small differences in development time between populations. however. we found large genetically-based differences in diapause propensity. Genetically differentiated reaction norms for diapause are required because photoperiodic eues for diapause are ambiguous among sites. In the zone of transition between phenologies voltinism is a conditional strategy, rather than a genetic polymorphism. First-generation females from this area can lay either direct-developing or diapause eggs depending on the likelihood that a second generation will have sufficient time to develop.
c 11
INTRODUCTION
Lire hisLory variation observed mllong POPU1.':ll ions of 11\l' S End1er 1986). This latter source of variation may b(' lIlC're]y li biochemical or physiological interaction of the 01"glllli sm wi tilt II(> environment. or may be adaptive if the variation in Lhe phenotypl' increases fitness in the enVirOl1l11cnts encountercd (SChlllHlhHIIS('1l lQ4CJ: Stearns 1989). Assessing the importance of thesc two sources of variétt iOIl is comp1icated by the observation t-hat genotypes flnd cllvi rotlmC' Il 1 s ('1111 interact in the expression of the phenotypc>, This int (>rA('! ion IIlf1y suppress genetic variation resulting in the appcanu1<'o Or:l sillf,l(' phenotype. or it may enhance sma11 genetical1y-bascd vRriéltioll ill Irnils (Berven et al. 1977; Gill et al. 1983). The observation o[ gcnoLyp('s in a variety of realistic environments ls required to separAlc Ih(,9(, possibilities. In this paper we examine. [or a cricket. lif" hiHlory variation in relation to clinal variation in cl imate. Insect deve} oplIIent i s 1 El rgel y tempe rature dependent and the life cycle must be adapLed lof i tin t tIC' part of the season warm enough for growth and reproduct ion (DmIÎ 1 evskl i 1965. Roff 1980), Traits associated with phenology (i.e, dLapaus~. deve10pment time, migratory capabiliLy) arc often slrongly affC'clcd by the environment (Tauber et al. 1986). and are thus good candidates for the study of the expression of phenotypus resulting from a mixture or genetic differentiation and phenotypic p1asticity. In flddlLioTl, the adaptive significance of variation in Lhese traits Célfl be idcnllfjnd because the selective regime. the thermal environment. can be rCRdlly ( 12 quantlflcd from metcorological records. Allonemobius fasciat.us, the strlped ground cricket. ls common throughout North America in moist ditches and fields. This species overwlnLers in diapause in the egg stage. hatches in spring and develops Lo Lhe adult stage over the summer. It is univoltine in the northern part of ils range and emerging adults lay only diapausing eggs that overwinter until lhe next year. South of 35-36°N li. fasciatus is bivoltlnc: [cmales of the first generation reproduce in June and Ju]y. fllld 1fly non-diapause eggs which deve10p directly into a second gcnernLion (FuI ton 1931, Howard and Furth 1986; Mousseau and Roff 1989). Mousseau and RoH (1989) surveyed populations of A. fasciatus across the transition between univoltine and bivoltine phenologies for di[[ercnces Jn body size and diapause expression. Field sampling revealcd that adults [rom univoltine populations were larger than those from bivoltine populations. consistent with a model of phenological adaptation which predicts that development time in the univoltine reglons will be longer. resulting in larger adult size (Roff 1980, 1983). When reared in a common environment in the laboratory, there was a gcnetically-based cline in diapause propensity with northern or high nltitude populations laying higher proportions of diapause eggs. This suggests adapta tian ta the cl ine in growing season. al though 1aboratory rcared populations always produeed intermediate proportions of diapause eggs. In the field first generation females in the bi vol tine region will produce direct deve10ping eggs: the second generation and univo1tine populations and will lay aIl diapausing eggs. Here wc examine in more detai! the role of environrnent and genolyp~ in the expression of the cline in voltinism in A. fasciatus. Using more naturai environments. we test whether the genetie differences in dlapause found by Mousseau and Roff (1989) are modified by the environment to produce the univo1tine and bivoltine phenologies observed 13 in the field. We also partition cllvil'onmental and gelwl il' variation III development time and body size. relating them to tlH' plwnologieal lIlode1 of Roff (1980). In addition. we investigate life history VélrtaLion III the Il!'ell of transition between univoltine and bivoltinc 11fe cycles. Til Ihis tln'il both life histories are observed. although it is llllclear wheLher 11H'y eoexist as a genetie polymorphism. or if vol tini SIlI 1 s a ph011oLypil'1I11 y plastic trait eomplex. We look for genetie differclIces between 1 hO~l(' expressing univoltine or bivoltine life histories in the field in 1.1118 transition area. and test for developmental switches (Smilh-Gill 198'3) in development time or diapause that would sugges\: thal voltlnislll is :Ill environmentally-dependent conditional stl'ategy. MATERIALS and METHODS A eommon garden transplant design was used Lo panilion environmental and genetic sources of variation in life histol'Y Lrilils. Populations from 3 areas were reared from egg to adul t Ln 3 incubnLors that siml.l1ated their natal environments in a fully faetorial desll:VI. Tn addition. populations from the transition zone were reared III 3 environments which simulated a range of hatching dates to test [or phenotypic plasticity in voltinism. Samples of 8.. fasciatus were colleeted from moist fields lIL 1 sites: Dulles airport VA (39"N), Danville VA (36°40'N), Mid Covington CA (33°40'). These correspond to the regions of univo]ti.ne, LransiLion and bivoltine phenologies, respeetively (Mousseau and Roff 1989; Bradford unpubl.). Sampling was eondueted in late July in the transition anù bivoltine regions and in late September in the univoltine area. Al lûast 100 individuals from each population were brought back ta the , '. laboratory. At the transition site both adults and midd1e instar nymphs 14 were collected ln July. These represent individuals likely to be • cxpressing the bivoltine and univoltine life cycles, respectively. A second generation of the bi.voltine populations was reared in the laboratory. The adults collected from the field in July were allowed to mate in groups of 50-100 in 19 L plastic mouse boxes (Mousseau and Roff 1989) at 30 D C and 15/9LD. Eggs were collected and hatched in similar conditions. The newly hatched nymphs were reared for 30d in the same 1 ncubator befora tr:msfer to ' fall' conditions (26/16 D C thermoperiod, 12hr warm-12hr cool, 13.5/11.5 LD) to ensure that maturing adults would lay diapausing eggs. The middle instar nymphs captured in the transition site were initially reared at 25°C (15/9 LD) to slow development in order to maintain a univo1tine 1ife history and ta ensure temporal overlap of the adults with those of the laboratory reared second generation of the bivoltine population from the same site. In early October groups of adults were established from the collections and allowed to breed in the 'fall' environrnent. The oEfspring from the following 6 matings were then used in the main experiment: i ,~ 1) Dulles airport, ~ September collection (univo1tine) (UNI) ! 1 2) Danvi 11e. from nymphs collected in July (univoltine) (TRuni) ! r 3) Danvil1e, from adults collected in July (bivoltine) (T~iv) 4) Hybrid, [ema1es from TRuni. males from TRt,iv (TRuxs) 5) Hybrid, females from T~iv. males from TRuni (TRBxu ) 6) Covington, adults collected in July (bivoltine) (BIV). For each population at least 50 crickets of each sex were divided among 4 replicate mouse boxes. A roll of moistened cheesecloth. changed at 4-day intervals. was provided for ovipositioning. Eggs retrieved from c the cheesecloth were reared for an additional 20 days before being 15 t stored at 4 oc to break diapause. Aftel' 4 mOllt:hs in the c01d eggs Wl'l'l' rewarmed to 28° and hatched. Nymphs from the six experimE'ntal populations wel'c r-eared in Lhtee different envir-onments designed ta simula Le the na tai coll ec Li 011 siLos (see Chapter 2, Fig. 1 for an example). Photoperiods followed the natura1 cycle of daylengths, including civil t:wllight: (Beek 1980: National Almanac 1988) and were changed twice weekl y. A thcrmopet"Ï od was also used. with minimum and maximum temperatures taken [rol1l 10ng~lc"lII averages for the collection sites (Ruffner 1980). The tefUpel'ature cye1 c followed a square wave, and there was a 2hr transition between ('xlrellles. We used a 12/12 h thermoperiod, the daily incl'ease set at 1.5hr artel' 1ights on. Groups of 45~50 new1y hatched nymphs were plaeed in pl é1sl ie sandwich boxes (Mousseau and Roff 1989) and iceberg Iettuec and erllshcd cat food was provided as food. Six replicate boxes were estllbl ished for each environment~population combination. When the nymphs werc plaeod ln the incubator, the starting date for the simulated seasonal environmental cycle was that predicted for the average daLe of hatehlng in the field. We used the long term meteorological records and a thermal requirement for hatching of 200 degree days aboya >14°C (Tanaka 1986; Masaki and Wa1ker 1987: Bradford unpub1.). The four groups from the transition zone were l'cared in 3 additj onal environments. this time simulating early, average and bite hatching dates. The ear1y and late hatching groups were started 5 wceks before and after the average date. Food was changed and sandwich boxes cleaned every 4 days until adult eclosion began. At the first appearance of adults tho boxes wcre checked every 2 days, adults removed and their sex and wi.ngform (micropterous and macropterous) noted. Adults were stored in 70% ethanol. From each population-by-environment cross 2 sandwich boxes of 16 4-5 eltch of males and [emales were set up for the estimation of diapause propensity. Rach of these boxes was provided with a ro11 of moistened choesecloth, which was changed every 4 days. Eggs were collected over the 4 day periods starting at 8, 12, 24 and 28 days of adult age. The eggs were reared in the same environment from which they were col1ected until the non-diapause eggs hatched (approx. 30 d). The proportion of diaplluse eggs produced was ca1cu1ated from counts of hatched and hea1 thy non-hatched (i.e. diapause) eggs. Femur length was used as an index of body size (Mousseau and Roff 1989). Right rear legs were removed from the preserved adul ts and measured using a dlgitizing tabJet. Statistical Ana1ysis Femur] ength and development time were ana1yzed with ana1ysis of variance (ANOVA), with population and environment as fixed effects. Because 4 diapause estimates were made over time from each reproductive group, repeated measures ANOVA was employed for these data (see Mousseau 1991, SAS insl. 1988). Dlapause proportions (p) were transformed as x-arcsin(jp) prior ta analysis. The correlation between deve10pment time and body size was estimated using replicate cage me ans because individuals were not preserved separately, and therefore could not be matched to their deve10pment times. Two sets of analysis were conducted on each trait. For latitudinal trends, the 4 transition populations were grouped together, resul ting in 3 populations in the analysis, differing in voltinism. For the detailed analysis of the transition zone comparisons were made among the 4 populations (#2-#5, see above) from this zone. 17 RESULTS Latitudinal variation The temporal pattern of adult emergence in 1-11(-' expcri mCII!. closel y matched that found in the field sampI ing ([l'lg. 2). Adul 1 s emcrgod fi n.;\ in the bivoltine population, in mid-July. Th(' tnlOsitLol1 éldul Is l'closed 10 -14 days la ter, and the northernmos t popu1a li on adul ts [1 t"s L 1\ ppen ro The clinal trend in median emergence dates WHS large] y du(' ta l hc' different thermal regimes. When expressed as degree -days Lhe ra wu I-e 01\1 y small, albeit significant. differenees in the average numbcr of clcgrec- days required for development (from hatching to final mouI L) bcLwecn eaeh of the 3 environments (Fig. 3, Table 1). DevelopmenL was the longest in the northern environment, and shortes L ln the transi Li 011 elimate, although the difference was only 5% of the tota1 devl'lopmenl time (about 3.5 days). Thus most of the cUna1 varjaLion in Fig. 2 is due the difference in temperature among localities through its eff(~cL on the hatching date and the accumulation of degree days. There were also small, but significant, geneLie differences in development time between the 3 populations as welJ as s igni f iCllnt population-by-environment interactions (Table 1). Overall, the transition zone population hart the most rapid development, 1.2-2 dl1ys shorter than the other populations. In the bivoltine and univoltine environments natal populations had the longest development times compared to the transplanted populations. The environment and population effects on body size (as indexed by femur length) were also small (Fig. 4). There was a significant: effe<:l of environment (Table 1), with the transition environment proùuclng the smallest individuals, though the mean difference was O. 05rnm, or about 1% of the total femur length. There were also signifj cant populati on-by- 18 environmenL interactions, as indicated by the crossovers in Fig. 4. • although no trend re1ated to the population's phenol ogy or natal envi ronment cou1 d be detected. Tnterpopulational differences in femur length in Fig. 4 were largcly attributab1e to a positive correlation between deve10pment time and body size (ANCOVAR based on cage means, P (Table 2). F1gure 5 shows how the genetic differences between populations coup1od wilh environmenta1 effects produce the phenotype appropriate for the thermal regime. For example, in the southern environment the bivoltine population produces on1y non-diapause eggs (forming a second generation), while in the northern environment the univoltine population 1ays aIl diapause eggs. as expected for a univoltine 1ife cycle. A population 'transplanted' into a non-natal environment produces a different fraction of diapause eggs than the natal population. No population was able to produce the proportion of diapause eggs that the natal populations did in aIl environments. Transi tion Zone , The following analysis focuses on the 4 transition zone groups that were reared in three environments simulating variation in hatching 19 l date. The group col1ected as nymphs in .Tuly in thE' tnlllsltlon ~one (Truni ) had significantly longer developmE'nt ti mes thml lhe> t:)roup collected as adults in July (Trhlv: Table 3, Fig. 6) indicnting lhnl HOmE' of the difference in development observed in the field was due Lo genetic differentiation. The hybrid groups had development times intermedtate to the pure populations. Across a11 Lhe trans il ion zone groups, development was positively related to the date of halching; the early hatching groups required 10-15% fewer degree days ta develop Lhnn the late group (Fig. 6). There were no consistent di fferences in body size bel ween Lh(' transition zone groups (Fig. 7). but there were strong genoLypc-by environment interactions (Table 3). Femur length was signiflcantly smaller in the early environment than for the laLer cnvironlllents (P=O.0009), in agreement with the shorter development times in that environment (Fig. 7). The transition populations produced intermediate fractions of diapause eggs, except in the late environment, where aIl crosses produced almost exclusively diapause eggs (Fig. 8). Excluding the late environment from the analysis, the group, environment or interaction effects were not significant in the repeated measures ANOVA (Table 2). The diapause data for the early and average environments were highly variable, as indicated by the large standard errors in Fig. 8. 20 c TabJe 1. Summary of ANOVA results for development time and femur length along the J atitudj na1 cline. All other intl?ractions were not significant (P>0.10). The 2 df values for the error line correspond to analyses for devc10pment time and femur length respectively. Development Femur Source df P p ENV 2 <0.0001 <0.0001 POP 2 <0.0001 0.017 ENV*POP 4 0.0025 <0.0001 Sex 1 0.51 <0.0001 Wing 1 0.0001 0.28 Rep1icate 45 0.031 <0.0001 Error 2905/2654 21 t Table 2. Repeated rneasures ANOVA for diapausc. DiélP[llISO proport ions nrl' transformed as arcsin(jp). and are l'epE'1'lte-d ov(:'r Tnm. The fi l'st- "l'rOI' df ts for tests of the main factors; the second lS for t}l(' t ('sl- of '('IME effect. AH interacti ons betwE'E'1l TIME and other ~Olll'CC'S w"rC' Ilot- significant (P>O.OS). For the transition zone analysis only c Latitudinal Trnl1sit. iolt Source df p df p ENV 2 <0.0001 l O.BO POP 2 <0.0001 2 0.50 ENV*POP 4 0.45 3 0.99 TIME 3 <0.0001 3 0.008 Error 36/81 8/24 22 Tabl e 3. Summary of ANOVA resu1 ts for deve10pment time and femur 1ength • for the 4 transi tian zone groups. Al1 other interactions were not si gnHicant (P>O .10). The 2 df values for the error 1ine correspond lo analyses for development lime and Femur length respective1y. Deve10pment Femur df p p ENV 2 <0.0001 ( 23 ••• Va Ne Figure 1. Map of the eastern United States, showing the location of the three experimental populations. From north to south, they are in the univoltine, transition and bivoltine regions. o 24 { 1.0 .... ,.- -+-' ,Ii • ::::3 0.8 ,.. ... ~ ,. • 1 1 Co: 0.6 1 ... 0 fiii A' 1 .- 1 1 iii.. 1 1: 1 0.4 1 0 I. .6- C- .1 0 !Ii ... Univoltine ~ 1 1 ... 0.2 .' Cl. • Transition • Bivoltine 0 _._ .. -.-ri l • 10 20 30 9 19 29 July August Figure 2. Cumulative emergence curves for the three populations in their simulated natal environments. Samp1e sizes range from 163 to 317. Also shown are the mean proportions (±SE) of adults in field samp1es averaged over collections made 1984 (from Mousseau and Roff 1989) and 1988 (Bradford unpubl.) from the three pheno1ogica1 regions. Symbo1 shapes of field data correspond to experimental emergence curves ( 25 ~ CI) ~ 850 "C Univoltine 1 • Cl Q) • Transition "C Bivoltlne '-'" • Q) 800 E + J= ~c: Q) 750 E C- -O ~ 700 0 South Central North Environment Figure 3. Mean development times along the latitudinal cline for three populations differing in phenology. Vertical lines indicate 2 standard errors, average sample size per point ls 323. 26 6.8 ..-... E ~E 6.6 .c: +-' Cl ~ 6.4 '- ... Univoltine ::l E 6.2 • Transition Q) • Blvoltlne LL 6.0 South Central North Environment Figure 4. Mean femur lengths (±2SE) for the populations in Fig. 3. Average sample size lS 295. 27 al 1.0 tJJ À • •• :Jca 0.8 0 c. ..• 0 ca 0 .- , •. 0 IÎI Cl 0.6 • .0 .0. c: • ... 0 ,/ .- 0.4 .0 0 t! .0 0 C. 0.2 . o· 0 .0 . o- • • '- . • y • c.. 0 South Central North Environment Figure 5. Proportion diapause for the populations in Fig. 3. For each population-environment combination the points are the mean of two samples taken over time. 28 ...-... ~ ~ 775 "C 1 Cl :sQ) Q) 725 E ~... c: • TR uni ~ 675 a. • TR biv o o TRu x b o TRb x u 1Cl 625. Early Average Late Environment Figure 6. Mean development time (±2 SE) for 4 transition zone populations over 3 environments varying in the date of hatching. Open symbols are hybrids between populations differing in phenol ogy in the field. Average sample size is 160. 29 6.8 Ê g 6.6 ..... t~~- J: ~. ~- +J fi-':::~-<:::: ~~ ~...... Cl ~ 53 6.4 -1 • TR uni ~ :::l • TR biv E 6.2 o TRu x b If o TRbxu 6.0 Early Average Late Environment Figure 7. Mean femur lengths (±2 SE) for the populations in Fig. 6. Average sample size is 143. 30 ( Q) "--"~.I 1.0 .' " en ..... ~" ::::J ...... ~/ .. "' ;' ctS 0.8 .. -.,"'.- ;';','" C- .. - '/ ctS ., " ./ " i5 0.6 ... , / .' ...... ,,/ c: ...... ,,/ • TR uni o 0.4 " t ------,,~. • TR biv o o TRu x b C- 0.2 e o TRb x u c.. o Early Average Lata Environment Figure 8. Proportion diapause eggs laid for the populations in Fig. 6. Shown here are the means (±2 SE) of aIl egg samples taken over time for each population-environment combination (n-8 for each point). 31 DISCUSSION The cline in voltinism in A. fasciatlls observed ln thE' fjeld 18 achieved throllgh a mixture of genetic differentiati on and envlt'onmenté11 effects on the expression of the phenotype. We found genetically differentiated reaction norms for diapause that interacted wi th the rea1'Îllg environment ta produce tlH.' phenotypo appropriate for the environment. It seems likely that di ffercl1t1a tad reaction norms have evol ved because the photoper lods cncollntered du ri ng the reproductive period are insufficiently different for a single genotype to produce the correct phenotypes in al] the variolls environments. Photoperiods and phenologies interact: although the univoltine populations emerge later in the SlImmer, their more norlhcrly origin reduces the difference in photoperiod eompared to the emergance time of the bivo1tine populations. Ouring the period 20 days prior lo emergence (Fig. 2) when photoperiods are important in deterrnining the expression of diapause in adult fema1es (see Chapter 2) the differcncc in photoperiods experienced by the two populations ls only 5 lo 20 minutes. However, due to the difference in the growing season rerna1ning, the two populations must produce completely different diapause fractions. Geographie variation is a colt1mon featllre of interpoplIlatlonal studies of diapause (Taylor and Spalding 1986), althollgh in nearly aJl cases on1y critica1 photoperiods (50% diapause) have been estimated under conditions of constant 1ight and photoperiod. Whtle it ts reasonab1e to assume that such c1ina1 variation represents adaptation to local conditions, the actual expression of the trait rnay be quite different in the field. The interaction between the semi-realistic environments we used and the genetical1y differentiated react10n norrns for diapause resulted in the expected phenotypes being exprossed. In 32 contrast, for A. fasciatus collected across the same latitudinal range • reported here, Mousseau and Roff (1989) found that, when reared in a single environment, diapause expression varied only between 60-75%, a much narrower range than found here or in the field. The enhancement of genetie differences by the environment is an examp1e of cogradient selection (Gill et al. 1983) in that both genotype and en\ironment influence the expression of the phenotype in the same direction. In our case the environment increases diapause expression to 100% in the univo1tine population and prevents the production of diapause eggs in the bivo1tlne population. The pattern of deve10pment times for populations in their natal environmenLs ls suggestive of the saw-tooth model of latitudinal variation in development time and body size suggested by Masaki (1978), Rof! (1980, 1983) and confirmed in the field for body size for A. fasciatus by Mousseau and Roff (1989). In this model, deve10pment time in the bivoltine region is predicted to decline along a cline in growing season, as the time available for two generations becomes progessively shorter. At sorne point a sudden switch to a univoltine phenol ogy is predicted, with a long development tirne and subsequent]y 1arger (and more fecund) body size. Here we found the transition population developed the most rapidly, presumably the result of selection for shorter development in order to maintain the bivoltine phenol ogy in the shortest possible season. In the northernmost environment, the longest development tjmes in the experiment were found for the univoltine population. consistent with the predictions of the model. The data also suggest that adaptation to different season lengths is enhanced by genotype-by-environment interactions. The difference in deve10pment time between the univoltine and bivoltine populations is. however, much smaller than suggested by the f model of Roff (1980), where development time of the univoltine 33 population is predictect to be double that of the bi vol ti ne phenology. A possible reason for this difference is the assumptlon in the moclel or , big bang' reproduction at the end of the juvenil e deve10pment perlod. A. fasciatus has a long reproductive lifespan and there ls potentLal ror a trade off between the length of the nymph and adult periods wit.hln the available growing season. For a population in the southern pal-t of t.he univoltine zone, such as the one we used here, selection La 1en8t:hen the longer development time to fil1 a growing season R1most: long enough roI' a bivoltine phenology might be countered by the 10ss in potential oviposition time that wou1d result from maturing later ln the season. Selection for longer development time may only he favored if there 18 Il large increase in fecundity, especially during early adult ]j[e Our finding of large genetic differences in diapause propenslty, but on1y smal1 differences in development time or body size across a cline in voltinism is simi1ar to results of a number of other studios. Danilevskii (1965) and 1ater Tauber et al. (1987) have noted thal: deve10pment time is conservative across large geographic areas. Across a transition from univo1tine to bivo1tine phenologies in other insects both Ritland and Scriber (1985) and Pul1in (1986) found little interpopulational differentiation in development. The lack o[ systematic variation in deve10pment time may be the resu1t of weaker selection on this trait, since it can be considered a form of 'fine Luning' of the phenol ogy to local conditions. In contrast. inappropriate diapause decisions can have Immediate effects on fitness, and selection might be expected ta have a large impact on the diapause reaction norm. At the extremes of the species range, geographic variation has been found ln the A. fasciatus group indicating development can evolve under strong selective pressures. Populations in the northern extreme of the species range have significantly shorter development times than those studied here (Sarai 1967, Tanaka and Brooks 1986; Mousseau 1988), prohably the 34 result of conslstent directional selection imposed by the coo1er c]lmate. Although correlated with development time, the body size data showed litLle of the 'saw-tooth' pattern predicted by the model of Roff (1980, 1983) and observed in the field by Mousseau and Roff (1989). We may have fai1ed to deteet this pattern. as there is considerable interpopuladonal variability in the field data (Mousseau and Roff 1989) and we used only a smal] number of populations in our study. A1ternatively. environmenta1 influences not accounted for in our experiments may enhance the sma11 differences we found in the laboratory. Transition zone The transjtional area between phenologies might be expected to be a hybrid zone sinee the latitudinal cline in phenology appears ta genetica11y based. at least for diapause. In this case the transition population might be polymorphie, with the possibility of intermediates existing from the hybridization of the two morphs (Kidokoro and Masaki 1978). On the other hand, voltinism could be a conditional strategy, where individuals would have a developmental switch enabling them to modify their deve]opment time and diapause propensity to assume either phenology depending on conditions (Smith-Gill 1983). Our data do not support the hypothesis of a discrete genetic po1ymorphism of rapidly developing females laying direct-developing eggs and slow dcveloping (univoltine) indivjduals laying diapause eggs. Rather, the data from our two transition populations and their hybrids suggests the presence of continuous variation in development time and 1ittle difference in diapause. Kidokoro and Masaki (1978) found for a transition zone population of a Japanese cricket a bimodal distribution ( of development times (although in only one of a number of constant 35 1 photoperlods they used) and suggested the populatlon was polymot'phlc, consisting of fast and slow developing genotypes. Our emergence dRta were uni modal (Fig. 2) and there was no evidence fOI' disc)'elc wtrillt:ion in development time or adult hody size. The pt'esence of both small nymphs and adults in the transition are8 i.n July i~ most 1 ikf'ly duC' to environmental variation rather than such discrete genetlc differences. The nymphs collected in the field jn July that werc the parents of the TRuni experimental population were at least 2-4 weeks from emergenco. yot in our common environment experiment there was only 1-2 days di.EferC'IIC!€' in development Ume between their oEfspring and Lhose of crickets collected as adults in July. There was also no evjdence for fi trftit eomplex assoeiated with voltinism (e.g. long developm(~nl. and hl.gh diapause as a univoltine genotype); diapause propenslty did not clifrer between the two transition zone phenotypes. There was a strong effect of environment on pheno]ogy in the transition populations. The eat'ly hatchlng group were exposed to shortor photoperiods in the early instars: this prohahly was responslhle for the faster development (Tanaka and Brooks 1986). Shorter developlllHnL for individuals hatching early could be adaptive in jncreasing the likelihood of a successful bivoltine phenology, in, a warmer than average year or a warm mierohabi tat. The early and average gt"oups produced intermediate diapause fractions, and were partially bivol Li 110. while the late hatehing group produced only diapause eggs, presumably because the photoperiodie eues indicated to the [emale that insu[flclenL growing season remained for a second generatlon. Variation in voltinism can be summarized in the following way. While adult emergence is unimodal, there exists large environmentally induced variation in the date of emergence. Females are able to esLilllatc the date from photoperiod, and depending when the y are reproduclng, will adjust the proportion of diapause eggs that she lays. A female eclosing 36 early Ln the summer will lay direct developing eggs, and assume a bivoltlne phenol ogy , whereas a slower developing female will take the safer univoltine strategy. Thus voltinism is a maternally direeted conditional strategy, which allows the female to maximize her fitness by adjusting her life history, especially in response to the large environmental variation in development. CONCLUSION Life history variation in h. fasciatus is the result of a combination of genetic differentiation and phenotypic plasticity. Phenol ogy is large1y regulated by the thermal environment through its influence on the rate of development of eggs and nymphs. Like a number of other insects, only small differences were found in thermal requlrements for development between populations. although the genotype by environment interactions for development may be adaptive. Because photoperiodic cues are ambiguous along the cline in season length, selection has modified the reaction norm for diapause to suit local 1 ~ conditions. Variation in voltinism in the area of transition between the univoltine and bivo1tine areas is conditional strategy, and the strategies used by individual females will be the topie of investigation in the subsequent chapters of this thesis. , , {: 37 REFERENCES Allen, J.C. 1976. A moclified sine wave for calculaltng d<>grcp dHyS. Environmental Entomology 5:388-397. Beck, S.D. 1980. Insect photoperiodism. 2nd ed. Acad{'mic Press. New York. Berven, K.A. and D.E. Gill. 1983. 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Adaptation to seasona1ity in a cricket: patterns of phenotypic and genotypic variation in body size and diapause expression along a cline in season length. Evolution 43:1483-1496. Nautical Almanac Office. 1989. Nautica1 almanac for the year. U.S. Naval Observatory, Washington, OC. Pullin, A.S. 1986. Effect of photoperiod on the life-cyc1e of the peacock butterfly Inachis io. Entomol. exp. appl. 41:237-242. Ritland, D.B. and J.M. Scriber. Larval deve10pment rates of three putative subspecies of tiger swallowtai1 butterflies, Papilio &laucus, and their hybrids in relation to temperature. Oecologia 65:185-193. RoH, D.A. 1980. Optimizing development time in a seasonal environment: the 'ups and downs' of c1inal variation. Oecologia 45:202-208. Roff, D.A. 1983. Phenologica1 adaptation in a seasona1 environment: A theoretica1 perspective, pp. 253-270. In V.K. Brown, and J. Hodek (eds.). Diapause and 1ife cycle strategies in insects. Junk, The Hague. Rllffner. J.A. 1980. 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Oec010gia 47:291-298. .. 40 CHAPTRR 2: Bet-Hedging and Phenotypic Plasticity in the Diapause Strategies of the Cricket Allonemobius fasciatus. 41 1 ABSTRACT We investigated the roles of bet hedging and phpnotypic pln~t icilv in the diapause strategy of a partial1y bivolttnE' population of tl\(' cricket Allonemobius fasciatus, Whpn reared under nal-ueall y chang! ng environmental conditions. first-generation [emalps prOdlH'C'cI i lH't'('c1S j nr, proportions of dlapause eggs over their reproductive 1jfespnn. presumably reflecting the decreasing probabil i ty ovC'r 1 i nll' thal sufficient growing season remains for a second gcneration. 'l'ho transition between direct-developing to diapause eggs occllrrecl ovC'r 3~ da ys for the population suggesting a considerable degrec of beL hedglllg, The analysis of individuals. however, revealed a much more rflpid response and large between-female variability in the median (50%) diapause date, This rapid transition from non-diapause Lo dlapause emv'l implies that interannual variability in season length has not rcsu]ted in the evolution of a substantial risk-spreading strategy Al the 1 (~vcl of the individual. Our results highlight the need to study Ilfe-hislory variation at the level of the individual rather than the popula! ion. '. 42 t INTRODUCTION Bet hedgtng and adaptive phenotypic plasticity are two strategies of phenotypic diversification that can increase fitness in temporally variable cOtlditions (Stearns 1976, Bradshaw 1965). They represent two alternatives for dea1ing with the shifting selective regimes associated wi th temporal heterogeneity in the environment (Lloyd 1984). In an unpredictab1y variable environment an organism may produce a variety of offspring phenotypes to ensure that at 1east sorne will be suited to unknown future conditions (Cohen 1966). Thi.s is a bet-hedging or risk-spreading strategy because fitness may be reduced in any single generatlon due to the production of non-optimal phenotypes. However, across generations a genotype with this strategy may be favored because at least sorne offspring will be adapted to the unknown future environment. More formally. arithmetic mean fitness will be reduced, but the geometric mean over generations will be maximized (Gillespie 1977, Seger and Brockmann 1987). The classic examp1e of this form of bet hedging is seed dormancy, where the germination of an annual plant' s seeds is spread over a nurnber of years because sorne years will be complete1y unsuitab1e for growth and reproduction (Cohen 1966). In contrast, if cues are availab1e that a110w forecasting of the future state of the environment and appropriate adjustments can be made ta the phenotype, a reaction norm may evolve that maps the phenotype to the envi.ronment (Lloyd 1984, Stearns 1989). Adjustments to the phenotype may take place during the development of the individua1 (Stearns 1989). or may be transmitted to offsprino to ensure their phenotype i5 Bppropriate for the predicted change in the environment (e.g., insect diapause in response to the onset of winter; Tauber et al. 1986). We restrict ourselves here ta Bdaptive plasticity, or changes that will result in increases in fitness, rather than those changes that are mere1y consequences of interacti ons between the envl ronmont Rnd ttll' physio10gy or chemistry of the organ ism (Stearns 1989). These two strategies are. of course. not mul.lIal1 y ext'lus ive /llIcl are 1ike1y to co-occur in cases where the eue 1.0 prNtict Lhe (uturp environment Is weak (e.g., Hoffmann 1978). Although such H eue will permit sorne adaptive modificatlon of the phenotype, the considcrHblc uncertainty still existing might favor a degree of bet hcdgi nB (Coopel' and Kaplan 1982). An examp1e of 1ife-history evo1ution in a temporally variablC' environment which has reeelved sorne theoretica1 treaLmcnl 15 arthropod dormancy (Levins 1969, Cohen 1970, Taylor 1980, Istock 1981. lIairston and Munns 1984, Taylor and Spa1ding 1988). Insects living in a l:w;lsorléll environment must complete their life cycle durlng the porti on of the year in which conditions are suitable for growth and reproduction. In tempera te regions the first frosts of winter mark the end of lhe grow i nt) season and will kill aIl individuals not in diapallsc. the overwintering slate. Since diapause is usual1y specifie to one stage ln the li[e cycle, the choiee whether ta enter diapause or attempt Lo comploLe another generation must be made weIl in advance of the end of the SeHSOI1 (Taylor 1980). Thus the fitness of an insect will depend heavily on making appropriate diapause decisions. This is an examplc of phcllotypi c plasticity because photoperiod ls normally used as a eue [or diapausc expression (Tauber at al. 1986). If the end of the growing seuson ls noL variable there should be a sudden shlft to diapause for a11 members of the population when an additional generation cannot be completed (Cohen 1970, Hairston and Olds 1987, Philippi and Seger 1989). In realHy, unpredictable weather and microclimatic variations resull in Intcrannual variability in the length of the season and, thereiore, the optlmal date for switching ta diapause generation will be uncertain. lIence a mixture of bet hedglng and phenotypic p1asticity might be iavored, resulling in 44 a gradual increase ln the frac tion of indivlduals entering diapause. • wi th a fcw appearing early in the season ref1ecting the smal1 probability of an early frost (Seger and Brockmann 1987). In thls paper we examine diapause strategies of a partially bivoltine populatIon of the North American cricket Allonemobiuq fascia tus . 11. fasclatus diapauses in the egg stage and females can pr.oducc mixed batches of diapause and non-diapause eggs, making it an idesl organl sm to study phenotypic di versification. In eastern North America this species 15 univol tine in the north, and bivol tine in the southern part of its range, the resul t of adaptation ta the lati tudinal cline in season 1ength (Mousseau and Roff 1989). In the region where the transition between univoltine and bivoltine life histories occurs. voltlni sm 1s a conditiona: strategy that may depend on the growing condi tions experienced during early summer. First-generation fema1es that are able to reproduce early in the season lay non-diapause eggs that produce a second generation, whereas individuals reproducing later in the year are constrained to a univo1tine phenol ogy and produee only diapause eggs (Chapter 1). We examine here the production of diapause eggs by individuai fema1es under environmental conditions simulating those in the transi tion zone of conditional vol tinism. In accordance wi th the expectations of phenotypic plasticity, we predict that females should use the photoperiodic eue and switeh from producing direct-developing eggs ta diapause eggs as the season progresses. Because the exact date of the end of the season is uncertain, the transition between egg types lDay be graduaI as a form of risk spreading. or bet hedging. As we consider diapause an adaptation ta the ablotic thermal environment we prcdict that there is an optimal diapause fraction for each day during the reproductive season. and that aIl females should follow this (. strategy. Interfemale variation should be low, the resu1t of stabilizing 45 selection on the strategy that maximizes the geoll\(, 1 ric 11\()é\tl l'il 1 t) of increase (Seger and Brockmann 1987. Gillespie:> 1977). METHODS More than 100 A.:.- fascia tus adul ts wel"e coll cct cd fl'Olll Il 1110 isl field near Danvi1le VA (36°l~O'N, 79°25'W) dudng laLc .July 1988 and returned to the 1aboratory. Crickets were l"eared ln pl (30 x 17 x l4cm) and fed lettuce and erushed cat food (Moussellu and Rof f 1989), These field-collected adults were allowcd to bl't'cd rallCloml y ln diapause-averting conditions (30 o e. 161.: 9D): their eggs wcre col h'el('d and ineubated until hatehing. This second generation was also rcared ill mass culture, but under fall condidons (25°C, 13L:llD) Lo clIsun' production of diapause eggs. Eggs produced by second-generalloll mlull s were chilled for 3 months at 4°e and then incubal"cd al 28°C 1I11111 hatching. Mouse boxes each containing 100 new] y hatched nymphs wpre> pl él(!('(1 in an incubator prograrnmed to simulate the seasouRl eyel e of l0mpcr;llur(' and photoperiod (Fig.1 ). Photoperiods, inc1uding civil lwnjght (Beek 1980), were obtained from the Nautical Almanac Offi ce (1989), and werc changed twice weekly. Temperatures used were the Hverag!.' daily mi.nimulII and maximums from long-term records for the collection si Le (Ruffrwt' 1980) and followed a 12:12 hr thermoperiod, with the increasc set 1.5 hours after the lights went on. The temperature cycle followed a squAre> wave, with the transition between extremes taking 1.5-2 hr. The stélrlillg date for the incubator was the average hatehing délte prcdicted from the thermal summation of historical temperatures (AJ len 1976). Wf! used Il thermal requirernent for embryonic development of 200 dcgree days (DD) above a base of 14 oc found for this and other orthoptcran spec: i es (Tanaka 1986, Masaki and Walker 1987) for the calculaLion of lhe 46 (~ ha Lchi ng date. The nymph cages were /Ilaintained every 4 days unti1 the first lldul Ls Ilppearecl. Subsequent1y, the cages were checked on al ternate days and the newl y ec10sed adu1 ts removed. Adu1 t emergence was spread over a 2/~-day period. Fema] es and males emerging over the same 2-day period were paired random1y and reared in plastic sandwich boxes (Mousseau and Roff 1989). A roll of moistened cheesec10th was placed in eaeh box for oviposi tian. At 4-day intervals the cheesecloth rolls were removed and the eggs counted out onto moistened paper towe1 in a smaU petri dish. The eggs were then incubated in the same environment as the adu1ts for 30 days or until a11 the non-diapause eggs hatched. Dead eggs were removed, and the proportion in diapause was ea1eulated from counts of hatched and healthy non-developing eggs. Fema1es produced an average of 23.8 viable eggs each 4 day period. Nine egg batehes spanning 36 days of reproduction were co]leeted from each female. Males that died during thls porlod were replaced wi th spare males of similar age. Diapause is a materna1ly transmitted trait and the genome of the male parent has no detectable influence on diapause in the eggs (Tanaka 1986, Mousseau and Bradford, unpubl.). To de termine if the changing environmenta1 conditions during adu1t life affected diapause expression, 12 pairs (ca11ed Group 2) that emerged over a 4-day period early in the experiment were transferred to a second incubator where the photoperiod and tempe rature remained constant at the conditions when final ecdysis occurred. Eggs were co11ected and scored as above. At the end of the experiment the parents were collected and the lcngth of the right femur and wingform (maeropterous, micropterous. see Fulton [1931} for details) were recorded. Diapause response curves were modelled by logis tic (~ t'agression fit by the maximum likelihood technique (SAS Inst., 1988): ., 47 where R is the proportion in diapause and ~ is -'uU an cilly. AIl clj élpltUSf' proportions used in other analyses were transformed DS tIres! n(lE) pdol' to analysis (Neter and Wasserman. 1974). To estimate the proportion of dia pause eggs 1 a 1d j n the [1 el d. Il small number of pairs were collected from the sampling si te in Illtt> Jul y and mid September, 1988. placed immed ia tely in cages élnci all owed to oviposit. These pairs were kept in the back seat of a CRr [or ~ dllys during the samp1ing trip: temperatures and photoperiods wel't> sil1l11tll" lo ambient conditions. The eggs laid were incubated in the Lélborntory êlnd scored for diapause as above. RESULTS For Group l females, th~ proportion of diapause eggs laid increased from 0 to 100% over the course of the exper iment (sol ici circles, Fig 2); the transition occurred over a 35-day perloci, corresponding ta 1ate Ju1y and early August. The median logistic !.il date was August 4. and the central 60% of the response (i. e.. frolll 20 to 80% dia pause , Taylor and Spa1ding [1986 J) spanned 21 days (F ig. 2). Nine of 12 pairs a110wed to reproduce immediately aCter capture in the field in Ju1y were successfu1 and produced an average 15.7±6.0r. (SR) diapause eggs (range 0-55%). These eggs were laid between .lul ian days 207-211 and the proportion in diapause was similar la the üxperilllcntlil results for the same period (Fig. 2). AlI pairs co1lected in Seplember (n=ll) laid exclusively diapause eggs. The change in diapause proportion over time was partl al1y the resul t of changing photoperiod and temperature during adul t ] ife. Group 48 ( 2 pairs, reared in constant conditions after ecdysis (open circles Fig. 2), produced lower proportions of diapause eggs than those in changing conditions. Both the slope and intercept of the fitted logistic regresslon for Group 2 pairs were significantly different from those of the main group (t-test, ,E With date controlled. the propol'tion of dinpauso cggs lllid IMS higher for females which took longer Lo becoll1c adu1t s lhan Lhosc wh! ch developed more rapidly. For eggs laid betweell days 202 and 200. the average diapause proportion (arcsin J'i. trans[orrned) was stgn lfi CélnL 1 y correlated with the developmenL time of the mothC'r Cr=O. 50. }?=O, 0009. n=4l). The exact interval chosen for this illwlysis was not Impol't RuL : similar resuI ts were obtained later in the expt'rlmenL (d.nys 226 - 230. r=0.44, ~=O.007. n=37). Phenotypic correlations betwcen diapause and oLher traiLs of the female were estimated using the mean diapause proporLion OVO!' her fi l'st 16 days of reproduction as the dependeut variat e in <1 lIIull i pl e regression ana1ysis (similar resuI. ts were obLai ned if the avcrag ing wus done over the first 12 or 20 days of egg laying). Diélpause proportion Wc!lS positively corre1ated with ernergence date CE be expected given the strong l'ole of the changing photoperiod Oll diapause expression. Neither wingforrn (f=O.14) , Eemur length. Hn index of body size, Œ=0.37) nor totaJ fecundity (P'=0.42) were signific;Hll in the mu! tiple regression . .... 50 • [j gt c 51 "C 16 •..•...... •.•.....•. " .. - ..... o ...... 4 ...... " ...... -5315 '. '. a. '. -§14 .r:::. a.. 13 30 ~ 3 "C 20 ~ elc: L..-....L_...i.-----I---.J._....L----'----"_-'---"-_"------'--' 10 m 150 170 190 210 230 Julian Day Fig. 1: The photoperiodic regime (dashed line) and maximum and minimum daily temperatures (lower lines) used in the experiment. The average hatching date was day 140, and adult reproduction spanned days 190 to 240. 50 1.0 Q) • ~ 0.8 ca a. ca ë5 0.6 c::: • o ~ 0.4 o a. o 0 o d: 0.2 ...... ~ .. ' 200 210 220 230 240 Julian Day Fig. 2: The average proportion of diapause eggs produced by all females during the experiment. Solid symbols are Group l females that experienced continuously changing conditions during adult life; open symbols are Grollp 2 females that were kept under constant conditions as adults. Parameters bo and b l and sample sizes for the logis tic regressions are: Group 1: -28.1, 0.130, n-9l10; Group 2: -25.9, 0.111, n-l224. Also indicated is the mean (±SE, box) and range of diapause egg production for 9 females collected from the field. A second collection , of females made between days 255-260 laid 100% diapause eggs. "4\> 52 ....." 4 6 13 12 15 12 17 15 14 7 Cl) 1.0 CI) ::J ca 0.8 0- .-ca Cl 0.6 c: .-0 0.4 "t= 0 0- 0.2 0 '- a.. 0.0 6 6 8 9 10 8 7 6 5 1 190 200 210 220 230 240 8 1- [)( 6 Y)< )< 1- 4 x /<.'" t- )()(,)< x 'X '>& .x )< 2 1- -X < :x X' ~ ~ IXXXX Wxxx ~ ~>Q< 1 190 200 210 220 230 240 Julian Day Fig. 3: a) The diapause proportion for individual Group 1 females. Shown are fitted logis tic regression lines for 24 females that exhibited a transition to diapause egg production, as well as the average proportions for females producing largely (>90%) one egg type only (solid circles). Nurnbers indicate the number of females included in these averages; note that each female 1s often included in more than one average. b) Histogram of median diapause dates calculated from the logis tic regressions in (a). ~~------ 53 1.0 ...... - l,,'" -- Q) ... en 0.8 .. ::s 1 " ctS 1 c.. 1 " 1 " 0-ca 1 C) 0.6 1 c: / a / .- 1 1 t 0.4 1 a 1 " a. Population 1 " " e 0.2 L... ".. "," c.. .- '" Individuals -- .; 1 0 • -20 -10 0 10 20 Day Fig. 4: The average transition from non-diapause ta diapause eggs for 24 females of Fig. 3. Data for each female were centred by setting the Median day (i.e. when the proportion diapause predicted by the lagistic regression was 0.50) ta 0; error bars denote 1 standard error. Salid line is the logis tic regression derived by averaging regression parameters for the 2,~ females of Fig. 3. Parameters bo and b l are -90.8 and 0.422. Dashed line 1s the logis tic regression for the experiment fram Fig. 2. ( 1 1 DISCUSSION The switch from direct-developing ta diapause ('gg produclioll iH Cl reaction norm that presumably reflE:'cts the dccreasJng 1 ik01 ihoot! ov('r time that a second generation will be able ta succcssfully dcvclop nncl reproduce before winter. Reaction norms for diapause are largely cued by ('tlvieonmcllLétl conditions experienced during the nymphal and adul L stnges (THllbct' pl al. 1986). and thus are adaptive in allowing the insect to makc appropriate diapause decisions. according to when in the seasoll tflC' female is 1aying eggs. Our Group 2 resul ts. as WE'll as t-llOSC of Tnnnka (1986) and Mousseau (1991). suggest that induction of diapnllse ill fl. fasciatus is due to a combination of photoperiodic cues rccclv€'c1 during both the nymph and adult stage. as well as maternal age. Kidokoro and Masaki (1978), Dean (1982) and Ingrisch (1987) have a1so found ln ollwl orthopterans that decreases in photoperiod at eeloslon or duri IIg the adult 1ife increased the incidence of egg diapause. Although the graduaI transition from non-diapause to diapmlsc cggs for the population as a whole suggests that i nterannual varinb! 1 il yin the season has resulted in the evolution of a bet-hedging or risk-spreading strategy (Philippi and Seger 1989), we found thnl t~lC' graduaI slope of this curve is the resul t of the averaging of [emnl cs with steeper slopes and variable median diapause daLes (Figs. 2 tlncl 3). The steep average slope for individuals implies that the Interannual variability ln season lengLh has been insufficient to resu1 t in the ev01ution of a graduaI diapause curve as predieted by bet-hedginB Lheory (Seger and Brockmann 1987). In contrast, females of the crickcL Gryll Ul-l firmus, 1ay mixtures of the two egg types (Ibrahim and Walkcr 1980), and Walker (1980) hypothesizes that this is a bet-hedgi ng strategy J ri response to unpredictable variation in soil moisture and temperaLure. 55 The pond-dwell1ng copepods studied by Hairston and Olds (1987) cannot produce mixcd broods of eggs, and thus females are incapable of gradually altering the diapause production of their offspring in response to environmental variability. The large interfemale variability in the timing of the production of diapause eggs is inconsistent with the prediction that selection should erode variability around the optimal diapause strategy (Taylor 1986, Seger and Brockmann 1987). A1though the distribution of median dates was suggestive of normaJizing selection, the range spanned more than a month and there were many females which exhibited n0 transition between egg types. indicating their switching dates occurred before ec10s10n or after the experiment ended. Large interfema1e variation in diapause egg production has also been found by Walker (1980) and Halrston and 01ds (1987). We cannot separate genotypic and phenotypic sources of variation, but experiments with this and other species have often revealed large heritabi1ities for diapause expression, suggesting that sorne of the variation has a genetic basis (Hoy 1977, Tauber et al. 1986, Mousseau and Roff 1989. Hairston and Dillon 1990). Although extremely limited, the diapause data from the field- caught pairs corroborate our experimental results and lend support to our confidence that the artificial rearing conditions employed wer~ successful in mimicking the natura1 environment. Our method of changing photoperiods and temperatures was a1so successfu1 in reproducing the phenologies of three separate A. fasciatus populations (Chapter 1). Interfemale variability in diapause expression could have been amplified by our artificia1 photoperiod regime: we were unab1e to change the photoperiod at very small (i.e .. daily) Increments or reproduce the spectral changes in sunlight associated with dawn and dusk. Nonetheless, naturally changing photoperiods appear important in the diapause response: for ~. fasciatus the transition between egg types is much more 56 gradua1 for fema1es reared LInder constanl 11 ghl cond 1 t Ions «(' r "'Ir., ) and Mousseau 1991). Taylor (1989) also not(l8 thal vadl1billty ln diapause is mueh ]ower under naturally changing comparcd to conslllnl photoperiods. We found a correlation betwecn the durrttion of L1w lIylllphnl pl'r!Oc! (Le., development time) and diapause thal result-cd in slowcr-clt'vC'loping females producing more diapause eggs on él glven dély thl1l1 [('mnles whil'h developed more quick1y as nymphs. This is contrary la Ihe pl't'diel ions or mode1s of diapause (Cohen. 1970; Taylor, lqSO) thaL all rl'lIlalcs should 1ay the same fraction of diapause eggs on fi given day, rC'E,<1rdless or their age or other characteristies. The correlation Olély he> r.,enel ie. simil ar ta the genetie correlation between diapnllse and body si ZC' rOlllld for some populations of the same species by Mousseau clnd RoU (1989). Alternative1y. it may be partia11y explained by a physiologiel\1 mechanism as suggested by a current hypothesi s fol' the indue 1 i on of diapause (Taylor, 1986). Accordillg to this hypothes is, a ccrl/l i n 11111111>('1' of days with a photoperiod 1ess than a critieal day] cngth mmll I>e experienced during a sensitive period be[ore diapause is inducvd (Saunders. 1987: Taylor. 1989). If the sensitivE' perlod is élll ontogenetic stage such as the ul timate nymphaJ i nslar or 1 hl' P(·rj od between eelosion and sexua1 ma turi ty, slow-developing cr i eket s wUI lwv(' a longer sensitive period and will experience shorter photopcriodH ir the sensitive period oeeurs after the vernal equinox (Fig. 1). BoLh factors will contribute ta inereased diapause pl'opensity and will r~sull in a correlation between deve10pment and diapause. Non-gC't1etl c phenotypie variability in development rate due to envi ronlllC'rllal influences (e.g .• temperature, food) or random internaI physioJogical events will affect the length and the date of the sensitive pcriod, arul - will result in variation in diapause expression. Normalizing selection for an optimal diapause strategy will have little effect on rcducinr., 57 ( LhJs source of variation in dJapause unless it acts on the environmental variance of development. The djfference between the population and individual responses in our experiment emphasizes the need to work at the level of individuals when studyinB phenotypically variable life history strategies (KLngsolver 1989). Population-leve] results are most applicable if aIl lndlviduals or genotypes have the same response as the population as a whole. This was not the case in our study. and averaging ov~r aIl femaJes masked the shape and variabjlity in the response of individuals. In Chapter 4 wc investigate the relative roles of genetie and phenotypic sources of variation and if elements of the physiology or genetie architecture such as those deseribed above contribute to the maintenance of variation. The possibility that non-genetie phenotypic variability is adaptive and Ls maintained (Slatkin and Lande. 1976: Kaplan and Cooper. 1984: Bull, 1987) also needs ta he assessed. In summary. for a partially bivoltine population of A. faseiatus. experimental results indicated that a switch to the production of diapause eggs oeeurs in late July. The steep slope of the diapause l'esponse for individuals suggests that interannual variation in season length has been insufficient ta result in the evolution of a suhstantially graded risk-spreading response at the level of the individual. In the next chapter we model the life history of A. fasciatus to predict the optimal diapause response function and determine the fitness consequences of variation around that optimum. ( 5B 1 LITRRATURE CITED Allen. J.C. 1976. A modified sinE' wélve for cfl1cl\1nt ille dl'!:,"(,(' dnvs. Environmenta1 Entomology 5:388-3Q7. Beck. S.D. 1980. 1nsect photoperiodism. 2nd ed. ACHdülllic l'I'('S8. Nl'W York. Bradshaw. A.D. 1965. Evolutionary sigllificancc of phellotypic plnsticity in plants. Advances in Genetles 13:115-155. Bull. J.J. 1987. The evo]ution of phenotypic variance. Ev()l\1ti()1l/il:30·~· 315. Cohen. D. 1966. Optilllizing reproduction in il randolllly vélrylll~ environment. Journal of Theoretical Biology l?:llq-l?q Cohen, D. 1970. A theoretical model for the optimal 1 illlln~ o[ diélPllItS('. American Naturalist 104:389-400. Cooper. W.S •• and R.H. Kaplan. 1982. Adaptivp "coin f1 ippinr,": il decision-theoretic examination of nntural sf'lccLion l"andom individual variation. Journal of Theoreticnl Biology 94:135-151. Dean, J.M. 1982. Control of diapause induction by é\ chanel' ill photoperiod in Melanop1us sanguinipes. Journal of Insc('L Physiology 28:1035-1040. Fulton, B.B. 1931. A study of the genus Nemobius (Orthoptern: Gry11idae). Anna1s of the Entolllological Society of America 24:?05- 237. Gillespie. J.H. 1977. Natural selection for variances in offspring numbers: a new evolutionary principle. Arnerlcan Naturalist 111:1010-1014. Hairston, N.G .• and W.R. Munns.1984. The timing of copepod di an evo1utlonarily stable strategy. American Natural ist 123:711- 751. Hairston, N.G., and E.J. Olds. 1987. Population dif[ercnccs Jn the 59 timing of diapause: a test of hypotheses. Oeco10gia 71:339-344. '~'rslon, N.G., and T.A. Dil]on. 1990. Fluctuating selection and response in a population of freshwater copepods. Evolution 4It:1796-l805. Hoffmann, R.J. 1978. Environmental uncertainty and evolution of physiologieal adaptation in Colias butterflies. American Naturalist 112:999-1015. llowtlrd, D.J., and D.G. Furth. 1986. Review of the Allonemobius fasciatus (Orthoptcra: Gryllidae) complex with the description of two new species separated by electrophoresis, songs. and morphometrics. Annals of the American Entomological Society of America 79: 472- 481. Hoy, M.A. 1977. Rapid response to selection for a nondiapausing gypsy math. Science 196:1462-1463. Tbrllhi m. R., and T. J. Walker. 1980. Diapause and nondiapause eggs laid by individual Gryllus firmus females (Orthoptera: Gryllidae). Florida Entomologist 63:510-512. Ingrlsch, S. 1987. Effect of photoperiod on the maternaI induction of an egg diapause in the grasshopper Chorthippus btrnhalmi. Entomologia experimentalis applicata 45:133-138. Tstock, C.A. 1981. Natural selection and life history variation: theory plus lessons from a mosquito. In R.F.Denno and H.Dingle (eds.) 1nsoct life history patterns: habitat and geographic variation. Springer-Verlag. Berlin. p.113-127. Kaplan. R.H .. and W.S. Cooper. 1984. The evolution of deve10pmental plasticity in reproductive characteristics: an application of the "adaptive coin-flippinglt principle. Amer. Naturalist 123:393-410. Kidokoro. T .. and S. Masaki. 1978. Photoperiodic response in relation to variable voltinism in the ground cricket Pteronemobius fascipes ( (Orthoptera: Gryllidae) Japanese Journal of Ecology 28:291-298. 60 1 Kingso1vcr. J.G. 1989. Weat-her and the popu1ation dynlllllics of iruil'ds: integrating physio1ogical and popullltion ecology. PhYfliologll'al Zoo1ogy 62:314-334. Levins. R. 1969. Diapause as an adaptive stnlteBY. Symposin of Ihl' Society of Experimental Biology 23:1-10. Lloyd, D.G. 1984. Variation strategi<.>s or plants in hctE't'ogl'lll'OIIS environments. Bio1ogical J. of the Linnaenn Soc 1('ty 21: 157 -1Re). Masaki, S., and T.J. Wa1ker. 1987. Cricket lirE' cyclo!'>. Evolul iOlwry Biology 21:349-423. Mousseau, T .A. 1991. Geographie variation in maternaI élge p[rcc! H on diapause in a cricket. Evo1utioll 45:1053-1059. Mousseau. T.A., and D.A. Roff. 1989. Adaptation ta sCHsonalily in:l cricket: patterns of phenotypic and genotyplc vnric1tloll 111 hody size and diapause expression a10ng a cU ne in scason 1 ength. Evolution 43:1483-1496. Neter. J .. and W. Wasserman. 1974. Applicd linea .. regrl'sslon iJIw1ysls. Irwin. Homewood IL. Nautical Almanac Office. 1989. Nautica1 a1manac [or the yen ... [J.S. Ndvill Observatory. Washington. DC. Philipi. T .. and .1. Seger. 1989. Hedging one's evo1uLionary }>pts. revisited. Trends in Ecology and Evo1uLion 4 :/11-1,". Ruffner, J.A. 1980. Climates of the states. Gan Rescarch c.o .. Detroit. MI. Saunders. D.S. 1987. Photoperiodism and the hormoTIfl] control of Inseel diapause. Science Progress. Oxford 71:51-69. SAS Institute. 1988. SAS/STAT guide for the persona1 computer. version 6.03. Cary. NC. Seger, J .. and H.J. Brockmann. 1987. WhaL is bet-hedging? Oxford StJrvPys in Evo1utionary Biology 4:182-211. - Slatkin, M. and R. Lande. 1976. Niche widlh in a fluctuaI tne , 6] envl rortmont-rlcnsi. ty ind(·pendent mode1. Amer. Naturali.st 110: 31-55. SL0srns, s.e. 1976. Life-hjslory tactics: a review of the ideas. Quarterl y Review of Biology 51: 3-47. Stcllrns, s.e. 1989. The evo]utionary significance of phenotypic plastLcity. Bioscience 39:436-445. Tanaka, S. 1986. Developmenta1 characteristics of two closely re1ated species of Allonemobius and their hybrids. Oeco1ogia 69:388-394. Tanaka, S. 199]. Genetic compatibility and geographic profile of two closely related species of Al1onemobius (Gry11idae: Orthoptera). Anna1s of the American Entomological Society 84:29-36. 'rauber, M.J .. C.A. Tauber and S. Masaki. 1986. Seasona1 adaptations of insects. Oxford University Press, Oxford, OK. Taylor, F. 1980. Optimal switching to diapause in relation to the onset of winter. Theoretical Population Biology 18:125-133. Taylor, F. 1986. The fitness functions associated with diapause induction in arthropods. 1. The effects of age structure. Theoretica1 Population Bio1ogy 30:76-92. Taylor. F. 1989. Diapause induction in changing photoperiods. Journal of Theoretical Bio1ogy 139:103-116. Taylor. F. and J.B. Spa1ding. 1986. Geographica1 patterns in the photoperiodic induction of hibernal diapause. In F. Taylor and R. Karban (eds.) The evolution of insect life cycles. Springer-Ver1ag, Berlin, p. 66-86. Taylor, F. and J.B. Spa1ding. 1988. Fitness functions for alternative developmenta1 pathways in the timing of diapause induction. American Natura1ist 131:678-699. Wa1ker, T.J. 1980. Mixed oviposition in individua1 fema1es of Gry11us firmus: Graded proportions of fast-deve1oping and diapause eggs. Oecologia 47:291-298. 1 62 CHAPTER 3 Seasonality. environmental uncertainty and illsecl donnHllcy: An empirical model of dlapause strategies in the CI'idwl AllonCIllObilll'! fasciatus 63 ABSTRAC'f ~c rnodel1ed the effecL of heterogeneity in climate on the diapause strategies of a partially bivoitine population of the striped ground cricket Alionemobius fasciatus. First-generation females of this population produce mixtures of direct-developing and diapause eggs, and bet-hedging theory predicts a graduaI increase in the proportion of di apause eggs laid as the likel ihood that a second generation will be able Lo complete developmenL before winter decreases. We quantified the Lhermal regime using long-term meteorological records. and constructed an analytical model based on empirical components for growth and reproduction. The timing of the switch to diapause egg production predic ted by the model was similar ta experimental and field data, but interannual variation in climate was not sufficient for the evolution of a marked bet-hedging response of the production of mixtures of diapause and non-diapause eggs. Climatic variability was also not sufficient to select [or a strategy of rand am developmental variation in switching date ('adaptive coin flipping'). Selection against genetic and developmental variabili ty around the optimal diapause strategy is weak, and may contribute to the maintenance of intrapopulational variation in diapause propensity. INTRODUCTION A1though ev01utionary blologists havE' long n'cogni~-:NI tllllt ail organism's environment varies over time. il is ollly r(,cC'lltly t!till lire history theory has incorporated this featu,"e Ltlto models prcclicl illg Ilw evolution of demographic traits (Cohen lCJ66. Lpvins lQ69. l'arlJ"idgl' ill\d Harvey 1989). ln sorne cases theoretical analyses suggesl Lllé.ll oplÎllléI1 life histories may be differenl when l he envi rOlUllenl 1 emporal1 y variable. especially when that variation is on a slmilal" scale 10 Ihe generation time of the organism (Cohen 1966.1970. Laccy ('1 i11. 19!V!. Leon 1987. Venable and Brown 1988). Insects living in temperate climates must shnp(' thl'ir 1irt, histories around the fact that on1y a portiotl of Ihe year is suilahlL' for growth and reproduction (Tauber et al. 1986). TllP 5(';)501);1) il Y of their environment has resul ted in lhe evoluLion or donn/mcy or lIIigl"lIl i 011 strategies to enable the insect to survi.ve the unfavol"ahll' willl <>1' months. On the average the seasonal cycle is rel1able élnd lIIany i IIs('cl s have high1y developed photoperiod sensing systems for aùjusl illg IIlPi r life histories in advance of these changes (Saunders 1 (87). F'WIIII.;II i Vp adjuslments to the phenotype in response lo sueh a cup ill lhe environment are c1early examples of adaptive phenotypic plnsl icily. li subject of much current interest (Bradshaw ] 965: St(~artls 1989: MOllsseau and Ding1e 1991). A second aspect of insect seasonalily ls lhe uncertainty due lo interannual variabil Hy in the environment (Hoffman 1978). l~ec;\Use of fluctuations in temperatures, rain[a]1, plant phenology or ollier factors, it may not be possible to [orecasl exact1y when the envi ron/llcnl will become favorable or adverse to growth and reproduction. Theory predicts that uncertainty may lead lo changes j n the opLi nUI l 1 if e history, usually with the goal of minimizing the varJabl1ily in filness 65 1 acrOSR years (DcmpsLcr 1955; Gillespie 1977). This can be accomplished by two rclated strategies, known as risk aversion and bet hedging (reviewed by Frank and Slatkin 1990). Risk aversion increases fitness by "enderlng offspri ng ] ess vulnerable to environmental variation; this can be accompl ished by illcreasing offspring size or parental care. possibly with a corresponding decrease in offspring number (e.g .. Boyce and Perrins 1987; Schultz 1991). On the other hand. bet hedglng is the production of phenotypically diverse offspring to ensure that at least sorne wll] be suited for the unpredictable future conditions. The classic eXRmple of the latter strategy is seed dormancy in annua1 plants: seed produced in one year will germinate over a number of years in order to minimize the risk of a complete reproductive failure due to po or conditions in any single year (Cohen 1966: Leon 1987). In both cases the vartabiltty in fitness caused by temporal environmental heterogeneity is reduced, and the geometric mean 15 maxlmized. ln this paper we analyze the diapause strategies of a partially bivo1tine population of the striped ground cricket (Allonemobius fHsc1atus). Pemales of the first generation of this population must "IIIRke" R decision to 1ay either safe. but lower value. diapause eggs wh i ch will overwinter until the following spring or to lay direct dcveloping eggs which will form a second generation. The latter choice lws the potential of greater fi tness payoffs, but also is riskier if the year turus out to be cold. preventing the second generation from developing and reproducing before the onset of winter (Roff 1980). ParLially bivoltine insect populations have been the subject of a numbel' of theoretica1 models which predict the timing and nature of the switch from direct development to diapause offspring production in relation the me an and variance of the end of the season (Levins 1968.1969: Cohen 1970: Taylor 1980; Seger and Brockmann 1987). The models Indicate that in a constant environment a switch to diapause will t 66 occur al the polllt in the S€élSOn WIWll 1 hL' contl"ibuliol1 to fïllH'l>:-l of Il non-diapause individwll raUs b~low thal of 0Ill' tn tlillpmlflP. " s('collcl prediction is that wit-h variabilily ill LIll"' lellgth or ll\l' Bl'owil1B flt'111WI1 t here should be a gradual increase in the propol't i 011 cl 1 time. reflecting the lncreasing likelihood Ihal d wlnl(,\' fl'osl will terminate growth or reproduction of the SPCOIld gellPral Ion «(.{'vills 1961): Seger and Brockmalln 1987). The only test of these lIlodels has been provic!pd bv "ain;loll ""d Munns (1984) [or the [reshwater copepod Diaptolllus Sfll1gUittellH. whlch diapauses to escape increased preda t i on preSSlIl'C fnllll fi sh 1 1tél t bp('(llll(, active with a spring increase in wllLer lemper:ltIH-es. Fpllkl1es èll III 10 1 produce mixtures of diapause and non-diapause eggs. so thcy IlS(' il conservative or risk-aversi.on slrategy of swilching to diapausC' l'g1.\ production one generation beforl" the onset of prf'daLÎul\. lu CI1Hlln~ 1111 0[[spr1ng avoid this source o[ mortality. IlILcl"i-mllllal VIII'i;tliol\ in Ih(' timing of predation results in an earlier switching claU', Ml prpclicll'd by theory for a conservative diapause sLrategy (HairsLolI ;11Ie! MUllns 1984) . Here we describe an empirically-based model of diapéllHH! st r:1I (!1.\les [or a single population of Il, [asciatus to test the predicLiolls IIIfHlt· hy the general theoretical rnodels for an organism capahle of produc:inB mixtures of dlapause and dLrect.-ôeveloping offspt"Îllg. (j. [H~.;cialu!i iH therefore capable of utilizing boUI risk-aversioll 01' bet-hedging strategies to deal with env1ronmenLal lIncerLainly. Under lhe ilHHlImpLÏolI that diapause is an adaptation ta the thermal regLruü, wc use )ollg-Lerlll tempe rature records and laboratory derived life llistory parallleters Lo de termine the optimal strategies and the fi tness consequences of genet i c and random developmental variation around the opLimum. We Lhen compare the model results to those obtained experimentally for Lhis population (Chapter 2). 67 1 nie Bi 01 ogy of 11, fasciatus fb.. ftHiCiatus is a common ground cricket found throughout eastern NorLh America. Along the east coast of the United States, there is VlIrlllU on ln the 1 ife history that ls the resul t of adaptation to the cline in the growing season. In the northern portion of its range it has li unlvo]ljne life cycle. while in the south it is bivo1tine or possib1y mulUvo1t.ine (Fulton ]931; Mousseau and Roff 1989). There is a reglon of LnmsiLion between these life histories at latitudes 34-36°N: in this area vo1tinLsm is a conditiona1, phenotypically variable strategy and noL Il genet/cally-based poJymorphism (Chapter 1). In this transition areu overwi ntedng eggs hatch in May and the adul ts of the first generation mature during July and early August. Females can la)' eggs for up to 6 weeks, and can produce mixtures of diapause and di recL-developing eggs (Mousseau 1991; Chapter 2). Early in the summer [emales la)' non-diapause eggs presumably because there is sufficient gt'ow i ng season remaining for a second generation ta he completed (Chapter 1). Later in the summer (August and later) first generation females la)' diapause eggs, which overwinter until the fol1owing spring. Adults resulting from non-diapause eggs that are able ta complete develapment [orm a second generation, and 1ay exclusive1y diapause eggs (Fig. 1. ClulpLer 2). The Thermal Regime Ta characterize the growing environment we conducted a statistica1 amtlysis of temperature data ta estimate the mean and variance of the growlng season. Temperature data were availahle for 37 years for Danville, VA. the site of our experimental population (U. S. Dept. of Commerce National Climatic Center. Asheville NC; see Mousseau and Roff [1989] for details). We calculated degree-days from minimum-maximum ( .. Lelllperatures uslng the sine wave approximation of Allen (1976). Although 68 more elaborate m0dels of thermp.l summation have bCf'tl devl'lo~wd. Ihis simple procedure is sufficiently accuraLc ta quanti fy lI\(' gcrwral patterns of variabi1ity (Kingsol ver 1980). Tho/.'o wcro 1 \'10 pari s 10 IIH' procedure. First. fol' a non-diapause egg laid on LI givC'1I dHy durlilg IIIl' reproductive period of the rirst gencration. j ts hllLchi 1Ir, ddll- \lIilS predicted by summing 200 degree-days with a threshold Il.'mpcl"alUl"(- (Io) of 14°C (Masaki and Walker 1986. Tanaka 1986). Second. lite t1H'l'Iunl sum remaining for nymphal development and adult reproductio!l (Io-\:I°C. Mousseau 1988) was calculated from this hatching date 1.0 t11(' dllll' of Ihl' first ki1ling frost. arbitrarily set at -BoC (Taylor 1986). Pre-} Îmil1nry experiments suggest that adu1ts can survive 2h1' exposures 10 -')"G temperatures (Bradford unpubl.). Sensitivity analysis revPéllH IhHI varying the temperature of the killing frost by 2-3 dcr,ree::; IlHS 11111(· effect on the total degree-days summed because dud ng L!w 1 i me of YPIH when these frosts occur. temperatures Ilre nOI:ma11y lower 1 han IIIP thermal threshold (13°C) and add llttle ta the total ac('umulRt Ion. The sequence of calcul ation was repeated for each of the 37 yl~a rH of temperature data. The mean and standard deviation of the t1umber of degree days avai1able for the deve10pment for the second gellerl-lLioll WOlS plotted as a function of the date when the egg was laId. Linellt regressions fitted the data very well. and were used ta predict of Lhe mean and sn of season 1engths in the model (Fig 2a,b). First generation fema1es must base their diapausc dccisioll9 or. photoperiodic eues during a period when the change in cIayl cnet h i S SIIUJ II (1-1.5 min/day). Errors in the estimation of the exacL daLe have thp effect of increasing the uncertainLy in the length of the rcmaining season. Nonethe1ess, insect are very sensitive to small chang0H in photoperiod, and 15-30 minute decrease in photoper1od can eJidt li léjq~e increase in the incidence of diapause (Taylor and Spalding 1986). Wo simu1ated a date estimation error, normally distrlbuted, with li sn - /. 69 days, rcsulLing in a likely range (1:25D) of 8 days. equivalent to 10-15 IIIjnul<~s in dayleneth. [n midsummer this standard deviation corresponds 1.0 25 degrcc days; the SD of season length predic ted from Fig. 2 was incrcased by taking the square root of the sum of both the variance of the sposon lengLh and Lhe dayJength estimation error. 'rHE MODEI. Wc construcLed an analytical 1ife history model with the goal of eSLllllating the expected fitness of mixtures of dlapause and direct devp]oplng eggs. This model calculates the optimal diapause proportion for each day of the reproductive period of a first-generation female and Is an empirically based extension of early models of Levins (1968.1969) and Cohen (1970). We make the simplifying assumption of a haploid asexual organism, to avoid the details of sexual reproduction and heri Labi 1 j ty. The sequence of events and sorne of the parameters are il1ustraLed in Fig. 1. We defined (itness as the number of surviving eggs deposited at l he beg1 nning of winter (Roff 1980), Thus the fitness of .a diapause egg produced by a firsL generation fema1e is its survival from when it is laid Lo the beginning of winter. Ff'l a direct-developing egg, fitness is :l [unction of its egg-to-adult SUP:h,il a1',,1 the number of eggs it produces upon maturation. Fol] owing hwiH (1968.1969). the fitness of :ln egg laid by a first generation fema1e on day t during the summer roproductive season ls then: (1) Whf' l'e p j s the probabi 1 i ty of the egg entering diapause. Sd is the surviva1 of diapause eggs during the summer and fa11 , Sn i8 the egg-to Adul\ surviva1 rate of a direct developing egg. and F(t) is the total ( ntllubc>l' of eggs produced by a second generation fema1e resulting from an 70 -1 egg laid on day L. Because Lhf' pel- i 0<1 of ov i pos i t i 011 ur 1 hl' 1 i rs 1 generation lS shorL relativE:' 1.0 LIll' d(>vE'1opnH'1l1 1 lUit.' of tll<' sPcollcl \Vl' assumed. as dld Cohen (1970), thal the fila"vival of d diap.l\lsl' l't',~ dllrtllg the summer and fal1. Sd. was consLélnl i rrE.'sppcl ive' of wh('11 1 h(' ('~g W.lS laid. FecundiLy. F. Is li dE'clining [lIncLÎOIl LI' 1 l'''C''''8(, s('colld generation crlckeLs hatchlng fram eggs l have less time ta develop and repradu('(' bp/"ol'(, tlH' onsPI of willll'l'. The number of eggs laid by cl second gell0ral JOli ('(,111:111' WolS caleulated as: d r F( t) = f Fec(r) e Mr nI' U 2 Here Fec(r)= a+hr+cr and 15 Lhe ag(->-specifit: {('cltrldily l'WH·t jOli lor ,1 fernale El. fasclatus (described below). and M is IIH! insl:IlII,l\ll'olls martaliLy rate of adults (degree-days-l). ThE' inLegrnlion is OV('" the' number of degree days between adulL emergence cllld Llw ki 11 illg Iro!;I. 'l'Ill' upper limit of integration. d r • js the length of the rpprodllcliv(' period. and is the differenee, in degree d;IYs. between the loUt1 SNl!;OIl length and season e-lapsed when the second geuerat ion nylOph l;t j cl Oll d:ly 1 ecloses as an aduit. Integration of equatlon 2 yields: e Md, where a. band c are empirically derived constanLs or th(;! qUilùt"aLic ('ge production function deseribed below. In any single year the optimal transition belween eBg Lypüs wi 11 be sudden. and will oecur on the day when the fiLness of a direct-developing egg declines below that of a diapause egg (Cohen, 71 ( B70; Taylor 1980). For ùach of the 37 ycars of historical ternperatllre dilltl availahle (or the sLlIdy sile we calculated Lhe optimal switching dHtC' by compuLing the fiLnesses of the two egg types each day during the r<'producLlve seaS'ln and determining when the fi tness of a diapause egg surpasses LhaL of a direcL developing egg. The resu]ting 37 years of swi t-ching dates werù USE'd as a measure of the interannual variability in the opt imurn. WC' 8150 derived "the optimal diapause strategy across years by Ïtlcorporating j nterannual variation in season length in the model. VlIrlati on i Il season ] ength can be considered as coarse grained Wlrla tian, and in this case fi tness is most appropriately calculated by lAking the geornetric mean across generations (Cohen 1966; Levins 1970; Gillespie ]977): ln [W(p, t)] = !8L(X, t:) ln [w(x-y,p) ] dx, -00 where SL(x. t) is tllf' normal probabil i ty function of the season remaining [or development of an egg laid on day t , and w(x-y.p) i5 t~he fitness in il season of length x for an egg laid on day t and taking y degree-days ta mature as an adult (see Fig. 1). The term x-y is then the quantity of degree-days available for reproduction (dr in equation 2). The distribution of season ] engths rernaining for a non-diapause egg laid on clay t to develop and reproduce was assumed to follow a normal disll'ibuti 011: if" 72 1 where /-Lt and (Jt arc rhe mE'dll Llnd SD of 1 Ill' llumbpr 0/ dq',1 t'l' d.l\':; available for dE'velopment and reproduc.'t iOIl. '1'11('8(' p.tl'al1l('{(·I'S .1 rI' 1 ilH'dl functions of the date. l. when the egg is l,dei (Jo'If, n. Nl·g.lIIVI· St'.t!;Oll lengths are extremE'ly un] ikply bE>cause ill Ilw nlil/jp tlsvc! 1 hl' lllC'oIll i!-' 10 SD [rom 0 (Fig. 1) Phenotypic variabillty in Ihe devclopml'tll 1 Îml' 01 lIympllH wi Il .t1so af[ect fitness of a non-diélpause egg hy inll'odlll'ÎlIg fllrlill'I' varL:lbilllV in reproductive succC?ss of a second g(>neral ion /\11 inv('rsl> llOrlllal function was used to model the probabil lly disl ri but iOIl Il/ d('WJOpllll'lll time. since its inverse. development rate. tends lu hl' iliOn.' lIunrwllv distributed (Sharpe et al. 1977). The distributioll of dl>Vl'!O\llll(,r11 t illws was gi ven by: (ri l-ud'~ D(d) = __1__ e ;>O~, 2 V21t 0 d d where /-Ld is the mean clevelopment rate and (Jd' is its VariélTlè<:'. /><1r estimates are described below, The fitness of an egg laid by a first gencnlLlon [cma] (. 011 il gi V('/I Julian clay. t. with probability o[ entering diapause. p. cal('ulalod ovpr aIl season lengths and development rates was thon f ound by irllf'gnlt i ng over the two probability densi ty functj ons: In[W[p, tl 1 ={SL(X, tl ln [{D(Y) .,(x-y,p) dY]dx (1) where D(y) is the inverse normal func ti.on [or devolop/OcnL li mu and 73 c w(x-y,p) le filness [rom equalion 1 above. The upper limit of the intcgral of development lime 15 the season length available. x: females wi Lh developmenl ti IDCS ] anger than Lhis will not mature before the end of Lhe 8rowing season and will noL contribute to lotal fitness. 1~e fitness of an egg laid on each day of lhe reproductive period of the rirsl generation (days 190 to 230) was calculated using equation 3. For each day Lhe proportion of eggs in diapause was varied from 0 to 1 and Lhe value of p whieh maximized fitness found using numerical lDethods Lo evaluate the integrais. Genelle and Developmental Variation Wc also examined the consequences of variation around the optimal diapause reaetion norm. Following the analysis of Chapter 2. we assumed Lhat the shape of the funetion describing the switch from direct- dcveloping te diapause eggs was fixed, but the location of the median (50%) date (hereafter ealled the switching date) varied. The diapause function was modelled as a logistic function. with the slope (b1=0.42) [rom a composite curve derived from individual females from Chapter 2. Ta compute 1 Uetime fitness. we modelled 3 idealized females that cclosed on Julian days 190. 200 or 210 (mid July) , spanning the period of eelosion of this population (Chapter 1). We assumed that these 3 fi.rst-generation females produced eggs according to our empirical quadralic [eeundity funetion, described below. These females were assumed ta liva for 33 days, which is the 1 ife expeetaney if the adul t morLality rate is 3% d-1 (see Parameter Estimates section below). LifeLJme fitness for these first generation females was then the sum of fi tness of the eggs laid over the 33 days. Two types of variation in switching date were examined. In the Eirst case we cot\sidered the switehing date to be genetieally determined, and derived the fitness of various switehing dates around l the optimum, From this wC' cOllslruclL'd th,-, fitlll'SH fUlll'liLltl for SWilchillg date. The second source oi val'iat ion \ofE' mod,' Il et! was rt1lldOIll 11011- [,<"11('1 1 (' phenotypic val'ialion in li(e history Iraits th.!! has b(,(,11 suggl'sll'd ln be advantageous in a varlablc envi ronm('llt- (Kapl Lill amI Gooper 1CJgL,. Walker 1986: Bull 1987; Schultz l!)ql: but seC' MC<;inley ('1 al, IQHI). Wl' calI this developmenta1 variation to avoid confusioll wilh ('xtpl't1dl environmental variation such as season lengLh discusscd I!l'n'. 0111' objective was to test the hypothesis lhat the hi8h intl'rfenlllip variability wc observed in our experimenLal rcsul ts (Chapl<.'r n is illl adapti ve strategy. We rnodified the mode1. /.llld having a strategy of picking a switching date al: rando!1l 0<1('h r,erlC'l'illioli would be favored over a strategy of a fixed clnl e of Lite> opt irnulII. W" compared the fitness of our 3 idealizcd [cmales laying eggs fOl' 31 dayn. starting on days 190. 200 or 210 with either a constant or randolll switching d":lte, For the latter strategy we ass Imed lhaL Lhe swi Lehing date followed a normal distribution wi th mean equal ta the opU mutll IJlld II spec if ied variance. The fi tne5S of th 15 s tra tcgy i s : .., ... k_t x(k. z) In[W(tt;J] =IISL(Z)SW(S) ln E"Fec(k) l w(x(k,z)-y,s) [ -tG .. OI:Ii-CO t"t 33 where SW(s) is the normal probability density funcUon o[ SwlLching dates. The within-year fitness of a first gctlcraLlon [ontillc wl th switching date s. maturing on day te. is the product aL the nUlllbcr of eggs laid each day. Fec (k), and the fi tness of a singl c cgg. w. surnrned over the 33 days of reproduction. It was necessary La modlfy the calculation of season length; now the probabiH ty densi ty [unct j on SL j s 2 standard normal with parameters J.L=O and 0 =1. The season avai lab1e [or :1 75 di rect-devcl opi ng egg is a [unetlon of the date when the egg ls laid • (k), which depcnds on when the [emale emerges (te) and when she 1ays the egg. ScaSOll } (:)ngth. x(k,z) 'Jlas calculated as l'k+akz, where JL and 0 are L rom F 19. 2 for day k. and z is the standard normal devia te of the inlegratlon of SL. PARAHETER ESTlMATES Survival rates Estimates of the survival rates Sd and Sn are unavailable for this specJ.es. so we wC're guided by the literature values for other inseet sp~cles. and varied our estimates in sensitivity analyses. In the bascline model R value of 0.075 for Sn. the egg-adult survival rate was used. based on lhe following observations. Taylor (1986) has complied eslimates of egg-adult survival rates for a variety of inseet species (N=l7) from the literature: the geometric mean of these data ls 6.1%. Secondly. if the late nymph and adul t mortality of 4%d-1 estimated for a mixture of Nemobiinae (Tennis 1983, see be1ow) is extrapolated over the approximately 60d nymphal period. the egg-to-adult survival rate is 8.6%. This lDay be an overestimate because smaller nymphs were not included in the data and are likely subject ta higher mortality rates (Cherri11 and Begon 1989). The geometric me an of nymphal survival rates for various orthopteran species (6 studies) compiled by Uvarov (1977) is 15%: this may also be high for our model because the grasshopper and locust species studied have shorter development times than A. fasciatus and lhe estimates do not include the embryonic period. The survival of diapause eggs over the summer and fall, Sd, was approximated by estimating the overwintering survival of diapause eggs 111 li uni vol tine population. The expected total fecundity of a female was ( calculated from equation (2) with parameters described below is 106 76 eggs. If the egg-to-adull sUl."v!val 18 0.075 1 hl' oVl't'wÎnll'I' $lll'\'iv~d IIIlIst be 0.25 La achievc a sLationary populnLiotl (i.l'. ') Sll\vivol."s). WL' tlsl'd" value of 0.35 for Sd bccélllse the dppl'o:dmalely l, lIlo11lhs rl'om mid-stllllllll'l' to the onset o[ winter 1S shorler thall the ovprwinLt'ril1g perim\. V;t!IIPB of 0.2 and 0.5 were used in the sCIlBitivity ,1lHllysis. Sonte estimates or M. the m!ult- mor-lnlity l'dl l'. nrC' ,I\'ilildbl" ill the 1iterature for oLller orlhoplet'Lllls. IhC'sl' range bl'Iwl'l'fl n.() :lIId }.')% pel' day (l'eviewed by B~'lovsky et al. 1990). For cl'ickl'ls. <111 l'sI illl:lll' of nymph and adul t morta] ity Céit1 be dcrivl'd rnllli élbullclallCl' d,ll:1 or Nemobiinae sampled in él Pennsylvania field by Tennis (] ana1ysis of her Fig. 3 data for il perim! of POPUl11lioll dpd illl' yil'ldn il mol'tallty estima!.e of 4% pE't' clay. In our lI\odel tlIon"lily WilS cillcullllt·d on a degree day basis, and a valUl' aL 0.00:>51)1)-1 W!lS IISl'd. col'responding to a daily l'ale of 2.0 la '3,')% !)(,l' cilly. dl'pl'ttdillg 011 temperaLures. LUe History Parameters Development rlite and feclltldlty pal'amctcrs w~rc' t'st illlfllN\ fl'o", experiments using crickets [rom the Danville, VA populal jOli. Cri ckpl H wel'e reared in mass cul ture from egg tü adull under cllêIngiuB photoperiods and temperatures silllulatlng condiLions arler é1 ",id-sunullcr hatch date (Chapter 2), and checked dally [or adu1ts whcn c/JI('rgctlc(' began, Development rate was ca1cu1ated a. the i nversc of dcvelopmenL time. and when averaged (N=l66 [emales) yielded the pl.lramClerB JLd=0.00194 degree-days-1 and od=0.000146 dcgt'ce-days-l (!qg. 3). The fecundity [unction for second genenlt lon [omales "cqui rcd by equation 2 was estimated from egg production dala c:ollocled tram éHlOthcr experiment using adults from the same population. Mated pairs (n=108) were reared in 'Fall' conditions (13,5/11.5h photoperlod, 26/16° C thermoperiod) and the eggs were collected every 4 days and counLûd. A 77 « quadnJLj c equéll ion was used La descri he average egg production as a function of degree d;lYS (Fig. 4). The thermal threshold (to ) of l30c round for nymphs (Mousseau 1988) was used for degree -day calculations as 110 cOlllparahle délta exist for adults. RRSUl.TS JII any single year there is an optimal date for the switch from non-diapause to diapause egg production. For the 37 years of temperature data Lhls date fluctuated hetween Julian days 203 to 224. with an average of day 215 (August 3), and a standard deviation of 5.7 days (Fi g. 'l). The long term optimal switching date predicted for the model incorpotating environmental variability was Julian day 214, and a graded t rans ilion between egg types was predicted. However. the swi tch occurs quick1y. with the central 60% spannlng 5 days. With a constant environment the model predicted an abrupt switch on day 214 (Fig. 6). The proporti on of diapause eggs laid by a small sample of field coll ected fema] es (Chapter 2) was somewhat similar to that predicted by the mode1 (Fig. 7). The median (50%) date predicted by the mode1 was lIlso s imilar to resul ts from an experiment where crickets from this population were reared in changing photoperiod and temperatures sim1l1aLing the seasonal cycle at their natal site (Chapter 2; Fig. 7). The median diapause date for the experimental females reared under the simulAted environment was 3 days later than that predicted by the model. The location of the predicted diapause function along the time élxis is affected only slightly by the survival rates used in the mode1. A1tering Sd' the survival of diapause egg9 over summer and fa1l, by 0.15 shlfts the dlapause curve by 2-3 days (Fig. 6). Thus inaccuracies in this poor1y esti rnated parameter could account for much of t.he difference ( between the model and the experiment in the timing of the switch to 7H diapausf? The dÏélpause responst> Cllt'Vt> f or' The fitness function for alternative genet Lcally-basl'd sw!lchilllj dates araund the optimum wa,!:; damed slwped éllld WHS cünt t'I'ed ;Il'ound t hl' optimum (Fig. 8). If the region Brounô the optimllm is MlSltlTlPd 10 [ollow a Gaussilln function the variance ls an €'sLilUaLe or Ihe sLlt'lIgth of normalizing selection (Lande 1975). FiLLed by eyl'. the cslilllal('t! coefficient was 136. Random developmental varlahility in switchitlg dal(' W:lS \WVP!' favored over a constant date. and fiLness declined .. Il an illc)'uélsitllj 1';ll(' with increasing variation (Fig. 9). However. low lev(,1s of ullcarwli;,:ed random variation would only be weakly selecLed agai nsl; for eXil/llp) l! d SI) of 4 days would only incur a 2-4% cost in fiLness. yeL the swit('hill8 dates could range by 16d (±2SD)...... •. \ \ 79 Generation 1 Generation 2 Direct development . . 'y 'X L. Dlapause . t El ·z Degree Days 900 350 o 1 1 1 200 250 300 Julian Date Figure 1. The phenology of A. fasciatus females from the study area, Danville VA. The rectangles symbolize reproducing females, laying either direct-developing (open area) or diapausing (shaded) eggs; lines represent egg or nymphal development. First generation females emerging in July can either produce direct-developing eggs that will form a second generation, or diapause eggs that will remain dormant until the following spring. Also indicated are model parameters: t ft , the date of eclosion of first generation females; 2, the switch to diapause eggs production; Y, the point when second generation females emerge; X, the end of the growing season. (: 80 """'- 1200 1000 tJ) ~800 Cl Q) Q) 600 ~ ~400 Y=3225-12.41X Cl 200 r=O.99 0 180 200 220 240 100 en ~ Cl 90 Q) Q) ~ Cl Q) 80 Cl ~ 0 70 Y=160-0.41X Cl r=O.98 en 60 180 200 220 240 Julian Day Figure 2. Upper: Mean degree days available for the development and reproduction of a second generation as a function of the date the egg is laid. Calculated from 37 yrs of temperature data from Danville VA, using a thermal threshold of 13C for every fifth day during the reproductive season of the first generation. Shown is the fitted regression used by - model ta predict growing season length. Lower: Interannual variation in growing season, calculated from data of upper panel. Regression equation was used ta prediet variability in season length by the model. 81 75------~ tn Q) -ca E 50 Cl) LL '0 "- ~ 25- E I----.f.--..--Y :::J Z O~~LLLL~~~~LLL2~~ 480 560 640 Degree-Days > 13 Figure 3. Distribution of deve10pment times for 166 females reared under simulated summer and fall conditions of photoperiod and temperature. Also shown i5 a inverse normal probability function fitted to the data and used in the model. ( 82 0.7 ,..------..., ~ 0.6 Cl cO 0.5 ~ ~ 0.4 Cl 0.3 .C)en 0') Y=O.012+0.0034X -5.67x1 &-l w 0.2 r=O.95 100 200 300 400 500 o Degree-Days > 13 C Figure 4. Mean (±SE) fecundity of 108 females reared under fall conditions in the laboratory as a function of age in degree-days wi th fitted quadratic function used in the model. -",. 83 230.....------b 1 1 1 5 10 (J) ta Cl 220 C) c: 0-..c: ~ 210 en3: 200 1950 1960 1970 1980 Year Figure 5. Time series of optimal switching dates, computed froIn historical temperature =ecords from Danville Va, and the frequency histogram of those dates. " .... 84 . 1.0 .... Q) ... (J) ·· ::::J 0.8 ·· ctS ·· Cl. Sd=O.50 ··· .-ctS ·· Cl 0.6 .· ~ ~ 1 : · 1 : Sd=O.20 c: 1 : 0 1 : .- 0.4 1: t .t :1 0 : 1 Cl. : 1 .' 1 0 0.2 : 1 '- 1 Constant environment a.. 1 --l 1 ... 1 ~90 200 210 220 230 240 Julian Day Figure 6. Madel predictions for the swi tch fram direct-develaping to diapause egg produc tion by first generation females. Shawn are predictions wi:h and withol.lt the inclusion of variation in season length in the mode]. and with two alternative values of Sd, the survivai rate of diapause eggs duri.ng the summer and faii months. [ 85 1.0 • .. - (J) • CI) ••••• :J 0.8 . ca • c. •••• • .-ca 0.6 Cl • c .-0 1: 0.4 0 C- e 0.2 0- ...... a . ' . 190 200 210 220 230 240 Julian Day Figure 7. Comparison of baseline model predictions for diapause egg production with experimental data from Chapter 2. Population curve (dashed line) is the average diapause proportion for all eggs produced during the experiment; individual curve (dotted line) is the average response of individual females, centered over the model (solid line) resul t to facilitate cornparison of shape. Aiso indicated is the mean. standard error (box) and range (line) of the proportion of diapause eggs laid by 9 field-caught females from the Danville, VA sampling site in 1988 (Chapter 2). A second sample, taken between days 260 and 265 laid 100% diapause eggs (not shawn). 86 1.0 en ...... en ...... ,., Q) '. , c:: 0.8 ... ~:~ ~ u.. ..•...... Q) 0.6 > ' .ca- . -Q) 0.4 0: 0.2 0 190 200 210 220 230 240 Julian Day Figure 8. The relative fitness of three hypothetica1 females reproducing for 33 days, beginning on days 190 (squares), 200 (triangles) or 210 (circles). The region around the peak can be approximated by a Gaussian l function with parameters ~-214, a _ 132. 87 1 1.0~~...~------' .. rn rn 0.95 Cl) c: .-..- • U- . Cl) 0.9 > .-ca -. -Cl) a: 0.85 • 2 6 8 10 12 SO of switching date Figure 9. The relative fitness of alternative strategies of random non-genetic variability ('adaptive coin-flipping') in diapause switching dates. Switching dates followed a normal distribution, with mean at the optimum (day 214), and a specified standard deviation. Plotted are results for the hypothetical females reproducing for 33 days, beginning on days 190 (squares), 200 (triangles), or 210 (circles). • 88 J) J SCUSS JON Along wiLl! sped dOr/nancy. ilJsecl diapause has receivE'd an (·xl ('Tlsi ve theorel ica1 lreaLrnenl as an examp1E' of 1ife history èv01ution in il It'lIIporalJy variable environment (Levins 1967.1968: Colh'n 1970: Taylor 1980; Casw(>1] 1983; Hairston and Munns 1984) and has recently beell llsed as an C'xamp1c of beL-hedging (Seger and Brockmann 1987: Phil ippi and St'ger 1989). A prirnary result of the E'arlier dE'lerministic l!Ioôpls j s '-haL Lhe Hwi lch to diapause should occur when Lhe expected fi 1 Hess of fi non-diapause indi vidual deel ines la below that of an j nel i vi dun1 in dj apause. When environmental variabili ty is included the opl iena} swi tch occurs earl ier. the amount depending on lhe circumslances of lhe particular model (HaLrston and Munns 1984: Taylor and Spalding 1989). III these mode1s diapause is an all-or-nollE' trait. and the variéHlCC of fiLness can on]y be reduced by earlier switching dates. This reduclion oeeurs because the suecess of ~on-diapause individuals Ls more variable Lhan Lhose in diapause. These results are consistent wiLh the concepl of risk aversion in that a cotlservative strategy is taken thet lowen: 1 ho expecled or ad thmetic fitness. but maxirnizes the geometric mE:'élll fillless. Environmental uncertainly caused by interannual fluctuations in c] Lmélte resulted in the prediction by the model of a bet-hedging response by female A. faseiatus in the form of the production of mixtures of diapause and non-diapause eggs. Illdividual females can adjusL Iheir diapause fractions on a daily basis (Chapter 2). and thus CHIl pot ent Lally fine tune thel r response ta match the uncertainty in the 101lgl Il of the growi ng season. Seger and Brockrnann (1987) prediel a gt'adual Increase in the diapause fraction as the probability that the subsequenl generati on will succeed dE'creases; if the environment is highly variable a long graded transition between egg types is likely. ThLs Ls a bet -hedging strategy because a mixture of phenotypes j s prod\1ced. and this diversification minirnizes the effects of envi t'onlIIent al uncertaLnty 011 fi tness. The bet -hedging response predieted by our mode 1 WftS not large. however. as mixtures were optimal over only ", d l4-day period. This L'eslIll lS suppot"tl·d hy p:--fwriI1lPllt.i1 ddtd. thl' shape of the diapallSE' curvE' fOL" indivi.duals \Ilii.!-. ;dlllo~l Idl'l\t it';d (0 tl\(' predictions of th€' model (Fig. 7). Our analysis suge;ests that inlp)-alllllial flllctlWt iOllb ill clilllill(' IIId\' be relatively smaH. The coefficient of vari,1t iOIl in II\(' ll'lIIdillillf, season length avai.lab1,e for lhe sE'cond gellE:'rat lOti i s olllv ilbollt j(l- 1 r,7.. (Fig. 2). The relatiVE'ly high thermal tlut'shold of lJ"l~ (THylol IlIHI) for this species may also minimize thl' vélriabilily in gl"owillg IWdMlll because. although killing [rosts may be highly Vdli.lhl(> ill d.ll('. tlll';> occur when ternperatures Are largely below 10' and lhus IIlilkp lilill' differf'nce to the total degree-day dccumlllal ion (Taylol 1989). ln addition. the relatively long df'vdopm!'lIt t ilUt· o( cl'il'k(·t oS minimizes the impact of climatic variat ion: f>llvirnnmellLtl tlllet·rl.tÎllt v may be more important for shorter-lived organisms mélking dUIPélIlS(' dE'cisions closer to the end of tlw growing seélSOIl Killgsolv(·.' (Ill/II) found the interannual variabil i. ty in growi ng S0élSOll 1 ('ngt Il Wil~ slIIa 1 1 «10% of the mE'an) and was relativelv Ilnimportilllt in his dlhdysis 01 pitcher plant mosquito phenol ogy , TflY] or flncl SpélIding (II)!!I)) .!lso conclude that realistic values of seasonal variation arc· 11111 ikply 10 result in the evolution of a subsLantial risk-rlversioll or bpI -11l·dglng strategy. Thus although insecL diapause strAtegit·s arC' ;1 eommollly Il!-.(·d example of lUe history evo]ution in a quantifiab1y v:lriablc· environment. it may not always be true lhat the llwnnal rl'gilll(' i'; sufficiently variable ta se1 ect for fi marked rf'sponse. Bet hedging might be more important in parlicul environments. One example is temporary ponds. whcrc t IWl"(' 1 S ri I-i sk 01 il complete failure of fi generation by pond dry-up unI ('S5 SOlll(' i IId i vi dll sanguineus living in a temporary pond wiLhout fish prf>dalion. ('ma1(>5 produce both diapause and direct developing eggs Lhroughollt Uw y"ar because of the highly uncertain date of dry-up (JIairsloll el al lCJ8 r». A polyphenic lite history of a freshwater shrimp Si12ho.!:.!,ghorcli gJJlbU lws also been suggested as bet-hedging in response 1.0 wi nt er clry-up or free7.ing (Saiah and Perrins 1990). It is important to reemphasize that bel. -hcdgi ne. or adapLi VI' ------ 90 1 ph('rlOtypic div~rsificfltlon is not the rE'stdt of gpnetic diversity but is (\\1(.' 10 Ill(> procltwlion of diverse phpnotypes from a single genotyp€'. Thus (·videllcP for bct-hcdgjng can only come from studi(>s that indicate within-genotype variation (Blllmer 1q8/~: Bull 1987). This is E'vir{ent for ('ge-diapausing spcciei> becallse of the strong maternal influence on offspring phPllotypf' (MOllSS('f1U and Djngle 199]). For species diapausing HS 1 arvae or adul t s. ma 1.erna1 influences are frequenll y weak or lJoncxlstent (MOUSSPflU élnd Dingle 1(91). In these cases a graded diapause "cspo!lse in the population js not C'vidence fOl- bet-hedging unless this rl'SpollsC' CélTl bp shown 1.0 exist within genoLypes (e.g .. families). Pararncter Unccrtainty 'l'hl' cffccts oi poor parameter estimates on the conclusions of this pétper cali he di vi ded i nto twC) categoriC's: those affecting averages and thol'w ir.fluencing vflriabil ity. ParamE'tcrs such fiS average survi val rates. development times and ildul t fecundi 1 ies wi l] e[reet the location of the dia pause response {llong lhc t tmc l'IX i s. Al though the primary concern in this paper is the crfeet of environmenLa1 uncertainty on the diapause reaction norm. the gl'lwral agreem0nt bet ween the model. laboratory and field resu] ts (Chapler ?) suggest that our estimates of parameters thal affect when the Hwi t ch bctween I?gg types occurs is reasonable. In the case of the sut'vi val rate Sd. for whi ch we have no direct estimates, the exact value I1sect sN'med 10 havI? onl y a sI ight effec t on the location of the dia pause curve (Fig. 6). Olle likely factor not included is that second generation lIymphs CAn reduce thl? i r dcvelopment Lime by basking in sunlight (Whi tman 1988). which wi l1 delay the optimal sw; tching date predicted by the model. Nonelhe] ess. our resul ts provide further evidence that diapause i s i IIdpcd an adaplation to the thermal regime. supporting the correl HI ion belween diapause propensity and Sl:. 1 thE:' other. and they dn' ul1colTeliill'd, OliJl'I'wi1->l', 11H'II \-.lrldhilll\' wil! not affect the relative fitnpss. HS cdlcllL1ll'c\ ill Cqlldlioll 1 If 1l\'1llphal survival rates are extl"l:'mely vill'inb]('. illl'llldillf, thl' poso.;ibilil\' of !OUl, mortality. a pure bet-hedging stralE-'gy of ;l1wavs pI'()(hlclllg SOllll' dldPdllo.;(, eggs mighl be favored (Hai l'SIon et Hl. 1(85). Ifow('v('l". Olll l':-"P<'I 11111'111 d 1 results do not suggest this as fplIlc1lC:'s lny E:'XClllSlVP!V 11!111,dlolpa\l<'I' ("'./',!-> early in the season (Chapter 2). A similar argument can be L1sed [or Illlpl"éllll\lI.t1 \'dri:11 iOll III fecundity: variation across years in lhe [('CLllldi 1 V oi (Ill' Sl'('oIHI generation due to factors unrelated lo sC'nSOll 1 l'l1gl h. s\l('h d!-o dl'ollglll (lI' nutritional stress will lower lhe gcomelric meilll fillll'ss of .1 11011' diapause egg and will shift the diapausl' Clll'Vf' eHrl ipl". Tft(, !-ohdpl' of 11ll' diapause curve would only be expec Led lo chang(' lIld l'kpd 1 Y i r IIH'I (' 1:, sorne probability of a complete reproducLi v(' failli\"(' Vill'Î.IIIO/! III fecundityamong individuals of a gcnotype willtill a Y('iH will 1101 .dfl'l'I diapause strategies because the y should Iw HriLhll1('licdlly dV('l"dgl·d. 0I11c1 should therefore be similar lo the expet"Îmcllt cil daLI III Fi g l, The on1y paramelers which af[pct Ihp Sh,IPI' of 1 Ill' didPdll!->(' response are the interannual variation in cJilllalc : IIIcl 1111,' ('11'01 illllu· estimation of daylength, a] though lhe lall cr cOIII ribul (,~ 1 i III c, 1 () IIH' total uncertainty in season length, We Iw\'(' LlSP(\ t II(' !-o i mp1 ('0.;1 1111,1 hwl of estimating variation in the thermal regjlllü and hllV<, i~llon·d 11011'] IIIC:l1 models of insecl development (e,g. Wagner pL ill. lfJ!!lI) M, w(·ll dLl 01111'1' factors sueh as calibrating tempcratllrc'~ to Ih" dCllllll Ill/bi/dl 01 lhl' crickets. We think il is unI ikely Lhal cilly of Ihe!">l' '>Ollrl'Vl uf bj.l~, would substantially allpr our cbLimatc' of the variallc(' of ~r()wllI!j season. Genetic and Phenolypi,c Variabil Uy Random developmenlal variation (spnsu 'm!apl iv(' ('oill fllppIIIB'. Kaplan and Cooper 1984) in SWllchillg dcltl· WO Thus the large interfemale VdrLHbilily in swilchillg d;t!(, wc· fOlmd in l)11I' experimental data (Chapter 2) is nol éltl adilptivc !>lnJte'8Y. The·o\'"('IÏC'aJ work has indicated lhat random non-gl'nptÎc phlJlIOI ypic wlridhi] il Y (':111 }H' 9') 1 hl·!t·clpt! for WIHlI PIlVirOllllJ(-IlLa] vilriahi 1 1ty is Itigh (S1dtkill cilld L-Inclp 19!rJ. Bill 1 I r)R7. Schullz FJ9]). Uncl(·r tlte assumption that fitllE'st. follows d C;;'l\1SSÎ Ilwt d(·v(·loPIll(·nt;il vflriflt ion i5 favorE'd when thE' varianCE~ of the C'lIvirOllllll·nt.il oplllllUIII excel·d!> tll(> coefficient of stabilizine selection (· .... 1 illlat(}d frolll tll(' GallbSirlll fiLness functiol1, This condition was not met 101 OUI' 11 J optimal swilcltillg date «(12 ·,Î6. Fig 5) Wé-IS less Ihé'Hl the breadtll of the 1 illll'sS rUllel ion for non-optilIIdl sWltchlng dates (a 2 ~136. Fig 8) CO()~ll'r alld K c!uvf·l oplIIent. ltowE'ver the il' hypothf'ti cal envi ronmf'nt s iHf> much more vliriah1(- Ihfln Iheir fïtness functiotls, They achiE'vecl this in some cases hy ilS!-lumin8 a Ilnifol'ln dislriblltion of E'l1\rironmenls and a unilIIodal fi IIH'Si> runet ion (e.g. Kaplan tnel Cl)Oper 1984: p 405), Although l'llvinHllllenlill vadation following il uniform distrihution may in reality bp l'art'. infrequellt harsh or cal.asl rophic events lIIay be sufficient to Llvor dev(dopmelltal variability (Bull 1987), WhPlI the environment is not Itiùh1y varidble thf're is ]iltle need Co produce extrernely diversf' pll!'llOlypps ilS a bpl-hedeing slraLegy: the best stratf'gy is to follow L1lf' 10118-1 Pl'm lIIei-ill envi ronmenl TIH~ filrlPss fl.Tlction for alternativE' genetically based switching dilLt's was domed-shaped. similar to those det'ived from general models of didpHUSP straLeeies (Taylor 1986: Taylor and Spalding 1988). The n·lal iv<,ly f1at surfaces near the optimum suggest thal the erosion of g('tlPI je vdrinbil ity :lfOlllld lhf' optimum may be slow. For example. a 10 dilv l'Illet! i Il swi t chi ng daLes al'ound the opt i mum win a11 be withi ri 95% of tlif> lIIilximum filness. CONCLUSIONS W(' havE' analyzl,d. using an !;'xpedmental1y based model. the optimal diapéHlsc stralE.>gies of a pal:'tially bivoltine population of the cricket Ô [else i él tus, goth 1abol-alory and model resul ts suggesl that diapause is 1 .Ill ddélpt Althotl8h pat·t i.d1y l1i\'ol t illl' illM'ct h dl'l' (l'pqlll'lIt 1 \' lI·;('d tll motivale lheoretic ..1l modp1s of lifp hihtO)"V l'\Olllll(l11 ill Il'IIII'PI.t1lv vat'iab1f' ellvironlllellts, fOI' our POPU1illioll tl\(' PI1\'irOl1l11l'lIt ib Ilot sufficienl1y vat'iab1e to e1icit d SUh:-.tclllti.d bl'l-hl'dgillt', didpdllhl' responsf'. Rather thE' mode1 predicts ('('1Ilc-l1('<, sllol\l diapausE' egg produclion, iiS WélS o!JS('lvl'd ill tlH' l';"'Pl'l'illll'lItd! Il'Slllt·, The thermAl el1virOtllJlPllI ih d1~;0 Ilot vcll"ldhl<' <'IHHlgh ln ~,l·I(·l,t (lll" hl·t hedeing in the farm of randolJl phenotypic vnl'ldl 1011 III dldpdll',{' sLraLE'gies. TherE' are n tlIllllbE'r of meclléltlisms tll mainLenance of interfPJllall' varialioll in switcllillf, ddll'S WP fotllld (Chapter 2). Fi rst. eN1Pt ie vclriéit iOIl i Il diclpdll~;(, 11101)' h(' (,Ollt i 1I11.i1 1 \' introduced by migrdtlon (pfl.l-liculclt-ly hv willf,t'd imlivldll.t!·.) flOIll AdJacf'nt diffelt;'ntiHted popll1iitiotls DiilpdllhP il1 ~ '-<.h~(').• _LU,,·, (·xltihil', hoLlI laLitudinal and altituditléll c1ir.;tl Vdlidtl(JlI Ihdt IS 1~(·III,tl('.tllv based (MOUSS€élU élnd Rorf ICJ89, ChdptC'r 1) 1c 11 i j () (1 q H()) hd S demonslrdted lItaL eene flow ih ,-il 1 Impolldllt fill'tOI illf111l'rH'illij diolpdll!.I propensily ill Drosophild 1.ICE'rto!>d. Withlll tlil' P0!l"J.ltioll, developlllPnlRl val'idt ion may be f,elll'rlltl"d bl' .1 ptwl1olYI'IC COIll·Lit IIJlI bptween developmf'l11. tilllE:' and dléiPdlLSl" thal is cl ('OIlM-'qlll'IIC(' of 1 h( Il conllnon physiological paLhwayf:. (Chclpt (·r Î) ThE'sE' sources or vdriiition will Iw ('o\llltpn·d hy IIOlllldll/.illg selection around the oplimrll diapdllse stl-dtl'r.S A1tho\lgh tltl·l. 1'-. d distinct single optimulI1, selection lIIay I>p wPélk }WCdIlS(' of tll(' IHhlclth 01 the IiLlless fUIICLioll, and the reléitivl'ly low cosls of 1IIIl'dll,IIiY..d developrnent, The erosion of BE'IlE't ie vdriat iOIl will ;dso 1)(· hilldl'lO·d by tfn' (,1<'1 thAn diapause is a malerna11y trrlnsmitL<~d trdlt (Kirkpdtrtck dlld !..lIldl· 1989). Diapause is larBE'ly an autosoma1 tndt, so t}wl e"TH's fOl' diapause are also carried by malf's (Tanaka 19H11). IIowE'v('r <,(I(ctioll 011 diapause in thE' first eeneratioll clcls ollly OIJ fplllél1('s h{'l'dU"',' Ill(' 111<11(· parent genome plays no role HI the f;xpressiol1 of di Il p;J\1 'i(' ill Ids ..... offspring. In d cold year, for eXdmple. there will 1)(' h '.W 1 1 cid Ilg d.iI (> Sc 1 (·c 1 jOli (1) cl id P!H1!-.P gE'1I1' <, will 1 IlP rp fore be onl y hE" ",JI 1 d'ô st t"orlg dS '>pl('cIIOIl 011 fl>Hl.!l(-s cal't'ying those g(~lws. Thi.s ',IOI'dg(' d r ('('1 nI 1 rH' llIa11-S WI1l hol11 reduce lhe l'ale of erosion of g('fl(" ie vdl'idl iO/l, il~ w('ll él~ buffer tllP effpcls of shifting selection l'dll!>('c! by 1 lllc IUiI t i OIJ!-. i Il ,h(' cl i ma t l'. RI':FIŒRNCES 1\11(,11, J.C. 1976.1\ modifiE'd sim' \,.1\(, (01' l'dl\'ltl.ttilll', dq',II'I' Env i r a l1ml' Il t cil En t omo 1 0 gy S' ~ 88- "ll) 7 BC'lovsky, C.E .. ,I.B. JlcldC'. cllld B A Stockho[! lqt)(). Sll~I'l'l'l il> i II t \ 1 \1 predat i Oll for d L[[e! t'lit gt'dsshopPCI'S: .III l"'fll'I i 1II1'lIt.r 1 .',llId\'. Eco]ogy 71.624-614. Boyce, M.S. and C.M rC:'rcin~. Ic)R7. OplilllizllIg GI'('dl 'l'II l'Illich !;i.~l' in Il fluctuating environnwnt Ecoloty (,R 11,)-1r)\. Bradshaw, A.D. 1965. Evolutiollary 1,igllificiITlCl' of phl'l1otvpic pl.u;llcil\ in plants. Adv. GCl\(,t n Il,)-1'i'i. Bull. J.J. 19H7. The ('vol ut ion of plll'Ilotypic V Bulll1E'r, M.G. 1984 DeLlyC'd gE'rmindtlOtl o{ s('('d~ ('l)lll'I1'~ lIIodl' 1 revisited. Theorel ical PopuLlt iOIl Biology )(, 1(,7- UI Caswell, H. 1983. Phellolypic pl demographic effects dnd ('volutiolldry COIl1-.('qllt'lICl':; ÂIII('1 Zoo] 23:35-t~6. Cherrill. A.J And t1 Begon. lq89 !'r('(talioll 011 gl-dSS!JUppt'IS by spidt·ll. in sdnd dunC' grdsslands Entomologied l'xpl'l·impllt.d is dppl il'dl,'1 50:225-231 Cohen. D. 1966. Oplimizing reproducLion in fi rélmlomly vdryinr, environment. Journal of Theorl't j çal Biology 17: Il!) -1 7l) Cohen. D. 1970. JI. theoretical model for Ihp op! irncil 1 illlinf, of di:lpdUbl . American NaluralisL 104:189-400 Cooper, W.S. and R.II. Kaplan. 1982 AdHpliv(· "coin flipping": decision-theoretic examination of naturel1 S(')c'ct Ion l'tilldolll individua1 variation. Journal oL Theoretical Bioloe;y %:11.'>-lr)1. Dempster. E.R. 1955. Maintenance of genetic hel<..,rogetwÎty. Cold Sprin~~ Harbour Symposium of Quantitalive Biology 20:25-37. Frank, S.A, and M. Slatkin. 1990. Evolution ill <1 variab1 ( PIlVÎ rOTlIll('1l1 . American Naturalisl 136:244-260. Fulton. B.B. 1931. A study of the geTlus Ncruobiu§. (Orthopt( rd: Gryllidae). Anna1s of the Entomological Society of AJlI(!rÎcéJ 7/,:70')- 237. 96 Gil1ebpil!, J.JI. 1977. NélLural sclecLion for variances in offspring lIumbl!rs: a new evo1ulionary prLnciple. American Natura1ist 1 1 1 : 101 0 - 10 1lL lia i rs Lon. N. G. and W. R. Munns. 1984. The Liming of copepod dia pause as an ('voluLionarily stahle strategy. American Naturalist 123: 733-751. IIniîsLon, N.G., E.J. 01ds and W.R. Munns. 1985. Bet-hedging and eflviromnenlally cued Lande. R. 1975. The maintenance of genetic variability by mutat~on in a po1ygenetic character with linked loci. Genetical Research. Cambridge 26:221-235. Leon . .I.A. 1985. GerminaLion strategies. pp. 129-142. In P.J. Greenwood, P.II. Harvey and M. Slalkin (eds.), Evolution: Essays in Hc.nour of John Manyard Smith. Cambridge Vniv. Press, Cambridge UK. IA'vins. R. 1<)68. Evo1uLion in changing environments. Princeton Uni v. Princeton NJ. Ll'viIls. R. 1969. Diapause as an adaptive strategy. Symposium of the Society of Experimental Bio1ogy 23:1-10. f Lloyd. D.G. 1984. Vadation strategies of plants in heterogeneous envLronmenLs. Biologica1 Journal of the Linnaean Society 21:357- 1 385. Masaki. S. and T.J. Walkel'. 1987. Cricket lifl' c\'d('l-> E\'ollltllllldlV Bi010gy 21:349-421. McGinley. M.A .. D.H. TemmC' and M.A. Gt'IJl'r. 1987 l'cll'l'llt.il in\'l'l->Im('llt in variable environments: theol"C'tical dlld t'mpil'iccd cùl1sidl'l'dt iOllY American Naturalist 130:370-398 Mousseau. T.A. 1988. Life history l'vllluLIOIl ill:1 8(' Mousseau. T.A. 1991. Geographie variaLton in IlIéllerll,ll é1gP l'f [('cL!, on diapause in a cricket. Evolution 45: 1051-10')CJ. Mousseau. T.A. and H. Dingle. 1991. MaternaI dl'l'ds tll il1l-a't'l~,: Re] evance to life-cycle evolllLion. Annual ({E'Vll'W n[ Ent o III 0 1 or,y V, (in press). Mousseau. T.A. and D.A. RofL 1989. Adaptation 10 seélSUlI.lI ity in d cricket: pattern"> of phenolyp il' and gelloLypi c va r i ,1 t ion i Il body size and diapause expression along fi cl lne III S('dSOIl 1 ('lIl" Il. Evolution 43:1483-]496. NauLical A1manac Office. 1989. NauLical almalléle for the YP history evo1ution. Science (Wash. DG) 241 :14~q-I~,),). Philippi. T. and J. Seger. 1989. Hedging one's ev01uLÎowll'y !J(.u-;. revisited. Trends in Ecology and Evolution Il :/11-/I/L Saiah. H. and N. Perrin. 1990. Autumna1 vs spring Illitching in th(c (;IÎI'Y shrimp Siphonophanes grubbii (Crustacea. Anoslraca): divcrsifjp(\ bet-hedging strategy? Functional Eco1cgy 4:769-775. Saunders, D. S. 1987. Photoperiodism and lhe hormona] cont ro] of i WH'C t diapause. Science Progress. Oxford 71:5]-69. Schu1 tz. D. L. 1991. Parental investment in temporal1 y vary i ng environments. Evolutionary Eco1ogy (in press). Seger, J. and H.J. Brockmann. 1987. Whal is bet-hedging? Oxford Survl'Ys in Evo1utionary Biology 4:182-211. Sharpe, P.J.H .. G.L. Curry, D.W. DeMiche1e and C.L. Co]~. 1~77. Distribution model of organism deve10pment times. Journ;tl of 98 1 ThcoreLic developmental pathways in the ti.ming of d1apausl~ induction. American Natura1ist 131:678-699. Taylor. F. and J.B. Spalding. 1989. Timing of diapause in relation to temporally variable catastrophes. Journal of Evolutionary Biology 2:285-297. Tennis. P. 1983. Survivorship. spatial pattern and habitat structure of field crickets (Orthoptera: Gryl1idae) in two old fields. Envlronmental Entomo1ogy 12:110-116. Uvarov. B. P. 1977. Grasshoppers and locusts: a handbook of general acridology. Vol. 2: behavior. eco1ogy. biogeography, population dynamics. Centre for Overseas Pest Research, London. Venable. D.L. and J.S. Brown. 1988. The selective interactions of dispersal. dormancy. and seed size as adaptations for reducing :- l)q risk in variablE' E'nvirOnnlC'l1ts. Alll('I'ic,11l Ndllll',lli~1 131: 360-3H/•. wagner, 1, L. ,H. Wu, p, J . Il. Sh" l'lw 1111d R. N. Coul SOli. 1qtV.. ~10(k 1 i ng distributions of insE'ct dpvclop/IIPlll lilllP:.1 1ilpr.lllll'l' l'PVil'W ,1Ia[ application o[ the WE'ibull FUllel ion. AunaIs of IIH' Ellloll\ologic,t1 Society of AmeriCA 77:475-487. WaJker, T,J. 1986. Stochastic polyphE'nisrn: ('opillg wllh UIll'Pl't.tilltv Florida Entomologist 69:46-62. Whitman, D.W. 1988. Function and evolution of tlte nuo\'l'gul il 1 jOlI in tlt(, desert grasshopper Taeniopoda equE'.Ji. JOllrml1 of Allim.11 Ecology 57:369-383. 100 CIfJ\PTER l, 'l'hl' Qlléllllilcltivl' GClICUCS of Seasonality-Related LiEe History Traits in 1110 Crickcl A1Joncmobiu9 fasciatus. 101 1 ABSTRACT Many life hist"ory traits of illsccts 1 ivillf, ill SI'dSOIl,d environments possess phenotypic plast icily thd! i!-. ,ldolpl i\I', !I('I"I' w(' examinE'd the g0netic basts of tlH' [0i1ctiotl nonll fOI" didp.tllSI' of .1 partially bivoltine population of the crick(·t t\1JotLl2!!!~)bil!~ L.,s~j~,"UW III a two environment experlment. Ferndlf's of Ihe fin;! g('lwl'rllion or Ihis population first lay direct-developillg eggs dnd Ihl'II swilch 10 !hl' production of diapause eggs lélter in the summ(' 1" , Wc LOlllld siglli ('je.!ll! heritabilities for the timing of the switch 10 diHpausl' ('g!~ pl odlll' 1 iOIl. but not for the rate at which the switch OCCUl"S. This pHL\('l"1I oC variation is similar to findings of an carl iCI" inlcrpopul<1l jOll,il sllldv. Diapause was positively phenotypically cotTelat 0<1 wi th and the incidence of microptery: genet j c corn'l dt ions wPrt' ill!-o() pos i t i VI' but were not nignificant. The pattern of corn'lations is eOllslst('ut willt the physiology underlying the expression of thcs(' tn,ils iilld slIgg(,nl that a physiolùgical and developmenlal appro;jch lo Undl',"st and i IIg g<'II(, 1 i (' correlations wOllld be valllabie. 10/ l J N','ROJ)UCT JON KnowlC'dg(' of ttH' intE'nelationships between thE' traits concerned wilh filrll'!:>s is rwcpssary for undersLanding the evolution of life histl)}-ies. Succe!ssful adaptation to new or changing environments r{'(lIl i rcs the coordi nalcd change of a number of traits comprising an ddapl iVe! comp1 ex (Dingle 1986). The degrE'C' ta which this is achieved will depcnd on the genetle variation in the traits and their con-elnlions; if trails are negatively linked. the response to selection lIlay he hinderC'd (Lande 1982). Many 1i[e history traits vary phenotypically in different cnvirornnents. and have genetieally based reaetion norms that deseribe t he' express i on of the phenotype wi th respect to the environment (Schméllhausen 19/19. Slearns 1989). Such reaction norms may be adaptive. wi th thcir shape reflecting the result of natural selection (Bradshaw 196); Via 1987). The relationship between traits can become considerably lIlore eomplex when they are plastic: genetic correlations between such pl (lsLi t' trai ls may change between environments (Gebhardt and Stearns 11)88: Stearns et al. 1991). This restricts the generality of usuai quant itati v(' genet ic estimates of variance and covariance. unless the <'sLimatcs arc made in a varlet y of realistic environments (Clarke 1987). The long Lerm cffects of selection on such traits, and any constraints offcred by the correlations will depend on the frequencies at whieh dif[prent environmcnts occur. Various authors have therefore suggestE'd thal il pure quantitative-genetie approach may be limited by these di ffieu1 ties and that a greater understanding of the physiological and dcvdoplIIC'nta1 pathways may reveai more of the constraints to evo1ution (Clarke 1987. Pease and Bull 1988. Barton and Turel1i 1989. Stearns et ill.1991). Empirical sLudjps of the genetie basis of phenotypic plastieity in 10 ~ i naturaI poplllLHions art' rl'I:tl iVl'ly rl'cPl11 tl!l'v h.lvI' (l'l'qtll'llt 1." cOllfirmed thE' pyistt'IH'P of gPI1l'lic vdridhilily in p1.WIÏl'ltv (l' !", Cllpt.! and Lewontin ]982: Groctprs :md IHnglc \(88). ollltl in soml' l'.I:;t'" docurnented changes in genC'tic cotT<'li1tio\ls bdw<'('1\ tLdts in dlf ft'I l'Ill environments (Gebhanlt and SLNlrns Iq8~n, !IOW('Vl' 1" , lIlost (lf tllt'SI' st1ldll". lnvolve plaslicity for which Ihe dt/Hpl ive' Ililtlll"(' 01 thl' ,'.!rLII iOIl ih unclear: a common C'xampl (' i s llll' 1 t'lIlpl'l"al url'-dqwmh'lll n'oll't 1 PI! 1101"111:: 01 morphologi cal characLers (SN! n~vi <.'w i.n ScJlCi IIPI" t.hese cases plasticity measurcd lIIay bl' m('l"ply a bioc\ll'lIlic.tl COIISl'q\ll'IH'V of the interaction between t.he dcv('lOp/llPlltal sy~t('1II illld thl' l'nVII"OIlIllI'lIt raLher Lhan an adaptive change in t.he plwT!o!yp<' I\lIIollg thl' ('J(· .. rlv adaptive reaction tlorms thal havl' bet'Il eXHlIIllwd Bl'Ill't I('dllv .ln' Ihobl' that resu]L From changes in lire hislory '15S0Cldt(·d Wllh !lll'cIicl,,!>ll> changes in tbe environment (N0\'/man ]988: (;rO('lpLS éllld Dillg]l' ]CJ1l8, Kingsol ver anù Wiernasz 1991). lIowl'vl,t" 1 Il llwny ('1I5('S 1 Il t 11' 1:, kll()WII 1)/ Lhe developmenta] mechanislIIs l"egul al i ng II\(' 1 rd il, dllcl 1 hl' g(>IIl> 1 1 (' analysis has b€'en confined 10 the lradiliollal 'black box' dPPI"O.ll'lt ln this paper we eX3/111 ne th0 t'cac 1 i 011 llOrrll for l'gg cl i "Pdll~,(' i Il d partially bivoltine population of LIll' cl"ickc! 1\1101J(·lJlohl.!..1~i l.'-I!:>_(~L~"Ju; /11 this population the fi rst: generalioTl m3Lul"Ps in lIlid-stlllllll('r iIIlcI .1<11111 females ] ay mixtures of di recl dcv('] opi lIg eggs ({ orrni ng il !,('('OIHI generation) or overwintering diapause ('gg&; litt> proport iOIl 0/ dliIPdIlS(' eggs laid increases wi th the daLe (Cbapl(~r 2, Fig, la) The' :,11.'1)(' 0/ l/tc· diapause reaction !torm [or individual fE:'rnalcs resPlflhlt>s éI logisl ie curve. with the switch bctween the two (}gg typ0$ laking almlll ID ddy~, (Chapter 2), Theoretical analysis has shown that ttH' shape of t Ill' diapause reaction norm of [emale 11. fasciat.us f rom t 11(> l);1rl id J 1 Y bivol tine population studied here is very close to the opt jlll,il r.:xp(·ct (·cI given temporal variability in the thermal envirorunenla] (Ghllpl(;r 'n, As diapause is an adaptation La Lhe abiolic (;t1vironmcnt, all !cllIale!, art' 104 t (·xpecl(·d 10 fol1ow 1 he· Sllmc'. opLim phenolypic Vllriat Lon bctwccn [omales in mean switch date: vllrialion thLl'L wOllld I1pp('flJ" to be maladaptive (Chapt.er '1. Fig. lb). /\daplive rcacLioll norOIs will evo1ve through sE'leclion acting on 1 hl' IIVd i ) ,lb) (' genpLic variabil i ty (Via and Lande 1985). For 8, fasciatus the logislic rcaclion norm (or diapause can be decomposed into its two PdJ'HIJle>lers (dC't.ailcd in the Methods section) with one parameter !:ipccilying tllc st.ccpncss of the lransilion from direct developing ta di apause> cg8 producLi on. and the other determini ng when the swi tch uccurs, If ger.et.ic variability exists for both parameters (and they are Ilncorrc·l a t ed) the reac tian norm should he able to evol ve to the optimal shapl' accordi ng lo lhe select 1 ve regime imposed by the environment. IIl1vÎ ng rellch"d thc opLimum, normalizing selection will eventually erode gl'lleLi C vdriabil ity in plaslicity (Via and Lande 1985). IIere wc estimale the genetic variahility ln the two parameters of the diapause reaction norm and the correlation hetween diapause and other life hlstory traits associated with seasonality. We then ask whelhcl- cOlTelalions bctween traits change in sign or magnitude across t wo l'nv lronment 5 representative of the period when 8. fasciatus rcproduces. Gomplex changes in these correlations might he expected, givcn the pOLenlial for reaction norms for each trait to cross a10ng an cnvinmmenlal gn~dienl (Stearns et al. 1991). Finally WE' relate the pAtterns of correlation of these traits to our understanding of the dt'vclopmcnt"l and physiological mechanisms of insect ~pasonality. MAnlHALS and HETHOnS Experim~tltal design- A split family full-sih design was employed ta ('stimalc gC'nelic components and correlations (Via 1984). Although 10', 1 heritdbilities obLlÎllPd (ro/ll flll1-!-.ih "'\Ih'lillll'llt!-> l'lllil,lllI \.!lioll', dmounts of 1l01l-éHidit 1\'(' gl'lll'l il' \.11-1.11 illll (F.dl'OIH·1 ltJHl)), ';,I.ll'l' limilaliom' ~lI1d difficullll'S willt lII.lllllb pn'\'('lltl'd Il!-. tlolll ll~.llll',.1 Il.''' sib dvsign, lltal aIl cggs they prodllcec\ Wl'lL' ill di.l(hlll!H' Titi" !.\lll'Ili Illli ,'V!. tlll'II' devdopment such that Ill(' hatchillb of otfspflllb .1111'1- dl.lp.lll'-;(· i!-. conccntl-aLcd OV('t- é1 2-3 dily pel'iod, lJpoll IIdlC'hill[. IIvmph:. Il'Ollll'olcl! Lunily wer0 placed in of conditions expcrienccd dllring thl' l'''prot!lIcIIVL' S('ol!->Oll 01 Ihl' 111'.1 gC'ncration of this popu]i1lioll (Chapt PI' ')) /\ltllOllglt Wl' lIol'ltl"ll)' Il!-.(' natural1y changi.ng photoperiod!:; é\IKI Il-'mpt'I',ltU\'('l, 10 Sllldv dl,q)'II\~,l' 111/\ fascia tus , (see Chapter 2, Fig,l) the ('llvironnwl1t,d l'(lII'l'1.III()II~. ill by the changing conditions comp1icate 1 !Je l·t.tÎlIIctt 1011 01 gl'll('IIl' parameters, Therefore wc uscd COllstant ellvi nJlllll('tlt s 101' 11t(· IIYlllplloil period, and transferred new]y cclosl·d ndulls 10 d S('t'Oll whi ch the photoperiod was reduced by (), 5hr, A subsilmpl (' 01 .!(IIt! I~, fi 0111 each fami1y in each environmellt WilS allowN! 10 hn't'd. ;lI1d tlH' didPdW;(' propensity of Indlvidual [ellla] es est illlaLed frolll Il l'gg bdl clH':. coll t'cl t'ri over a 16d period. In our 'early' environment the pholopC'riod WHS 1'J,'J'El C)III I.D loI' nymphs and 15:9hr LD [or adults. corrc.spol1dillg dpprOXlllIall'ly \0 photoperiods (including civil twilight. Beek 1lJ80) bctWN'1I JlIl ilill 195 and 210 al the collection sitp of U\l' cxpprilllef!L;11 pOpll];11 iOll, III the 'late' environment conditions were 1'):9 LD roI' l1ylllphs LD for adults. simulating days 210 and /25, In a11 envi l'OIlIll(,I1\!; d 31:19°C thermoperiod was used on il 12hr Cyc1l" with the· ill('r'e:1S(' in temperature set at l.Shr before lights on, Thes(' are aVl'rdgc mid-Slllfll/l('r temperatures for the natal site of the study populHI ion (Ruflm'r 1(80) 10() l':xpt:ril/lt:tltdl 1II('Lhods- Approximatcly 100 1':1. fasc.JJJlllS were coll(·cled in J\lly (rom DIlTlvill(! VA (lal-l(mg) and t"l'turllcd to the ldboratory fOl" '3 ~(·fl(·!"élli()I1S of I/IHSS culture. The culturC' conditions emulaLed a bivo]til1f- 1 i fp hi sI ory. wi Lh rearing envi l"onmenLs al ternat ing beLween midsummet" (dillpllU!H' averl ing) and {all (diapause inducing) photoperiods The IHlI"PIlI al generaLioll for t his experiment was hatchcd [rom diapauso eggs :lTId !"earec! in e}"oups of 100 nymphs in plastic mouse boxes under a IhC'rmoperiod of 26:H)oC (l2h cycle) and a photoperiod of 13:11h LD. Upon pc1osiotl adulls wc}"" collecLec! [rom the nymph cages and palred randomly. Pnin; werc re:lrcd in plastic sandwich boxes and were fed carrots and c !"ushcd ca t food. A roll of moistened cheesecloth was provided for ovipositionJng. Every four days the food and cheesecloth roll were c1wtlged and the cggs deposited in the cheesecloth were collE'cted and placcd on moistened paper towel in a petri dish. The eggs were then incllb:lled for an additional 10 days before being transferred ta a [~OC coolpr (or 3 monLhs ta break diapause. Afler Lhe chilling period the eggs were incubated at 28°C until hal chlng began. lIatchlings were counted out daily from each family inlo groups of /,0 and placed in sandwich boxes wi th carrots and dry cat food d:ly. élnd sorne from an adjacent day had ta be added to make up a full box. There were 55 famUies that had sufficient eggs ta make up the 1+ hoxPS (7 replicates in each environment). and 9 families that yielded otlly '} boxes; these were placed in the early environment only. TIlf> tlyrnph boxes were cleaned and food changed every 4 days. When final eclosion began boxes were checked every 2 days; adult males and fClllaies werf' removed and counted. The newly eclosed adults were t nmsfern.>d to lheir reproductive environment and reared, sexes separatc. for 8 days ta allow sexual maturation. 1 For each environment 6-8 females were chosen from each family and IO! 1 \1\ dlll'mpl wdt-> lIkllh· lu ; dk( r('Jlld]l''> 1 dlidollll \ fI (llll 1111 tlllr,lt'"11 1 hl' 1"'1 Il'(\ ,d t>rnOJ"gl'I)L'C' ùf ('deI! I,Hllilv dllhllllgl! Ihi~, W.I'; 1101 .llllIdV', ~"Il'I'l,:,',(111 '1'" (IISUt'(' !->t1t't'l'ssful IIldlll\t, l'dl'Il ft'lI\dll' ...... 1:-; \,1 \1\ ,d, tl \,11 Il ) • .1 \Id (\111 1 \' ,'Il\l';"ll rnalL'b, l,i 111l'r f r,JIII tlll' pool ul lII,dl'S l'III,'rglllg (1'(11111 Ill' "'l'l'I IlIIl'lIl (\1 f1"<)l1I a S<:'p,Hdte rc'dl"ilJ~ or IIIl !->tllpll\S Il VIII pit t, 1'1)(' 1 l'III,d,'<, .Illd 1""11 mates Wl'rl" n'al pd in 1'1 d!->I il' dl ~plls.J1)ll' II)(lml llt'·.';l'l'I 1'111"" WII" ".1111'1', illle! eat food Lib beforc' A holtll' l'dp (III (cl wi 1 II mol ',1 "111'<1 :.,111<1 IVoi'. Pl"c'vided for oVlpositl01l1llg 'l'Ill' boll J(, l'd\,S Wl'11' ,hdllgl'd l'\ll\ Il .1.1\''' l, rE:'pllcdtlJS WPH' Llkoll from c,lCh [(,111<11(" '>P,lllllillg j() ddyh ul ll'production. The deposill'd l'~g~, Wl'I-,' :-"'pdt"dll'd Irolll th., >,.Illd ,1IIt! 1'1"11<1 on papel" towel ln p(,tn dl~h(JS dS h('fol-, Egg<, Wl'l, 11I('\lb,llld 1111111' sam(' envÎrorunent ,IS the 1IJ0t]wn; for \/,-IB ddV,> , .II WIll'" p011t1 dI0lI'.I11''''. dirl'ct-devf>JoT))ng And infprLil(' (ggb Wl'I'(' Idl'lIltfidhll. COIIIII'. W('I' 111,1<1,· or the two viable egg Iypps ,111d Ill(' pl-opoJ"liOIl dldp,tll~;)lIg l'"Jl'ltl"I,cl Batehe!. of [ewc"1" than l, l:ggs Wf'\"(' l· ... cludc,d (1"0111 II\(' .llIdlv'.I·. Statislical Analysi1i- TIl<' lfdils 01 prlllldry illll'f(",1 h('ll' .III dt'veloprnent t ime, wi ngform ,Incl d iapdllS(' propl'II!-> il Y Wh Il (, 11,,- .111.1 J y~, l', of development time dnd will8 mOI"ph is str,dghllor\>Jdl"d, di.lPdll"(· l'. 11101 ( compJicated because of thL' strong lIIéltern,tl miel (·tlviroIlIlH·nlnl 11tflll1l1('(", on its expression, In a prl'ViOllS dtléllysi~, MOW,:-'('dll ilnd RoI f (11)1\'1.1) considered diapause in !:J, fasciatus 10 })(, il Ihr(·:-,hold Ir,1Ï1 qf 11t(· q~l',. aIld used techniques appropriate for hindry Irail!:> Exp(·rirnltlt.d cldld. however. suggesls the paternal genot ype of Illl' Elgg pl.ly~, no d('1 ('1'1 dhl (. role in diapause (Tanaka 1986) and that conlrol i~ 1.Iq~(,ly dllf 10 IIJ(' influence of the rearing €'nvirollnlùtlt during both Ih!' lIylllplt:i1 .IlHl.tdnl1 stage of the fema1e (Chaptf'r 2). élS weIl 1991), We therefore consider diapause to bl' é.l tnlll of 1 h!' //Iollt('r .Incl can be consldered continuous when estimated ilS the· proporl ion of IOH ( Th(· t 1 III ( t n·ml III IlH' /1 S(!fjIH'nl 1,11 ('88 bal clws co] ll:cl ed from f·ach le·III.lle· W:I'; sllIflllldri/.l·d by (illlllg Ihe logu." ie regressjoll equell ion' p. (br.,b,X) p 1 'e (bu·b,X) wltl!'e· l' l', 111(· plOpoltioll didJlélllb(J ,Incl X lb lim(· from tlw collecUolI of cI:lys Irol/l ('deh v:!ltlP. ~lilh Ihis codillg lhe intercept parameLer. b o • Ci1n he' lI!,pd 1 Il ('sI i IIInl (. 1 h(' proport ion diApaUSl' hal fway lhrough the egg LIVllle Jl(·rlod. ,mtl is slatistically independent of the s]ope. b l (Net-er (>1 ,11. 1990). T!tC' rq~rcssioll was fittcc\ by thl' weighted least squ.Jl'es proc(dur(· ol Net!'f ct ;.1. 0990. p.361). llsing on]y fema10s \'.hich produc('d 1 of 1. cgg bélt elles. Diapause thus was analyzcd as 2 trét ils: Lhe IIll'didll di;lpéiUSC' pl·oporLion. calculatN\ from the lagistic regrer;sion al 1 he· ('od('d X·O, éind ttH:' eslimated s]ope> of the [unction. hl' '1'11<' 1 wo diapausl' t rdi t8 and developrnent Lime werc ana] yzed using a lIIi».(·d-model analysis of variance (ANOVA). with environment lreated as fi f iXl'd l'(fl'cl allli family LIS a randam e[[ect. In the case of development lillll'. replicél\.(' (cage ('[[('ct) was also jncluded as a rand am effect IlPsll'd wi t hi Tl fdmi 1 y dnd cnvironment. The SAS GLM procedure was used ta l'si illlilte the' lIIeaTl squares and the RANDOM option was used for F-tests of tll<' v"dous factors (SAS insl. 1986). Although sorne cantroversy slirrollnds t II(' spe'ci [i cation of l he ANOVA rnadel for a mixed design (Ayres dncl Thol1las 1990), Fry (1991. suhmitted) suggesls that the SAS fOlïll\lLIII011 is 1I('rj t ,1"1 111 les [or diapause and development time for females were c,t!C1I1:ltpd ln ('Hch C'twironment using the SAS GLH procedure with Type III StllllS of sqllan's élnd stclndard [onnulae pravided by Falconer (1981) and Bcckl'r (1976) Gpnetic correlations werp calculated using family rneans, l nt) 1 a proceÙllrc which t ptlds to Ill' conse' Vdt iVl' (Vi.I Iqil/,) hl'l'''"~(, l'~,t illldll", (Jf thE' \ëlflalWC of f IWllIp] i ng ('n'or or Ill(' IIlC,tll Bl'CdU!-.P 11If' t ('III,lll'5 chosPII tOI' di .1 (1.1 II: il' ('sliméttioll W("'(-' not ,1 comp1elely nllldolll s [amily mPiHls [or de\lcloplIlelll ,met witl/:,Iolïll WPl'(' b.1M'd 011 .Ill l'IIIPI'gl'1l1 [ernalcs (dVE'rage n-l'3) from Ill<' fdrni1y. ratlwr IIteill Ill(' ~ll"~l't ChWH'1I 10 rl'prOdULe (n--6), For wlngtorm. heritabil ities étnd sléllidani (""1"0'8 W("l' l'illCIII.JI(d by Lite Lhree d ifferenL mcLhods d('scr i bed by RoU () Roff (1989b), The eslimatE's oblairlPd wcre very slIlIildr, ,1Ild WPI(' iivE'raged Logethcr, RdW p,'oportiolls wc)'t' uspd i Il 01 )1('" élL'csin(jp) lransform Illétde vel'y ItLtlp dif[p!'Pllcp to Il\(' l'Pf.lIll!-' Rcsults population means- Tltere was a di[ferPTlce of "bout 0.10 in IIH' dLlp.IW,(' proportion as a result of the 30 min. diff(>t'pt\c(, III pho\op<'l'Îod )}(,tW('('1l thf' two environments (Fig. 2) Repeated lDeéJSurt's ANOV/\ U10lls1-H' revea1ed that this differencc was signific,HlL (p-(J.OOO'), ilS W!lS 1111' increase in the proportion diapause over t tille (P 0.00(6). 'l'Il\' interaction between time and cnvironmcnl WélS nol sie"ilïc,ltIl (1' () III) d', evidenced by the paral1el lil1es of Fig, 2. The rearing environments had on1y a SHI;]11 ('ffùcI on LI", du!':" iOIl of the nymph stage: development of femalcs was slieht1y mort' Iiipid IlTld(,t the shorter photoperlod (61.5!O.43d vs. 60.710.41d IJSEl, P-O Ob,), paired t-test on fami1y means). MicropLcry Wü5 more COlllIIIon in Ihe 1al(' environment. as would be expecLed by the shorLer phoLop(!I'iou (68% vs, 78%. P=O.0016. paired t-test OP ramily means). MicropturollS f(·lIIH1(·n lwd slight1y longer development Urnes than macropLers «(jl.610.'~ v:,. 59.lJ:O.2). 110 '. Plwllotypic corre]Hlions- Wc found significant phenotypic correlations hl'I Wl'en lIll'd i an diapausC'. wingform and development time. Later median diapausl' waS associated wiLh longee deveIopmenL times and rnicroptery (ANCOVAR; all P dl'vc>lopllll!nL Li me wert' a1so poslti vely corre] ated; the slope of the dillpWlSl' reacUon norm was negatively correlated with microptery (See 'J'abl e 1 for li LJ bivari ate correlations). There was little change in t IH'Sl~ pllPnoLypi c corre] a Lions across read ng environments. IIerl Labi 1 i Ly- The ANOVA resul ts [or the median diapause proportion t'CVl'é.l1 PÙ significant: environment. family and GxE effects (Table 2). The corre laU on of family means across environments was highly significant (r~O.47, P-O.0004. Fjg. 4). Therefore the genetic differences among familles were consistent across environments. Families producing more diopause eggs under lhe early conditions tended to have a higher diapausc [racLjon in the late environment. The significant GxE effect suggcsts Lhat there was. nonetheless. variation among families in the response 10 the environment. On1y Lhe family effects were significant in the ANOVA of 10gistic l'agression slopes (Table 2). The cross-environment correlation of family 111('<111 slopes was low. and non-significant (r=0.22. P=O.l1). As noted above Lhere was no significant effect cf thp rearing envlronment: on devcIopment time. a1though the family and cage effects wiLhln famUy were significant (Table 3). As expected the GxE term was not s ignif icant because. for development time, the two environments can b(' consldered almost as replicates. There was a significant correlation of family mesns across the 2 environments (r=O.53. n=55, P The proportion of macropterous females in each family was a1so posilivcly correlated across environments (r=O.61. n=56, P .. Separate heritabillties were calcltldlcd fOL l'.!l'h l'm'i t'Ollllll'Ilt liS i Il~', one-way ANOVAs. 8ignificanl fuL!.-slb ht'rllabilit [PB \\101'(' {oulld [01' development time. wingEorm and tl\(' IlIcdian dl apdllSl' pl'OpOn i l>1I i Il l':lell 2 environment (Table 4). howE?ver h v<üues fOl" tIlL' di 10wand not ~ignificant, For both c1evelopmclll liwe amI Will~rC.lïIlItJ values were very similar across thE' ellvirolllnC'lIfs. The IH'rit mc:èdlan diapause in the ear]y environmenl was higlwl' Ilwt1 fOl' tlll' Lill'. To test for the prE'sence of genetic wlriélt iOIl 111 1II('diHII cli:lPdIlS(' independent of the corre1ated effeets of wing[olïll .lIld d('wloplIll'lll 1 illll' we estimated the heritability of diapause using éJ lincéll' lIloc!('l incorporating development Ume and wlllg[onn as wdl as félluily ('11'('('11->. For both environments, wingform. devel apment t i!UC' and félmi 1y w<'n' a Il strong1y significant (a11 P:sO. 0005). 8j gni r LeanL lwl' i 1 ahi 1 i 1 i L'S wc' l'l' found for both environments (Early h 2 --O.601O . .13. Lale hl O.lIBIO Il). suggesting that gE'netic variabi1 ily in diapmlsC:' Î s not Ill\' n'sul t of 1 hl' correlations with other genetically variable lraÎts, Genetic Correlations- AlI of the genet1c eorrelaLiolls baM'cl OJJ fillIIi Iy means were positive. hut none were signifie.ml (Tab]p ]). ;tlthough 111(' number of families rnay have been too small lo delcct very w('ak correlations. There were no changes in sign or magui tude ilCrO,ss lhp lwo rearing environments (Table 1). .- 112 Tllhll! 1: PhNlOLypi c (above diagonal) and genetic (below diagonal) corre·laLions betwecn [emale life history traits for the two (·nvj ronmenls. Samplc sizes, by environment, for phenotypic correlations lIIoans, Early: 64, Latc: 55. ~~ P<0.05. ** P ERrlyenvironment: DrA b l DEV WNG ME'd ian di apause (OTA) 0.07 0.30** O. 35,h~ ) S10pe (bl -0.04 0.04 -0.15*k D('vp 1 opment (DEV) 0.13 -0.09 0.11* Mi c roptery (WNG) 0.21 -0.05 0.15 Lal'C' C'nvi ronmC'nt: DIA b 1 DEV WNG M('düm diapéluse -0.08 O. 27,b~ O. 29'~** Slopc (bl ) -0.04 -0.08 -0.28** Developmcnt 0.08 -0.06 0.11* Microp\:ery 0.15 . 0.20 0.16 1 l 11 ,1 Table 2: Mixed model ANOVA n'sulls for th" 1II('di,11l di.lp.lll~(, pl0!l0!"t iOIl and :,lop~ of the logistic diapause r"é\ct ion l1orm. Jo' j',l( ios \v(,ll' constructed from TypE' IV surns of sqttéHeS pro\' i dNI hy SAS. liS i Il!'; 1 Ill' Sutterthwaite approximation. Note thélt thel"(' WC'l°(' (,') fillllil ips in t IH' early envlronrnent. and 56 in the latt'. l"csulting in SOli!\' lIIissillg cl'ils in the analysis. Med i.<1lI Source ct[ HS F P Environment 1 2.367 30 () o.nool Family 64 0.331 I~ . 1 q O. 0001 Family*Environrnent 53 0.133 1.68 O.OO?') Error 68i. O. 07 Q S10pe Source dE MS p Environment l 0.057 1 .09 O. '30 Family 64 0.078 l. 50 0.011 F amil y*Envi ronrnent 53 0.044 0.8/4 0.79 Errer 679 0.052 11/4 1~bl~ 1. Mix0d mode1 ANOVA results for dev010pmenl time. using the 1IJ(:tllods dpt;erihcd ill Table 2. tif MS F p Envi ronment 2]8.0 ] . 7/. O.] 9 FRrni 1 y 6/J 291.6 2.57 0.0002 FllIni 1 y''<"Envi ronmpnl 55 ] 15.7 1.10 0.32 Rep] icatc 120 107.4 4.52 0.0001 r':rror 2291 23.7 115 Table 4: Heritabilities (with SE) of [L'male lIIl'di,tll di slcpe (bl ). dcvelopmcnl time And willr,forIn il\ tl\l' two Çll\'il\)}lllll'llll> Source Early Median Diapause 0.61 (0.12) O. '3t) (0 1 1 ) Slcpe 0,011 (0.08) O. 1'3 (0 (1)) DeveIoprnent 0.29 (0.06) O. 30 (Cl. 06 ) Wingrnorph 0.4') (0,07) 0.59 «(l.IO) -- 116 ( Q) 1.0 en ffi 0.8 c. ctj ë5 0.6 c: .-0 0.4 l: • c.0 0.2 0 '- a. 0 190 200 210 220 230 240 0.6 t- 0.5 t- • Q) • u 0.4 - • -.. c: e •• • .-ctj • '- 0.3 e ctj 1-. • > 0.2 t- e t- • 0.1 t- • • • t- •• 0 ., 1 • ,a- I. 190 200 210 220 230 - 240 Julian Day Figure 1. Upper: mean proportion of diapause eggs laid by 108 females reared under environmental conditions simulating their natal environment of Danville VA (from Chapter 2). Lower: Phenotypic variance of diapause for females in the upper panel, illustrating that the variability in diapause is temporally variable. ( 117 4 2 0.9 ,..------ CI.) en ::3 ('Cj 0.8 Late )----1------1 c. .-('Cj I____ 1 ~~~ l Cl l ------I~ l c: 0.7 o t o Co 0.6 a..e EarlY) 0.5 12 16 20 24 Adult Age (days) Figure 2. Mean diapause proportions (± 2SE) for aIL females in the experiment, for the 4 egg batches collected from each female. 118 , '1 J Q) 1.0 (J) :::J ------I ca ____ -,------__ 'T' C .-ca 0.8 t Cl '" Micropterous c: o :e 0.6 o C ~ Macropterous O a.t- 0.4 Early Late Environ ment Figure 3. Mean diapause proportions (±2SE) for the two wing morphs in the two environments . • 119 . 4 1.0 / •• / • • • +"""c: • • • Q) • E 0.85 ,-. c: • .. .:, 0 •• os;'- • •• c: .. UJ • • CD 0.50 • r=O.51 ta ....J • • P Figure 4. Correlation between family mean diapause in the two environments. Preponderance of points ab ove the 1:1 line shown indicates most families produce more diapause eggs in the late environment. 120 ( 0 0 Q) 0 Q) en !9 0 :Jm c. m êS Early c: 0 t c.0 0 0 Q) 0 ~ 0 a. !3 0 Season Early Figure 5. Hypothetica1 reaction norms for diapause, showing the results that might be obtained from sampling at 2 discrete intervals (e.g. Fig. 4). Upper: parallel reaction norms for diapause, varying in location, will result in a strong correlation across environments. Lower: variation in shape but not location can lead to a negative correlation across environments. 1 )1 DISCUSSION Diapallse- The two cnvirOnmf'llts. cdthollgh t\'pic,t! ot tltl' ldllgl' (lI temperatllres and photoperiods CIWolllltl'r<.'d bv (i rst gl'I1l'r.lt l(lll t\ fasciatus (Ghapter 2) produccd only slIwll c1ilrl'll'lll'l'!> ill diolPdll!W .llId 1111 effect on development lime. The dif(Prt'IH'l' ill dLIPdllS(' WdS Il'ss tltolll \\'.!', originally anticipatE'd and SllggCSt s that t IH' ch.mglilg phot llpl'I'l {)d~. Ilhl'd Chapters 1 & 2 are extremely important in el ici t ill[\ t Ill' sigllloid.1l diapallse reaction norm. Nonetheless the candit ions Wl'n' SIIt'Cl''>!>!1I1 III producing variability in diapause. allowllIg the t'st imdt IOll or gl'II(·t i(' parameters. Our results emphasize the IH:'l'd whcn stlldvilll', pLlsti(' tr.!lt!> to emulate. as much as possible. thE' naturcll (;:'TlVlrOnlU('lIt 11\ 01'<.\('\ \0 produce realistic phenotypes. The heritability of median diapause (t!tp l0E,isl ie Inll'rel'pt) Wd'. high. consistent with other studies which have l"l'vl'al l'd hi gh hl Vd ) Ill'''' . sometimes in excess of 0.70. [or various aspects of di al. 1976: Dingle et al. 1982: Palmer élnd Dîlle1 (' 1qH(): M()IIS~.l' 1989a: Bradshaw and HolzapfeJ 1990: Hairston and Dillon 19(0) i\lrhollgh our full-sib design prevents us from [ully distinguishine addit iVl' .Incl dominance sources of variation. resu] ts From parE'1l1 -of [::;prillg regressions and selection experiments suggcst thal ntllch of the vilriml('l' in diapause observed in other species is additivC' (e 8 .. /lov )CJ/7: Bradshaw and Holzapfel 1990). The most common method used in the 81l81ysis of plnsliclty in recent evo1utionary studies lS the Lwo environrnenl IflPl.hod origlllHlly proposed by Falconer (1952) and recently popularjzed by Via (198/1) where the trait measured in different envîronrnenls cart he> cOllslden·d as two separate characters. While entirely appropriate [or di seret (~ environments such as host plants (Via 1984), most envi rOl1lrlpnt S vary continuously, and the reaction nonn is better described by a f ulle 1. i on 12/ lIIi1pplllg Ilu· ptwTlOlypf.:· 10 Ihe environment (de Jong lQ90) By this .. ppn>flch!l mon· dc,'ILliled exarnillation of the components of plasticity can he· ubl ,Ii ned hy cons i deri ng I-he pa rameters of the reaclion norrn as 1 r,Ji 1 s For LllC' diapause reaction norm of A. fasciatus the relative IlwgllÎtllde 01 Ihe' genetic vRriatiol1 in the logistic parameters can result ill ql1il(' diffprent expectatlons for the cross-environrnent correlation of family ml'illl diflpause in a two environrnent experiment (Fig. 5). If the g('rJ('t le vnriat ion is confined mainly to the intercept or location pé\rarnet(~r the:' correlation will be positive. the genetic component of the ANOVA will be largE' and the GxE variance will be smail. A smaii GxE Intel"Hction lS usually interpreted as a lack of genetic variability in plasticity (Via 1987: Scheiner and Lyman 1989), though in our case it reff'rs ouly to a lack of variation in shape. Evolution of the reaction 1I0l'm along lhe time axis wauld not be constrained because the intercept pardmeh>r Ls still heritable (lIpper panel. Fig. 5). At the other <'Xl ,-pme' . if t he variation in diapause is entirely due ta differences in Ill<:' slope. the cross -environmental correlation of family mean diapause wi Il be negative. the family effect will be smaH in the ANOVA and the GxE variance will be large (Fig Sb), The large GxE interaction doesn't Lmply thill the reaction norm Ls free to evolve: the greatest potential for evolutLon comes when theTe is variation in both parameters (Via \487). We found a positive cross-environment correlation for median di apd\lSe in our data. suggesting that most of the variance in the reaction norm is due to variability in the location parameter. The significanl GxE interaction for diapause indicates that there Is a1so sorne genetic variability in the shape of the diapause reaction norms (lower panel. Fig. 5), al thour,h the low herHabili ties confirm that only n small proportion of the variance in sI ope has a genetic basis. Tht> heritability of median diapause was much lower in the late eompared to the early environment. The reduction was due to the lower 1 123 among famlly variance: LhE' wlthin [éllnily variclllCl' difft'Il'd Ilv 1\,0.,-; tlt.lll 5% between envjronments. This tentatlvcly suggl'sts tltdt tht' h"I-il.lhllil\' of diapause may be grealest Hllder conditions lililt rl'sttll III inl "I"lllt'di.ll t· diapause frequencies (Fig. 1). The dependencf' of Ill(' hl'ritilbil it\' 011 Illl' E.ovironment may explaill Lhe maintenance o[ lClrg(' ,"\lottnt s of gl'l1<' 1 il' variation found for diapause in many sluclies (e g .. Bl'adshaw élnd Holzapfel 1990). For f1. fasciatus. phenotypic vélricll\{'l' for di.lp:tII~,1' is only exposed for a 30-day period in rnidsumnter (Fig. 1). 1 f. dS Ollr results suggest, heritahility is highE'sL al intcrmcdiatp diapausl' levels. the period of high heritability (or diapause Indy 1)(' conf ltlpd 10 onlya portion of that period. Givcn this, normal izing spl,'cl Ion 011 diapause may be ineffeetive in reducing geneLic varinbilitv dllrilll', Ibis short time period. The patterns of genetie variabil j ty in di npHllSP pclr;tIi!C'1 prs Ilwy he> related to the adaptive nature of this reaction tloml. 'l'hl' 1 wo !lcll-,llllPI (·l·~. of the logistic function will be seleeted on hy di f[PI-('nt "sppct '; of Ill<' thermal regime. The intercept (or median) determines when 1 JI(' swi t ('Il between egg types occurs. and should he relélted to the élv~raf,(> S('asoll ] ength. This was confirmed in an interpopulational st udy whcrp w(' (ollml significant genetically-based variation in thp inc idencp of di f1patlsf' (i.e .• the logistie intercept) along a cline of growing season length (Chapter 1). The presence of genetic variation for the inu:rcppl impl i,·s that adaptation to new habitats would be rapid and unconslrairwd. The second diapause parameter. the logistî.c slope. rnay ref 1 ('cl the degree of bet-hedgi ng favored by interannual vari abi] i ty in t Ile c1imate: for exarnple, if variation was absent a sudden lraT1S i t i on l'rom direct -developing to diapause eggs wou] d be expec ted. We f ol1l1d t hr· average slope estimated under naturally changjng envi ronmC'ntaJ conditions (Chapter 2) was very simHar lo lhal predicted by rI 1II00üd incorporating environmental heterogenej ty (Chapt er 3). Thl'n~ WH <, ;JI SO lIO 124 ( h(~IQrogpT1(~ity rllnong pOpu]RLions in the slope of the di.apause reaetion nonn (Chapl (Ir 1). Thi sis consistent with the observation that there i8 ] ilile cl inal variation in Lhe variability in season length (Taylor 19R6b). 90 Ihat the optimal slope will be the same among populations. Though olher explanations are tenable. these observations suggest that t he> 1 [lck of Iwri tôble variation for the slope may be the result of stabillzing selection that varies litt1e Beross a broad area. Qth~r Traits- We round signifieant ful1-sib heritabi1ities for wing form and development lime. Mousseau and Roff (1989b) found heritabi1ities of wLng type for 9 populations of A. fasciatus varied between 0.31-1.65: our estilllates fell within this range. The development time estimates we obtai ned are similar ta those compiled for other insects by Roff (1990). élnd were unaffeeted by the rearing environments we used. Corrplations- Slgn changes in genetie correlations can occur if the rf'ôc\'ion norms for the traits under examination cross at sorne point along the envi ronmental gradient (de Jong 1990. Stearns et al. 1991). In our experiment we rE'stricted ourse1ves ta a narrow range of environmental conditions. typical of the period in the field when vélriabil ity for diapause is exposed (Fig. 1). The consistency of the correlations does provide evidence that the functional relationship lwtwC'en ttaits we hypothesize below are relatively stable (Scheiner and Lyman 1990). Because phenotypic variance for diapause is exposed only lInd(lt" a small range of conditions (Fig. 1), the weak genetie correlations between these traits can on1y be considered a slight l'onstraint to evolution. Stearns pt al. (1991) suggest that a more meaningful undcrsUmding of the variability in plastic traits and the correlations b(,tween them will occur when the traits are placed in the context of the l.?'i orgEmism/s development and physio10gy. Wilh éinlllldl'rstclllClillg or tl\(' Diapause. wingform. and to a 1 esser ext en! developmPlI1 t i m(' :lrI' aIl traits whose plasti city is slrongly j nf] Ilcnced by phot operi od (Tanaka and Brooks 1983: Tauber et al. 1987: Danks 1987). PhotoPNiodic signaIs are received. processed and stored in the braill bplorp bl'il1g transrnjtted. hormonally. to the target tissues (SaundlJr 19B7). Bl'('!llIS(' these traits share this method of sj gna] transdllcli on. posi 1 i v(' correlations between them are expected duc to bol il genet Le éllld environmental variation in the common pathway (RLskn 19R(,) Tite si rC'llr,t Il of the correlation will de pend on the l'elative IIwgnitlldE' 01 Ih(· variances in the segment of the pathways that are spùeific 10 (' To our knowledge a positive correlatj on wi thj Il an enVi ronm('nt between microptery and maternally-controlled egg diapflUSl! has not b(!(Jll 1/6 f oh:·;crvpd in other inscct species. WC' sugges!. that the correlati.on ~'1wuen wingform and median diapause is evldence of a common nouroondocrinological pathway which may or may not be adaptive. The ('xpn'ss i on of both wing [orm and diapause can be described by reaction norms with photoperiod: both the incidence of diapause and microptery i nC'rc'8se wi th shorter photoperiods. If there is a common pathway of photorcccption and transmission. a positive correlation between traits will occur. Vurther, if the variation has a genetic basls, the1'e will also he a genetic correlation. In addition ta neuronal mechanisms hormonal regulators Buch as Juvenile hormone may be involved. High 1itres of Juvenile hormone (JH) are positively correlated to the incidence of microptery in ft number of species (e.g. Zera et al. 1989: reviewed by Fairbairn and Roff 1990). and high titres of JH can a1so dolay pclosion or metamorphosis. This will lengthen the sensitive period for diapause, increasing 1ts occurrence. Experimental manipulations of .lU titres in the 1ate instars wou1d be useful in confirming this link. The negative phenotypic correlations between microptery and the 10gLstic slope parameter of diapause may be the result of a statistica1 constraint imposed by the fact that diapause is not likely to fo110w (>xact1y the 10gistic function at the tails of i ts distribution. Many micropterous fema1es laid exc1usively diapause eggs, and their estimated logistlc s]opes were close ta O. The result was a negative correlation hetwccll microptery and the ]ogistic slope. The positive phenotypic correlation between diapause and dcve]opment time is expected based on our current understanding of the lI\C'étsurement of daylengths and the determination of when to diapause (Taylor 1986a: Scott and Dingle 1990). The most widely accepted model for diapaltSe proposes an ontgenetical1y defined sensitive period during which the number of days the photoperiod is 1ess than a critical c daylE'ngth is counted (Saunders 1982). When the number of short days ln exceeds a certain valuE'. diapause is induced, Tf tlll' dlll'dt ion of tlll' sensitive period lS 1engthened. the likC'\ ihood thal Pl10ugh short (!.\VS will be counted increases and diapé-luse will hl' marC' pr0vall'1I1 l'os i t i V(' correlations between diapause and developmC'tlt t i m(' have' éll HO \)('('11 observed in other studies (Tstock et a1, FJ76; HC'nl'Îck mu! 1)('111 ill~l'\' 1982: Hollingsworth and Caphlera 19R3), ln clddit ion. IIl deve10pment rate during the sensitive' period by élltprilllj f('l'dillg conditions have been sllccessfu] in eliciting él changE' ill dinpallsP in II\(' predicted direction (Saunders and Bradley l q81~), 'l'hi S II1PC!WIl i sm lIlily br' important in generating non-gE'netic phE'notypi c var i nhi 1 i t Y i Il cl i npi1\IS(' in the field because deve10pment time is mon' Vélriahlp in Wltlll'C' Ihall in the laboratory (Chapter 1). Although weak. the r,cnetic corrpl between development lillle and diapause were é11so posi t iV0. which lIlighl hl' expected if the traits Involved in the proposed physiolor,icéll 1II('('l!llllitml possessed sorne genetic variation. In sllmmary. diapause 1.s an excel]ent exampl(' of ;1 plnst je 1 nlll for which the ecologi cal rel evance is evi dent. Al thoueh i ncornpl ('1 (>. 0111 knowledge of the deve10pmental systems leading to t!t" cxprc'ss i ort or diapause allows a more insightful invcstigati on of 1 h" t ra il pl (Isl je j 1 Y based on physiology. rather than hypothetical 'g"no cff"cts', To Ihis end, we found significant heri table va riation in the 1 ocaLi OTt parame 1 ('1' of the diapause reaction norm. and suggest ,-hat select ion i mposcd by different climates may act on this pararneter. The shape' of LI\(' r('/I(" ion norm was on1y weakly heritable. This is consistenl with the hypol!lPsis that the optimal slope may be s imilar across a broad e;()()f,raph i en 1 rHJlr,(·. 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