And Heritability of Wing Dimorphism in the Striped Ground Cricket, Allonemobius Fascia Tus
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Heredity 62 (1989) 3 15—318 The Genetical Society of Great Britain Received 25August 1988 Geographic variability in the incidence and heritability of wing dimorphism in the striped ground cricket, Allonemobius fascia tus Timothy A. Mousseau* and Department of Biology, McGill University, Montréal, Derek A. Roff Québec, Canada H3A 1BI. We examined nine Allonemobius fasciatus populations for variation in the incidence and heritability of wing dimorphism. When reared in the laboratory, the incidence of long winged forms varied significantly among populations (from 9 to 74 per cent), and sexes, with females usually producing a larger proportion of long winged individuals than males. The heritability of wing length, averaged across populations, was O52±O14formales and O72±O15 for females, and did not vary significantly among populations. There was no apparent relationship between either the incidence or the heritability of wing length and the geographic origin of the founding population. INTRODUCTION interpopulation variability in the genetics of wing polymorphism has not until now been addressed. Wingsusually bestow the power of flight and the The purpose of this study was to examine geo- ability to disperse in search of food or mates, graphic and sex linked patterns in the heritability or to migrate when environmental conditions of wing dimorphism in the striped ground cricket, deteriorate (Roil, 1984, 1986a, b).Yetin many A1!onemobiusfasciatus. This species is widely dis- pterygote insects wing dimorphism is common tributed in North America (Alexander and (Roil, 1986b). Recent evidence suggests that Thomas, 1959) and has been found to exhibit possession of long functional wings often entails genetically based geographical variation in body a cost to fitness, usually in the form of reduced size, ovipositor length, diapause propensity, fecundity and/or a delay in the time at first repro- development times, and the thermal requirements duction in the long winged morph (Dingle, 1980; for development (Mousseau, 1988). In addition Harrison, 1980; Roil, 1986a). Roil (1986b) has there is evidence to suggest a significant cost to proposed that "the frequency of the two wing the possession of functional wings in this species: morphs (long and short) in a population will short winged females (which cannot fly) produce depend upon the stability of the habitat, the on average more eggs and intiate egg production benefits such as increased fecundity of being flight- at an earlier date following the final molt than do less, and the genetic basis of the trait". long winged females (Roil, 1984). Long winged Relatively little is known of the genetics of wing males may also suffer delays in maturity and dimorphism. Of the 23 insect species examined by reduced sperm production (Roil, unpublished Roil (1986 a), eight appeared to inherit wing length data). However, variation among wing morph in in a Mendelian fashion, while fourteen exhibited nymphal growth and development rates may tend polygenic genetic determination of pterogomorph- to compensate for these apparent costs (Mousseau, ism. Within the sand cricket, Gryllus firmus, the 1988). family means of wing length incidence were found Here, we present the heritability of wing to vary continuously within a population, and to dimorphism in nine widely distributed popu- possess high heritabilities, on the order of 055 to lations. We test for geographically related 068 (Roil, 1986b) clearly indicating the potential heterogeneity in the incidence and heritability of for selection to act within populations. However, wing expression, and determine the coincidence * Presentaddress: Department of Entomology, University of between sexes in any discernible patterns of geo- California,Davis, USA 95616. graphic variation. 316 T. A. MOUSSEAU AND 0. A. ROFF METHODS removed from cages and scored as to sex and wing length (long or short). Allonemobiusfasciatus was collected from nine The heritability of wing length was estimated locations chosen to span much of this species using a two-tiered approach. First, the intraclass latitudinal and altitudinal range along the east correlation (t) of family incidence of long winged coast of North America (table 1). A minimum of offspring (0:1 data) was estimated using three one hundred individuals was collected from each methods which we refer to as the ANOVA method population, and upon return to the laboratory (Elston, 1977), the Maximum Likelihood method, crickets were placed in plastic mouse cages (30x and the x2 method (Robertson, 1951).Usingthe 17 x 14 cm) and provided with iceberg lettuce, ANOVA method, the intraclass correlation is crushed Purina cat chow, a cotton corked water calculated as: vial, strips of kraft paper for cover, and a plastic sandwich box (12 x 12 x 3•5 cm) filled with steril- t —(MSaMSw)/(MSa+(k— 1)MS) ized potting soil for ovipositing. Glass lids and where elastic bands were used to seal the cages. Mainten- ance was provided twice weekly. These stock popu- MSa=( m/n—( m)2/N)/(C—1), lations were permitted to breed randomly, and to MS = C), complete a minimum of two generations prior to ( m,—m/n1)/(N— the initiation of experiments. k=(N— n/N)/(C—l), Between 45 and 74 mating pairs were estab- and m, is the number of long winged individuals lished for each experimental population. Virginity of females was assured by collecting within one in family i, n, is the total number of offspring in day of final ecdyss. Each mated pair was placed family i, C is the number of families, and N is in a plastic sandwich box (12x 12x35 cm) con- the total number of individuals in the study popu- taining approximately 1 gm of crushed Purina cat lation. chow and some iceberg lettuce. Females laid their Using the x2 method t is calculated as: eggs in a 25 x 30 cm piece of cheese cloth that had been loosely rolled, moistened and placed into the deep half of a 37 mm petri dish. Boxes were cleaned where twice weekly and eggs removed every five to seven x2=( m/n—( m)2/N)/(p(1—p)) days. Eggs were extracted from the cheese cloth, put onto moistened paper towels, placed into 7 cm G= n-( n/ n)-(C-1) petri dishes (egg dishes) and incubated at 30°C and p is the mean proportion of long winged and 14:10 hrs LD for fifteen days. Egg dishes were individuals per family (Bull et al., 1982): checked daily; hatched nymphs were counted and placed into plastic boxes which had been equipped p=(1/C)(m/n1). as those of the parents (minus the egg dish). All The maximum likelihood estimate of t is estimated nymphs were reared at a maximum density of sixty as: per cage and maintenance was provided twice weekly. Following development, individuals were t =(2p(l—p) K,)/ n(n, —1) Table 1 A description of the study populations. Season lengths were calculated as mean annual degree. days> 13°C, except for populations denoted by * which were estimated from latitude and elevation. Transition zone populations contained mixtures of both univoltine and bivoltine crickets Elevation Season length Population Latitude (m) (°.d> 13C) Life history Montréal, Québec 45°30'N 30 851 Univottine Licklog Ridge, North Carolina 35°32N 1500 1200* Univoltine Asheville AP, North Carolina 35°37N 700 1435 Univoltine Horsegap, North Carolina 35°50N 600 1500* Univoltine Richmond, Virginia 37°32'N 30 1857 Univoltine J. H. Kerr Dam, Virginia 36°35'N 70 1833 Transition Oxford, North Carolina 36°20'N 100 1968 Transition Toccoa, Georgia 34°34N 300 2007 Bivoltine Winder, Georgia 33°59'N 290 2011 Bivoltine GEOGRAPHIC VARIABILITY AND HERITABILITY OF WING DIMORPHISM 317 where K1=05(a+b+c), a =(m1(m,—1))/p2, — — — b =((n1 m1)(n, m, 1))/(1 —p)2, > and C0 ci) c= — (2m(n,m1))/p(l—p). a- The heritability is then estimated from the intra- ci) class correlations using the methodology outlined U- by Bull et a!. (1982) and by Robertson and Lerner >1) (1949) where: -I-- h2=2tp(1 —p)/z2, = E and zisthe ordinate on the standardised normal = curve which corresponds to a probability p. The 0 standard error of the estimate was calculated as: S.E. (h2) =2p(l—p)(l —t)(1+(k—1)t)/z2 xJ(2(N —1)/k2(C—1)(N—C)). Incidence of Long Wings RESULTSAND DISCUSSION Figure 1 The cumulative frequency distributions of the In incidence of long winged offspring in 242 full-sib families. fig. 1 it is shown that the distribution of family S x S indicates crosses between short winged parents, Lx L incidence of long winged offspring (i.e., the pro- denotes crosses between long winged parents, while S x L portion of long winged offspring within a full-sib indicates families of mixed parentage (i.e.,longx short and family) is continuous regardless of parental short x long). The distributions are plotted separately for male and female offspring because of the sex related phenotype. However, short winged parents did differences in the incidence of long wingedness. produce on average fewer long winged offspring than did crosses between long winged parents, while "mixed" parents produced a distribution of family incidence intermediate between those of the nificant (paired t-test: D=018±007, df=8, t= monotypic parental crosses. These observations 257, P <0.04), though there were indications that suggest a polygenic basis to wing dimorphism in the estimated heritability for males and females the striped ground cricket. within a population was associated (r=0.62, t= In all populations there were more long winged 21, df=8, P<O.04). There was no significant females than males, though comparing across relationship between the incidence of long wings populations there was a good correlation (r =O93, and heritability in males (r=—0.27, t=074, P> =67, P<00O1)between the proportions of male 0.4) but macroptery and its heritability were and female macropters within a population (i.e., strongly correlated in females (r =—082,t= —38, a population that produced a high percentage of df= 8, P <0.007).