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Survivorship and Growth of Sexually and Asexually Derived Larvae of pometaria (: Geometridae)

Lawrence G. Harshman University of Nebraska - Lincoln, [email protected]

Douglas J. Futuyma State University of New York

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Harshman, Lawrence G. and Futuyma, Douglas J., "Survivorship and Growth of Sexually and Asexually Derived Larvae of (Lepidoptera: Geometridae)" (1985). Faculty Publications in the Biological Sciences. 273. https://digitalcommons.unl.edu/bioscifacpub/273

This Article is brought to you for free and open access by the Papers in the Biological Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in the Biological Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Harshman & Futuyma in American Naturalist (June 1985) 125(6). Copyright 1985, University of Chicago. Used by permission.

SURVIVORSHIP AND GROWTH OF SEXUALLY AND ASEXUALLY DERIVED LARVAE OF ALSOPHILA POMETARIA (LEPIDOPTERA: GEOMETRIDAE) A substantialbody of theoryis devoted to understandingthe relative advan- tages of sexual and asexual reproduction.It is generallyunderstood that asexual formspotentially have a higherrate of reproductionbecause theysave the cost of producing males. The microevolutionaryconsequences of sexual and asexual reproductionare less clear. Sexual reproductiongenerates abundant genotypic diversitywhich may be adaptivelyadvantageous (Williams 1975; Maynard Smith 1978). Asexual reproductionmay perpetuate combinations of genes that are coadapted (Templeton 1979), heterotic(Suomalainen et al. 1976; White 1979), or specialized (Vrijenhoek1979, 1984). Thus, it is possible thatthe fitnessof a sexual populationmay be lower thanan asexual, in part,because recombinationtends to break up especially favorable genotypes (Williams 1975; Hebert 1978). If it is generallyobserved thatasexual reproductionhas an immediateadaptive as well as a reproductiveadvantage, then it is difficultto see how sexual reproductioncan be maintainedby short-termadvantages (Williams 1975). A comparisonof closely relatedsexual and asexual formsis a promisingavenue of researchto evaluate experimentallythe consequences of both modes of repro- duction (Maynard Smith 1978). In this study,geometrid larvae (Alsophila pometaria) derivedfrom both kindsof reproductionwere rearedon differenthost plants. The goal was to assess larval viabilityand growthin an ecologically relevantcontext and thus partiallycharacterize the fitnessof sexual and asexual reproduction.

Biological Background Alsophila pometaria, the sole Nearctic memberof its (Inoue 1943), is a species of moth whose unusual mode of reproductionhas been studied exten- sively withelectrophoretic markers. Most females are gynogeneticand requirea matingwith a nonspecificmale in orderto reproduce.Gynogenetic females of the fall cankerwormtransmit the maternalgenotype intact to theirprogeny (Mitter and Futuyma 1977). A varietyof electrophoreticallydefined multilocus genotypes (clones) are foundin asexual populations,and it has been establishedthat there is a coexistingsexual population(Mitter et al. 1979). Electrophoreticsurveys have revealed thatthe same alleles are presentat similarfrequencies in the sexual and asexual populations (Mitteret al. 1979; Harshman 1982). The species is univoltine.Eggs are laid on treesmostly in earlywinter and hatch about the time of budbreak in the spring.There can be extensive dispersal of hatchlinglarvae by ballooningon silk threads(Futuyma et al. 1981). This larval

Am. Nat. 1985. Vol. 125, pp. 8%-902. ? 1985 by The Universityof Chicago. 0003-0147/85/2506-0010$02.00.All rightsreserved. NOTES AND COMMENTS 897 dispersalcommonly may resultin movementamong different host trees(Futuyma et al. 1981). The larvae feed on the foliage of numerousdeciduous tree species untilearly Junewhen they drop to the groundand pupate. The correspondence between the maturationschedule of the larvae and that of the foliage on which theyfeed is criticalbecause, like the European wintermoth Operophtera brumata (Feeny 1970), theyare unable to completegrowth on excessively maturefoliage.

MATERIALS AND METHODS Identificationof ReproductiveMode Larvae forthese experimentswere obtainedfrom egg masses foundon trees or directlyfrom mating adults collected in the field.The collectionsites were wood- lots dominatedby oak (Quercus spp.) and red (Acer rubrum)near the Stony Brook campus of the State Universityof New York, which is located on the northernshore of Long Island. A numberof eggs fromeach egg mass were set aside at room temperaturefor early hatchingand subsequent electrophoretic analysis. The remainderwere leftoutside for the winterin 1-dramglass vials to be used in experimentsafter larvae hatched in the spring. It is not possible to determinethe reproductivemode of adultfemales from their externalmorphology. Thus, it is our procedureto electrophoresea numberof sibs froman egg mass to determinedirectly the mode of reproductionof the mother. First-instarlarvae that hatched in the lab were homogenizedin 12-14 kl of the grindingbuffer used by Mitteret al. (1979). Afterelectrophoresis, we stainedfor two of the polymorphicenzymes employed for adults (PGM and PGI). Usually, 10 larvae were run from each egg mass. Sexual reproductionwas detected by observingrecombination of electrophoreticmarkers among larval sibs (Harshman 1982). Larvae of specific asexual lineages were isolated fromfemales whose multilocusgenotype correspondedto that of an established clone (Mitteret al. 1979; Futuymaet al. 1981).

Growthin a ControlledEnvironment In the spring of 1980, larvae from a sexually derived sibship (HL75) were compared to two asexual lineages. One lineage was the widespreadand abundant asexual genotype2(B), which is associated withoak in the fieldand is apparently an oak specialist (Mitteret al. 1979). The second was asexual genotype48(Z), which is abundantin mixed oak-and-maplestands. Mitteret al. (1979) suggested thatthis genotypemay be more generalizedin its feedinghabits than the oak- or maple-associated clones. Larvae of the maple-specialistgenotype, 26(A), were not available for this experiment. Larvae were rearedentirely on eitheroak or red maple leaves obtainedfrom the field.All larvae reared on oak were exposed to a mixtureof two species (Quercus coccinea and Q. velutina). Hatchlinglarvae fromeach sibship employed in this study were split evenly between oak and maple foliage. The experimentwas conductedwith approximately 10 larvae per 5-litercardboard container. Food was 898 THE AMERICAN NATURALIST replenishedevery otherday in the formof freshsprigs of foliage,with the twigs insertedinto flasks of water. The containerswere placed in an environmental chamber adjusted for 18 h of lightat 24? C and 6 h of darkness at 16? C. Larval survivorshipwas not recorded in the experimentbecause a major source of mortalitywas uncontrolleddesiccation of first-instarlarvae. After3 wk to 1 mo of growth,prepupal larvae leftthe foliageand began burrowing.When an individual reached this stage its weightwas recorded. Prepupal weightis correlatedwith fecundityin thefall cankerworm (Schneider 1979),and generallypre-image weight is highlycorrelated with fecundity in (Engelmann 1970). Body size is not necessarilya primarydeterminant of fitness,but it is correlatedwith fecundity, which is a componentof fitness.

Survivorshipand Growthof Larvae in the Field In the springof 1981 an experimentwas conductedto evaluate the survivorship and growthof sexually and asexually derived larvae. The tested genetic groups consisted of a sexual sibship (VW1 18), the oak-specialist [2(B)] and maple- specialist [26(A)] genotypes,and two clones thatare uncommonon Long Island. The site of the studywas a woodlot on the Stony Brook campus in which five oak trees (Quercus coccinea) and five red maple trees were selected as host plants. Soon afterbudbreak, larvae thathad just hatchedwere placed on foliage enclosed by finemesh bags as describedby Futuymaet al. (1981). Each host tree supportedat least two bags withthree larvae fromeach geneticgroup. Afterapproximately 1 mo of growth,when larvae began to reach the prepupal stage, all rearingbags were broughtin fromthe field,and larval survivorshipwas recorded. Larvae thathad not attainedthe prepupal stage were raised for a few days in the laboratoryon vegetationsimilar to thatavailable to themin the field. All prepupal larvae were allowed to pupate in a mixtureof sterilizedsand and pottingsoil. After2 mo, pupae were removed fromtheir subterranean cocoons and sexed. They were thenlyophilized and the dryweight of each individualwas determined. Several aspects of these studiesdeserve comment.First, they are not designed to quantifytotal fitness.Moore (1976), in a comparisonof the gynogeneticfish Poeciliopsis monacha-occidentalis with a related bisexual species, describes three aspects of fitness.One is the twofoldreproductive advantage accruing to asexual reproduction,another is mating success, and the third is "primary fitness,"which includes fecundity and survival.In the presentstudy on Alsophila pometaria, we are concerned with measures that are analogous to "primary fitness." A second pointis thatthe size of our studieswas limitedby the scarcity of sexual reproduction.In one survey only 2 sexual females were found in 117 progeny-testedfemales collected frompopulations near Stony Brook (Harshman 1982). Thus, our experimentswere constrainedin sample size and the numberof treatmentsemployed. Finally, it was observed that the egg masses used in this studyhad no egg parasites, and larvae derived fromboth modes of reproduction appeared to be free of disease. NOTES AND COMMENTS 899

TABLE 1

PREPUPAL WET WEIGHT OF LARVAE REARED IN A CONTROLLED ENVIRONMENT

ON OAK ON MAPLE

GENETIC GROUP Y(mg) N S Y(mg) N

Sexual sibship 54.88 17 92.485 50.06 17 75.184 Asexual genotype2(B) 55.16 57 51.564 27.92 51 56.554 Asexual genotype48(Z) 49.88 24 56.984 27.44 25 24.340

NOTE.-Interaction (genetic group X plant): x2 = 40.7, P < 0.001.

RESULTS Growthin a ControlledEnvironment Table I presentsthe average weight,variance, and sample size forlarvae reared in the laboratory.The larvae cannot be sexed and consequentlyboth males and females from the sexual sibship were weighed, which may have inflatedthe variance of samples. Wilson's nonparametricmulti-way method (Wilson 1956) was used to analyze the data. There was a significantinteraction between genetic group and host plant, which was presentbecause red maple was an inferiorhost forthe asexual genotypesbut not forthe sexual sibship. There was no evidence that genotype48(Z) was more generalized in resource utilizationthan the oak- specialist genotype. The lengthof the larval periodwas recordedin thecourse of theexperiment. On red maple, the average larval periods were 27.0 days (sexuals), 27.3 days [2(B)], and 28.1 days [48(Z)]. On oak, the larvae maturedfaster: the averages were 18.2 days (sexuals), 20.6 days [2(B)], and 21.8 days [48(Z)]. There were no significant differencesbetween geneticgroups in the rate of development,and it is clear that largerlarval size is not a functionof an extended feedingperiod.

Survivorshipand Growthof Larvae in the Field Table 2 shows the proportionof larvae thatsurvived in fieldcages. The average survivorshipacross plantswas 50% forthe sexuallyproduced progeny and ranged from29% to 39% forthe asexual geneticgroups. The data were evaluated using log-linearmodels (Sokal and Rohlf 1981), which revealed no significantthree- factorinteraction. The lack of independencebetween variables (table 2) indicated that larval mortalitydiffered among host plants and genetic groups. Red maple was again an inferiorhost, and as demonstratedby Futuymaet al. (1981), the oak- specialist genotype2(B) has poor viabilityon red maple. The average dryweights of pupae are shownin table 3. Data were available only for oak-reared larvae because of high mortalityon maple. Only females were weighed fromthe sexual sibship to make the data as comparable as possible. Nevertheless,the sexual sibship was significantlymore variable than the pooled 900 THE AMERICAN NATURALIST

TABLE 2

PROPORTION OF LARVAE SURVIVING IN FIELD CAGES

ON OAK ON MAPLE

Survival Survival GENETIC GROUP Rate N Rate N

Sexual sibship .80 30 .20 30 Asexual genotype2(B) .55 60 .03 60 Asexual genotype26(A) .64 45 .13 45 Uncommon asexual genotypes .62 45 .13 45

NOTE.-Overall interaction (genetic group x plant X mortality): G = 2.183, NS; independence of mortality and host plant: G = 118.585,P < 0.001; independenceof genotypeand mortality:G = 12.893, P < 0.05.

TABLE 3

DRY WEIGHT OF FEMALE PUPAE DERIVED FROM LARVAE REARED ON OAK

GENETIC GROUP Y(mg) N S2

Sexual sibship 12.68 10 34.388 Asexual genotype2(B) 11.44 10 9.860 Asexual genotype26(A) 8.37 19 5.138 Uncommon asexual genotypes 7.63 11 13.943

NOTE.-Kruskal-Wallis testfor heterogeneity among ranked loca- tions: H = 19.840, P < 0.01. asexual genotypes(F = 3.28, P < 0.05), whichindicates that considerable phenotypicvariation is producedby sexualreproduction. A Kruskal-Wallistest showsthat there is significantheterogeneity among ranked locations of genetic groups.In part,this heterogeneity exists because the top 3 larvae,and 6 ofthe 10 heaviestlarvae, were from the sexual sibship.

DISCUSSION A numberof studies have compared sexual and asexual reproduction in closely relatedanimals. Numerous factors have been considered,including competition (Wilbur1971; Cuellar 1979); fertility and fecundity(Uzzell 1964;Thibault 1975; Lamband Wiley1979; White and Contreras 1979; Schall 1981); tolerance to stress (Lindroth1954; Danzig 1959;Bulger and Schultz1979); and colonization(Wright and Lowe 1968;Jaenike and Selander1979). In general,it is difficultto knowif differencesbetween related taxa are due to modeof reproduction alone instead of othergenetic attributes. For example,a comparisonof asexual forms of Poeciliop- sis (Schultz1977) with bisexual progenitors is complicatedby hybridizationand polyploidy.This does notseem to be a problemin Alsophila pometaria, for which bothsexual and asexual populationsare quitesimilar in geneticconstitution. NOTES AND COMMENTS 901

Apparentlypopulations of A. pometaria are not composed of exceptionallyfit asexual genotypes. There are a numberof factorsthat could act to reduce the relative fitness of gynogeneticlineages. For instance, it is possible that the gynogeneticlineages carrynumerous deleterious mutations (Muller 1964; May- nard Smith 1978). Another possibility,suggested by the distributionof pupal weightin our fieldexperiment, is thatsexual reproductionproduces especially fit ephemeral genotypes (Williams 1975). There is no indicationthat, on theirop- timumhost, the specialized asexual genotypesare morefit than a sample fromthe sexual population. Moreover, there is a preliminarysuggestion that the sexuals may have a broader niche thanthe asexual genotypes.This observationwarrants furtherinvestigation, especially in view of the argumentthat a batteryof asexual genotypes may have greater niche breadth than a freely recombiningsexual population (Roughgarden 1972). In summary,this study on A. pometaria has produced no evidence thatasexual reproductionhas an adaptive advantage.

ACKNOWLEDGMENTS We would like to thankJames Rohlf for statistical advice. We would also like to express our appreciationto Rob Armstrong,Lev Ginzburg,Ed Golenberg,Jerry Hilbish, Dave Innes, Charles Mitter,Lorenz Rhomberg,Peggy Saks, Woody Setzer, Dan Wartenberg,George Williams,Dan Yocom, and Tony Zera formany helpfuland enjoyable research-relatedconversations. This is contribution511 fromthe Programof Ecology and Evolution at the State Universityof New York at Stony Brook. This researchwas supportedby grantsfrom the National Science Foundation (BSR 8306000) arndthe WhitehallFoundation.

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LAWRENCE G. HARSHMAN* DOUGLAS J. FUTUYMA DEPARTMENT OF ECOLOGY AND EVOLUTION STATE UNIVERSITY OF NEW YORK STONY BROOK, NEW YORK 11794 SubmittedMarch 23, 1984; Revised September19, 1984; Accepted November 12, 1984

*Presentaddress: Departmentof Genetics, Universityof California,Davis, California95616.