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Published in final edited form as: J Evol Biol. 2010 Oct 7;23(12):2677–2684. doi: 10.1111/j.1420-9101.2010.02127.x

Phenotypically plastic traits regulate caste formation and soldier function in polyembryonic wasps

Margaret S Smith 1, Ian Milton 1, Michael R Strand 1,*
PMCID: PMC3057481  NIHMSID: NIHMS238259  PMID: 21040070

Abstract

Polyembryonic encyrtid wasps are parasitoids that have evolved a clonal form of embryogenesis and a caste system where some progeny become reproducing wasps while others develop into a sterile soldier caste. Theory based on the biology of Copidosoma floridanum predicts the primary role of soldier larvae is to mediate conflict over sex ratio, which also favours female-biased soldier production. Other data, however, suggest female-biased soldier production reflects a developmental constraint. Here we assessed whether female-biased soldier function by polyembryonic wasps reflects sex specific adaptation or constraint by conducting comparative studies with Copidosma bakeri; a species that produces clutch sizes similar to C. floridanum yet rarely produces broods associated with sex ratio conflict. Our results indicate the oviposition behaviour of adults, development of progeny, and function of soldier larvae differs greatly between C. bakeri and C. floridanum. These findings indicate that caste formation and soldier function in polyembryonic encyrtid wasps are regulated by phenotypically plastic traits. Our results further suggest the primary function of the soldier caste in some species is defence of host resources from competitors while in others it is resolution of sex ratio conflict.

Keywords: caste formation, sociality, parasitoid, altruism, spite, competition

Introduction

Social insects form groups comprised of castes that perform either reproductive or altruistic helper functions (Costa, 2006). The greatest diversity of social species occurs in the order Hymenoptera (bees, wasps, and ants), which exhibit haplo-diploid sex determination and usually form colonies of related individuals (Hughes et al., 2008; Boomsma, 2009). Polyembryonic wasps in the tribe Copidosomatini (family Encyrtidae) are unique because they are the only social species that are also parasitoids whose offspring develop by feeding inside the body of another insect (the host). These wasps are also unique because caste formation occurs through a process of clonal division from a single egg (Ivanova-Kasas, 1972; Strand & Grbic, 1997; Strand, 2009).

The development of polyembryonic encyrtids is best understood from studies of Copidosoma floridanum, which parasitizes the egg stage of plusiine moths (Strand & Grbic, 1997; Strand, 2009). Each C. floridanum egg undergoes a proliferative (clonal) phase of embryogenesis that generates as many as 3000 embryos as the host egg hatches and the larva grows to a fourth instar (Grbic et al., 1998, Corley et al., 2005; Gordon & Strand, 2009). Up to 24% of embryos develop into soldier caste larvae during early host instars, while the remaining embryos develop into reproductive caste larvae during the host’s final (fifth) instar (Grbic et al., 1992; Harvey et al., 2000; Giron et al., 2007a). Reproductive larvae rapidly consume the host, pupate in the remnant host cuticle to form a ‘mummy’, and later emerge as adult wasps. Soldier larvae in contrast die following consumption of the host by their reproductive caste siblings. Mechanistically, asymmetric parcelling of the germ line determines caste fate with embryos inheriting germ cells developing into reproductive larvae and embryos that do not forming soldiers (Donnell et al., 2004; Zhurov et al., 2004; Gordon & Strand, 2009). Development of other Copidosoma species proceeds similarly to C. floridanum although the number of progeny produced per host varies from less than a hundred to several thousand (Doutt, 1947; Cruz, 1981; Walter & Clarke, 1992; Carroll et al., 2007).

Soldier larvae are hypothesized to have evolved in response to two factors. The first is defence of local resources (the host) from intra- or heterospecific competitors (Cruz 1981; Harvey et al., 2000; Yamamoto et al., 2007; Giron et al., 2007a; Gardner et al., 2007; Segoli et al., 2009a). The second is resolution of sex ratio conflict that can occur if wasps oviposit both male and female eggs into the same host to produce mixed-sex broods (Grbic et al., 1992; Giron et al., 2004; Gardner et al., 2007). Under such conditions, no sexual conflict exists if brothers only mate sisters (i.e. complete local mate competition (LMC) (Hamilton 1967)), whereas conflict would be predicted to increase as opportunities for males to obtain additional mates rise (Grbic et al., 1992; Godfray, 1994). However, soldiers (either female or male) could resolve this conflict by killing embryos of the opposite sex. The model framework of Gardner et al. (2007) assumes that polyembryonic wasps lay a single female and male egg per host, and predicts female eggs will produce more soldiers than male eggs if their primary function is to manipulate sex ratio in favour of their own siblings. This is because increasing the number of wasps produced per clone benefits females, who only reproduce by finding new hosts, more than males where each individual is able to mate multiple sisters. Males on the other hand are predicted to produce more soldiers than females if the primary function of soldier larvae is defence. Since clonal assemblages are expected to show group-optimal behaviour (see Gardner & Grafen, 2009), we would further expect both sexes to produce soldiers equally if wasps allocate male and female eggs to different hosts provided 1) no sex differences exist in the benefits of soldiers to the clone, and 2) no sex differences exist in constraints affecting soldier development.

Consistent with model assumptions, C. floridanum usually lays one female and one male egg per host and produces mixed-sex broods with strongly female-biased sex ratios (Grbic et al., 1992). Although Gardner et al. (2007) showed that a male clone in mixed broods might be selected to voluntarily reduce its own proliferation, experimental studies with C. floridanum show that female clones produce in total more soldiers than males, and that female soldiers also begin emerging much earlier males (Giron et al. 2007a). Moreover, female soldiers greatly reduce the abundance of brothers in mixed broods by recognizing and killing them which manipulates the sex ratio toward their reproductive sisters (Grbic et al., 1992; Ode & Strand, 1995; Giron et al., 2004; Giron et al). Together with evidence that surviving males mate sisters but also disperse and outcross (Ode & Strand, 1995; Giron et al., 2007a), these findings indicate the primary function of soldiers in C. floridanum is to resolve sex ratio conflict although it is important to note that female soldiers also recognize and kill other competitors (Harvey et al., 2000; Giron et al., 2007a, b). Soldiers likewise develop only from female eggs in the congener Copidosoma koeleri (Doutt, 1947; Kaesar et al., 2006). Unlike C. floridanum, however, C. koehleri females lay only one egg per oviposition event, producing single sex broods, where no sex ratio conflict is expected. This finding raises the possibility that female-biased soldier production may not reflect an adaptive response to sex ratio conflict, but rather a developmental constraint prevents or severely reduces the ability of male eggs in polyembryonic wasps generally to produce soldiers (Doutt, 1947; Giron et al., 2007a). If true, such a constraint could also favour the oviposition of mixed sex broods in species like C. floridanum where the male clone is numerically reduced by female soldiers, but surviving males secure large numbers of matings with sisters while benefiting from the protection soldiers provide against other competitors.

Distinguishing between 1) whether soldier larvae evolved primarily in response to local competition for resources or sex ratio conflict, and 2) an adaptive versus developmental explanation for female-biased soldier production clearly requires additional comparative data. Yet obtaining such information is extremely difficult, because most polyembyronic wasps parasitize hosts that are either unknown or cannot be reared (Zolnerowich, 1995; Gurrier & Noyes, 2005). In addition, many species are uni- or bivoltine, which makes collection of material from the field and detailed behavioural or developmental studies nearly impossible. Here, we conducted studies with Copidosoma bakeri: a species amenable to culture that produces clutch sizes very similar to C. floridanum, yet in the field rarely produces mixed-sex broods (Byers et al., 1993). Our results indicate that female and male C. bakeri produce similar numbers of soldiers, which strongly attack a heterospecific competitor but exhibit little aggression toward conspecifics.

Methods

Insect rearing

The C. bakeri culture we maintained was founded from Y. Saeki (University of Kentucky) who provided approximately 20 mummies from parasitized black cutworms (Agrotis ipsilon: Lepidoptera: Noctuidae) collected at field sites near Lexington, Kentucky (USA). We reared A. ipsilon on pinto bean based artificial diet at 27° and a 16 h light-8 h dark photoperiod. Moths were fed 10% sucrose. We maintained C. bakeri as a randomly mating population by introducing male wasps from broods into glass tubes containing female wasps from other broods. Twenty four h later, mated females were provided A. ipsolin eggs for parasitism. After hatching, parasitized host larvae were reared as described above until formation of mummies.

Oviposition behaviour and soldier development

The oviposition behaviour or C. bakeri females and development of offspring were monitored using methods previously developed for C. floridanum. In one set of experiments, adult wasps from 10 different broods were provided six hosts for oviposition. Half of these hosts were squashed on microscope slides to determine the number of parasitoid eggs laid per host (Strand, 1989) while the other half was allowed to complete their development and emerge as adult wasps. In another set of experiments, wasps were allowed to oviposit into hosts, which were subsequently dissected in physiological saline as first-sixth instars. The total number of soldier larvae present in the hemocoel of dissected hosts was determined by counting. The assemblage of embryos produced during the clonal phase of embryonic development is called a polymorula (Strand 2009). We sexed broods by taking a portion of the polymorula collected from each host and preparing chromosome squashes stained with lacto-aceto-orcein as described by Strand & Ode (1990).

Soldier behaviour toward competitors

We compared male and female soldier behaviour toward intraspecific competitors using previously described in vivo assays that quantified the aggression of C. floridanum soldiers (Giron et al., 2004; Giron & Strand, 2004; Giron et al., 2007a, b). Briefly, polymorulae of unknown sex were collected from fourth instar hosts, labeled with carboxyfluorocein diacetate succinimidyl ester (CFSE), and injected into fifth instar hosts containing a C. bakeri brood. Each donor polymorula derived from a female wasp that was not a full sister of the mother that parasitized the recipient host. Thus, each transplant was classified as being a ‘non-relative’. Recipient hosts were dissected 24 h post-transplantation and the soldiers were examined by epifluorescent microscopy while the developing resident polymorula was karyotyped to determine the sex of the recipient brood. Soldiers that attacked a CFSE-labeled polymorula contained the fluorescent tracer in their gut, while non-attacking soldiers did not. To assess whether C. bakeri soldiers recognize and attack a heterospecific intruder, we repeated these experiments by injecting a CFSE-labeled polymorula from C. floridanum into hosts.

Microscopy and data analysis

Chromosome squashes, polymorulae, and wasp larvae were examined using either a Leica DMRXE or DM-IRE2 epifluorescent microscope. Images were captured using Leica or SimplePCI software followed by figure assembly using Adobe Photoshop. All statistical analyses were carried out using the JMP 8.0 platform (SAS, Cary, NC).

Results

C. bakeri produces clonal, single sex broods

C. bakeri parasitizes noctuid moths in the genera Agrotis and Euxoa (subfamily Noctuinae; tribe Agrotini) that lay eggs in surface debris or soil (Mitchell et al., 2006). Byers et al. (1993) collected more than 400 larvae of the army cutworm, Euxoa auxiiaris from multiple field sites that were parasitized by C. bakeri. The number of C. bakeri adults emerging per host varied from 520–2400 with 92% of broods producing offspring of only one sex. Saeki et al. (2009) reported that most broods produced in the black cutworm, Agrotis ipsilon, also contained all male or female offspring. Taken together, these findings indicate that C. bakeri primarily produces single sex broods, but whether all offspring develop from a single egg and are full siblings was unclear. To address this question, we allowed individual C. bakeri females to parasitize either one (n= 50 females) or five A. ipsilon eggs in succession (n=50 females) to simulate conditions of high or low host density in the field. Each oviposition event was observed to assure that no hosts were self-superparasitized (i.e. female oviposits into a host more than once). Following wasp oviposition, half of the host eggs were crushed on microscope slides and examined to determine the number of wasp eggs laid while the other half were allowed to form mummies. Our results indicated that 93% of host eggs from the low and high density treatments were parasitized and each contained a single C. bakeri egg, while 7% of host eggs contained no wasp egg(s) and were presumably non-parasitized. Correspondingly, all resulting mummies that developed from hosts that we reared in this experiment produced adult wasps that were entirely of one sex or the other. By counting the adult progeny that emerged from 15 male and 15 female broods, we determined that all-male brood sizes 2130 ± 27 (mean ± SE) were on average larger than female broods (1821 ± 46.7) (t-test; P<0.01). Viewed in relation to the field data of Byers et al. (1993), these results suggested that C. bakeri offspring rarely experience within-brood sex ratio conflict among siblings.

Males and females produce similar numbers of soldiers

As previously noted, polyembryonic encyrtids clonally produce increasing numbers of embryos as the host larva grows from its first to penultimate instar. Visual inspection indicated that C. bakeri formed a large, contiguous polymorula that in the host penultimate instar consisted primarily of embryos which emerge as reproductive larvae in the host’s final instar plus several embryos in the process of developing into soldier larvae (Fig. 1a). To assess whether male and female eggs produce different numbers of soldier larvae, we used karyotyping methods. Like C. floridanum, squashes prepared from C. bakeri contained an abundance of chromosome spreads due to high levels of cell division associated with clonal development. Inspection of polymorulae from hosts parasitized by unmated C. bakeri indicated the haploid (male) karyotype for C. bakeri, was n=11, while inspection of broods produced by mated wasps indicated the diploid (female) karyotype was 2n=22 (Fig. 1b, c).

Fig. 1.

Fig. 1

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C. bakeri embryos can be sexed by karyotyping. (a) Phase-contrast micrograph of a C. bakeri polymorula dissected from a fifth instar host. The polymorula consists of thousands of embryos at this stage of development. A majority of embryos are rounded, 80–100 µm in diameter, and will develop into reproductive caste larvae (R). Interspersed among these embryos are embryos in the process of developing into soldier caste larvae (S). Scale bar= 1 mm. (b) Lacto-orcein stained chromosome spreads from a male polymorulae. (c) Lacto-orcein stained chromosome spreads from a female polymorulae. Scale bar in C= 30 µm

We then dissected individual fifth instar hosts and counted the total number of soldiers present in the hemocoel, while simultaneously noting the location of the polymorula and its sex. An average of 18.2 soldiers (range 6–51) were present per host with no differences between male and female broods (Fig. 2a). The polymorula for each sex was also usually attached to trachea in the host thorax and formed a contiguous sheet in the haemocoel that was not covered by fat body or other tissues (Fig. 1a, Fig. 2b). This pattern distinctly differed from C. floridanum where female polymorulae develop in the thorax fully exposed in the haemocoel of the host, while male polymorulae usually develop in the abdomen surrounded by host fat body, which may reduce vulnerability of discovery by female soldiers (Grbic et al., 1992). Dissection of hosts during earlier instars indicated that one soldier larva was usually present at the end of the host’s first instar, and that small numbers of additional soldiers emerged during subsequent instars to yield the total number of soldiers present in fifth instar hosts. Like C. floridanum and other polyembryonic encyrtids, C. bakeri soldier larvae had an elongate, serpentine morphology (Fig. 2c). Measurements of the head capsule widths and lengths revealed no differences in the sizes of male and female soldiers (data not presented). Reproductive larvae in contrast had a rounded body form at emergence and thereafter greatly increased in size as they consumed the host (Fig. 2d).

Fig. 2.

Fig. 2

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Male and female broods produce similar numbers of soldiers and develop in the same location in hosts. (a) Mean number of soldier larvae per male and female brood (N=30 female and 30 male broods; t=0.43; P=0.67) in the host fifth instar. (b) Location of polymorulae in fifth instar hosts. Location was scored as being either connected to tracheal branch in the thorax or abdomen of the host. The proportion of polymorulae developing in the host thorax did not differ by brood sex (N=30 for each brood type; two-tailed binomial test, P=0.7). (c) Phase-contrast micrograph of a soldier caste larva. (d) Phase contrast micrograph of a newly hatched reproductive caste larva. Scale bar in (d)= 50 µm.

Soldiers exhibit higher aggression toward hetero- than intraspecific competitors

Given that C. bakeri oviposits only one egg per host and polymorulae of both sexes produce similar numbers of soldiers, we assessed the behavioural responses of soldiers toward potential competitors for host resources. This was accomplished by injecting a non-relative C. bakeri polymorula or a potential heterospecific competitor into hosts containing either a female or male brood. For a heterospecific competitor, we used C. floridanum because its polymorula is very similar in size and morphology to C. bakeri. Preliminary studies also indicated that CFSE-labeled C. floridanum polymorulae survived and remained viable for multiple days when injected into A. ipsilon larvae (data not presented). We thereafter dissected each host, sexed the resident brood, and scored soldier attack levels by recording: 1) the proportion of male and female broods that contained at least one soldier with fluorescent tracer in its gut, and 2) the mean proportion of soldiers per host with tracer in their guts. Both measures indicated that soldiers more strongly attacked heterospecific (C. floridanum) than intraspecific (C. bakeri) competitors (table 1). In contrast, we detected no differences in the responses of male and female soldiers to each competitor (table 1). We reasoned that if the low level of aggression toward non-relative C. bakeri accurately represents soldier behaviour under natural conditions, then hosts superparasitized by different wasps should result in coexistence of their progeny. Such a finding would also explain why Byers et al., (1993) collected some mixed-sex broods in the field even though our results showed that female wasps never oviposit more than one egg per host. We tested this by allowing a C. bakeri female to oviposit into a host followed one hour later by allowing a second female to oviposit. Examination of a subset of superparasitized host eggs by microscopy confirmed that each female laid a single egg. We then allowed the remaining superparasitized hosts to form mummies. A total of 46 broods produced adults of both sexes indicating that one mother laid a female egg while the other laid a male to create a primary sex ratio of 0.5. More importantly, the average secondary sex ratio of the adult offspring that emerged from these broods was 0.47 indicating a high level of coexistence. More than 100 broods produced in the experiment were single sex (all male or female but from two eggs) but we were unable to determine the parentage of these offspring due to a lack of molecular markers for this species. Taken together, these results indicate that C. bakeri soldiers infrequently attack intraspecific competitors and that progeny from different parents often coexist in superparasitized hosts.

Table 1.

Proportion of CFSE-labeled competitors attacked by C. bakeri soldiers in vivo.

Brood type (N) Competitor Proportion of hosts
with a labeled
soldier		 Mean proportion of
soldiers labeled
per host
Female (12) C. bakeri 0.25 0.032a, b
Male (20) C. bakeri 0.30 0.025a
Female (11) C. floridanum 0.83 0.156c
Male (20) C. floridanum 0.70 0.097b, c
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The proportion of hosts containing at least one labelled soldier varied significantly with brood type and competitor (Full model, G2=17.5; d.f.=3; P<0.0006). Calculation of liklihood ratios for each effect revealed attack rates toward non-relative C. bakeri were significantly lower than toward heterospecific C. floridanum (G2=16.7; d.f.=1; P<0.0001), but attack rates toward non-relatives (G2=17.5; d.f.=3; P<0.0006) and C. floridanum (G2=17.5; d.f.=3; P<0.0006) did not differ between female and male broods. Similar trends were obtained measuring the mean proportion of soldiers per brood that attacked the competitor. A higher proportion of male and female soldiers attack C. floridanum than C. bakeri (F3, 63=7.8; P<0.0002; arcsin transformed data; means followed by the same letter were not significantly different (Tukey-Kramer multiple comparison method).

Discussion

Given the difficulties in experimentally working with polyembryonic wasps, we were attracted to C. bakeri because the literature suggested it provides a distinct contrast to C. floridanum for assessing whether the primary function of soldiers differs among species and asking if developmental constraints affect soldier production. Our results show that C. bakeri females always oviposit one egg per oviposition event in the laboratory which results in single sex broods comprised only of full siblings. This finding is consistent with prior field data while also suggesting the small number of mixed-sex broods collected by Byers et al., (1993) reflects low-levels of superparasitism. Our developmental data further reveal no sexual asymmetries in where polymorulae develop in the host, the timing of soldier development, or in the total number of soldiers produced by C. bakeri, while our behavioural assays show that soldiers of both sexes exhibit higher attack levels toward a heterospecific competitor than non-relatives. When viewed in relation to prior studies with C. floridanum, these results indicate that adult oviposition behaviour, embryonic development, and soldier behaviour differ among species of the Copidosomatini. This also suggests key traits regulating caste formation and soldier function are phenotypically plastic and that differences in selection regimens experienced by members of the tribe have resulted in the evolution of different functions for the soldier caste. In the case of C. bakeri the primary function of soldiers appears to be defence against competitors for host resources while the primary function of soldiers in C. floridanum is resolution of sex ratio conflict.

Low-level aggression by C. bakeri toward intraspecific competitors likely reflects an evolutionary history in which adult wasps rarely if ever allocate multiple offspring to the same host. Risks of superparasitism may also be low due to specialization on moth species that oviposit in cryptic environments (soil and leaf litter) where location of host eggs may be difficult. Together, these conditions have potentially worked against C. bakeri soldiers developing a kin recognition system as acute as that of C. floridanum where soldiers commonly encounter close relatives (brothers) and also experience high risks of superparasitism from non-relatives (Giron & Strand, 2004; Giron et al., 2007a). As recently suggested (Segoli et al., 2009b), limited kin discrimination favours tolerance while also explaining the near equal sex ratios of the mixed broods we produced by superparasitizing hosts in the laboratory. Although C. floridanum exhibits no tolerance of non-relative competitors (Giron et al., 2007a), studies with other polyembyronic wasps and social species like gall-forming aphids have identified benefits from the coexistence of offspring in multiclonal groups (Ode & Strand, 1995; Abbott et al., 2001; White & Andow, 2008; Abbot, 2009; Bryden & Jansen, 2010). We think it unlikely, however, that tolerance and multiclonal group formation is actually functionally important to C. bakeri given the infrequency of mixed broods found in the field.

In contrast, the high levels of aggression C. bakeri and C. floridanum soldiers exhibit toward heterospecific competitors are consistent with both species experiencing local resource competition from other species of parasitoids. Survey data of agroecosystems indicate the hosts parasitized by C. bakeri and C. floridanum support large guilds of other natural enemies including multiple species of larval endoparasitoids (Capinera, 2001). Experimental studies indicate that some of these endoparasitoids consistently outcompete C. floridanum when soldier larvae are absent but rarely survive when soldiers are present (Harvey et al., 2000). A weakness of our heterospecific competition experiments with C. bakeri is that we had no other species in culture that are known natural enemies of its host A. epsilon. However, our results using C. floridanum as a competitor show at minimum that C. bakeri soldiers aggressively attack this foreign intruder and are capable of discriminating a congener from clonemates.

In light of our results, it remains curious that C. koehleri exhibits a female bias in soldier production, yet wasps rarely oviposit mixed-sex broods (Doutt, 1947; Kaesar et al., 2006). However, this species parasitizes a pest, Phthorimaea operculella (Lepidoptera: Gelechiidae), which can build to high densities in spatially restricted habitats (Rondon, 2010). C. koehleri females readily superparasitize hosts in the laboratory that have been parasitized by themselves or conspecifics resulting in high frequencies of mixed-sex broods. Behavioural studies also circumstantially suggest female soldiers recognize and kill non-clonemates including brothers that bias the sex ratio toward sisters (Kaesar et al., 2006; Segoli et al., 2009a). Thus, ecological conditions associated with high levels of mixed brood formation through superparasitism may underlie female-biased soldier production in this species.

Polyembryony has independently arisen at least four times in the Hymenoptera in different lineages of endoparasitoids, but a sterile soldier caste has only evolved in copidosomatine encyrtids (Strand, 2009). Following the evolution of polyembryony itself, one key step in the evolution of a soldier caste was the asymmetric parcelling of the germ line to embryos which provided a developmental mechanism for producing reproductive and sterile progeny from the same egg (Gordon & Strand, 2009). The other factor favouring specialization of a soldier caste must have been development under conditions where local competition for host resources frequently occurs via interactions among siblings as found for C. floridanum, or competition from hetero- and conspecific competitors as occurs for both C. bakeri and C. floridanum (Giron et al., 2004; Giron et al., 2007a; Strand, 2009; West & Gardner, 2009). Systematic studies suggest copidosomatine encyrtids form a monophyletic assemblage of wasps specialized to parasitize the eggs of primarily foliar and leaf-mining Lepidoptera (Zolnerowich, 1995; Gurrier & Noyes, 2005). Unfortunately, our current findings cannot distinguish between whether soldier larvae in the common ancestor of these insects evolved in response to competition from siblings or risks from other competitors. Phylogenetic data are too incomplete to identify the basal members of the tribe, but the limited comparative data available suggest more species of polyembryonic wasps produce single sex than mixed-sex broods in the field (Strand & Grbic, 1997; Strand, unpublished). Combined with the broader trend for foliar and leaf-mining Lepidoptera to serve as hosts for multiple species of parasitoids (see Godfray, 1994), we think it more likely that soldier larvae first evolved as a defence response such that both sexes similarly invest in the soldier caste in species like C. bakeri. Thereafter though, shifts in adult oviposition behaviour or other conditions that create sex ratio conflict resulted in sex-specific selection in species like C. floridanum that now exhibits female-biased development of soldier larvae and pronounced sex-specific differences in soldier behaviour (Giron et al., 2007a, b).

Acknowledgments

We greatly thank Y. Saeki for providing the insects used to found our C. bakeri culture. This work was supported by support from Ruth Kirschstein Fellowship Award from NIH to MSS and the Georgia Experiment Station to MRS.

References

  1. Abbot P. On the evolution of dispersal and altruism in aphids. Evolution. 2009;63:2687–2696. doi: 10.1111/j.1558-5646.2009.00744.x. [DOI] [PubMed] [Google Scholar]
  2. Abbot P, Withgott JH, Moran NA. Genetic conflict and conditional altruism in social aphid colonies. Proc. Natl. Acad. Sci. USA. 2001;98:12068–12071. doi: 10.1073/pnas.201212698. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Boomsma JJ. Lifetime monogamy and the evolution of eusociality. Phil. Trans. Roy. Soc. B. 2009;364:3191–3207. doi: 10.1098/rstb.2009.0101. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Bryden J, Jansen VAA. The impact of clonal mixing on the evolution of social behaviour in aphids. Proc. R. Soc. B. 2010;277:1651–1657. doi: 10.1098/rspb.2009.1876. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Byers JR, Yu DS, Jones JW. Parasitismof the army cutworm, Euxoa auxiliaries (Grt.) (Lepidoptera: Noctuidae), by Copidosoma bakeri (Howard) (Hymenoptera: Encyrtidae) and effect on crop damage. Can. Ent. 1993;125:329–335. [Google Scholar]
  6. Capinera JL. Handbook of Vegetable Pests. San Diego: Academic Press; 2001. [Google Scholar]
  7. Carroll MJ, Lampert EC, Berenbaum MR, Noyes JS, Ode PJ. New records of Copidosoma sosares (Walker) (Hymenoptera: Encyrtidae), a parasitoid of the parsnip webworm (Depressaria pastinacella (Duponchel)) (Lepidoptera: Elachistidae) in western North America. J. Kansas Entomol. Soc. 2007;80:309–318. [Google Scholar]
  8. Corley LS, White MT, Strand MR. Both endogenous and environmental factors affect embryo proliferation in the polyembryonic wasp Copidosoma floridanum. Evol. & Devp. 2005;7:115–121. doi: 10.1111/j.1525-142X.2005.05013.x. [DOI] [PubMed] [Google Scholar]
  9. Costa JT. The other social insect societies. Cambridge: Harvard Univ. Press; 2006. [Google Scholar]
  10. Cruz YP. A sterile defender morph in a polyembryonic hymenopterous parasite. Nature. 1981;294:446–447. [Google Scholar]
  11. Donnell D, Corley L, Chen G, Strand MR. Inheritance of germ cells mediates caste formation in a polyembryonic wasp. Proc. Natl. Acad. Sci USA. 2004;101:10095–10100. doi: 10.1073/pnas.0403625101. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Doutt RL. Polyembryony in Copidosoma koehleri Blanchard. Am. Nat. 1947;81:435–453. [Google Scholar]
  13. Gardner A, Hardy ICW, Taylor PD, West SA. Spiteful soldiers and sex ratio conflict in polyembryonic parasitoid wasps. Am. Nat. 2007;169:520–533. doi: 10.1086/512107. [DOI] [PubMed] [Google Scholar]
  14. Gardner A, Grafen A. Capturing the superorganism: a formal theory of group adaptation. J. Evol. Biol. 2009;22:659–671. doi: 10.1111/j.1420-9101.2008.01681.x. [DOI] [PubMed] [Google Scholar]
  15. Giron D, Dunn D, Hardy ICW, Strand MR. Aggression by polyembryonic wasp soldiers correlates with kinship but not resource competition. Nature. 2004;430:676–679. doi: 10.1038/nature02721. [DOI] [PubMed] [Google Scholar]
  16. Giron D, Strand MR. Host resistance and the evolution of kin recognition in polyembryonic wasps. Proc. Roy. Soc. B Biol. Letters. 2004;271:S395–S398. doi: 10.1098/rsbl.2004.0205. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Giron D, Ross KG, Strand MR. The presence of soldier larvae determines the outcome of competition in a polyembryonic wasp. J. Evol. Biol. 2007a;20:165–172. doi: 10.1111/j.1420-9101.2006.01212.x. [DOI] [PubMed] [Google Scholar]
  18. Giron D, Johnson JA, Harvey JA, Strand MR. Male soldier caste larvae are non-aggressive in the polyembryonic wasp Copidosoma floridanum. Biol. Letters. 2007b;3:431–434. doi: 10.1098/rsbl.2007.0199. (doi:10.1098/rsbl.2007.0199) [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Godfray HCJ. Parasitoids. Princeton Univ. Press; 1994. [Google Scholar]
  20. Gordon SD, Strand MR. The polyembryonic wasp Copidosoma floridanum produces two castes by differentially parceling the germ line during embryo proliferation. Dev. Genes & Evol. 2009;219:445–454. doi: 10.1007/s00427-009-0306-8. [DOI] [PubMed] [Google Scholar]
  21. Grbic M, Nagy LM, Strand MR. Development of polyembryonic insects: a major departure from typical embryogenesis. Dev. Genes. Evol. 1998;208:69–81. doi: 10.1007/s004270050156. [DOI] [PubMed] [Google Scholar]
  22. Grbic M, Ode PJ, Strand MR. Sibling rivalry and brood sex ratios in polyembryonic wasps. Nature. 1992;360:254–256. [Google Scholar]
  23. Guerrier E, Noyes J. Revision of the European species of Copidosoma Ratzeburg (Hymenoptera: Encyrtidae), parasitoids of caterpillars (Lepidoptera) Syst. Entomol. 2005;30:97–174. [Google Scholar]
  24. Hamilton WD. Extraordinary sex ratios. Science. 1967;156:477–488. doi: 10.1126/science.156.3774.477. [DOI] [PubMed] [Google Scholar]
  25. Hardy ICW, Ode PJ, Strand MR. Factors influencing sex ratios in polyembryonic wasps. Oecologia. 1993;93:343–348. doi: 10.1007/BF00317876. [DOI] [PubMed] [Google Scholar]
  26. Harvey JA, Corley LS, Strand MR. Competition induces adaptive shifts in caste ratios of a polyembryonic wasp. Nature. 2000;406:183–186. doi: 10.1038/35018074. [DOI] [PubMed] [Google Scholar]
  27. Hughes WOH, Oldroyd BP, Beekman M, Ratnieks FLW. Ancestral monogamy shows kin selection is the key to the evolution of eusociality. Science. 2008;320:1213–1216. doi: 10.1126/science.1156108. [DOI] [PubMed] [Google Scholar]
  28. Ivanova-Kasas OM. Polyembryony in insects. In: Counce S, Waddington CH, editors. Developmental Systems, Vol. 1. Insects. New York: Academic Press; 1972. pp. 243–271. [Google Scholar]
  29. Keasar T, Segoli M, Barak R, Steinberg S, Giron D, Strand MR, Bouskila A, Harari AR. Costs and consequences of superparasitism in the polyembryonic parasitoid Copidosoma koehleri. Ecol. Entomol. 2006;31:277–283. [Google Scholar]
  30. Mitchell A, Mitter C, Regier JC. Systematics and evolution of the cutworm moths (Lepidoptera: Noctuidae): evidence from two protein-coding nuclear genes. Syst. Entomol. 2006;31:21–46. [Google Scholar]
  31. Ode PJ, Strand MR. Progeny and sex allocation decisions of the polyembryonic wasp Copidosoma floridanum. J. Anim. Ecol. 1995;64:213–224. [Google Scholar]
  32. Rondon SI. The potato tuberworm: a literature review of its biology, ecology, and control. Am. J. Pot. Res. 2010;87:149–166. [Google Scholar]
  33. Saeki Y, Crowley PH, Fox CW, Potter DA. A sex-specific size-number tradeoff in clonal broods. Oikos. 2009;118:1552–1560. [Google Scholar]
  34. Segoli M, Harari AR, Bouskila A, Kaesar T. Brood size in a polyembryonic parasitoid wasp is affected by relatedness among competing larvae. Behav. Ecol. 2009a;20:1262–1267. [Google Scholar]
  35. Segoli M, Kaesar T, Harari AR, Bouskila A. Limited kin discrimination abilities mediate tolerance toward relatives in polyembryonic parasitoid wasps. Behav. Ecol. 2009b;20:761–767. [Google Scholar]
  36. Strand MR. Oviposition behavior and progeny allocation by the polyembryonic wasp Copidosoma floridanum. J. Insect Behav. 1989;2:355–369. [Google Scholar]
  37. Strand MR. Polyembryony. In: R Carde R, Resch V, editors. Encyclopedia of Insects. 2nd Edition. San Diego, CA: Academic Press; 2009. pp. 928–932. [Google Scholar]
  38. Strand MR, Ode PJ. Chromosome number of the polyembryonic parasitoid Copidosoma floridanum (Ashmead) (Hymenoptera: Encyrtidae) Ann. Entomol. Soc. Am. 1990;83:834–837. [Google Scholar]
  39. Strand MR, Grbic M. The development and evolution of polyembryonic insects. Curr. Topics Devp. Biol. 1997;35:121–159. doi: 10.1016/s0070-2153(08)60258-6. [DOI] [PubMed] [Google Scholar]
  40. Walter GH, Clarke AR. Unisexual broods and sex ratios in a polyembryonic encyrtid parasitoid (Copidosoma sp.: Hymenoptera) Oecologia. 1992;89:147–149. doi: 10.1007/BF00319027. [DOI] [PubMed] [Google Scholar]
  41. West SA, Gardner A. Altruism, spite, and greenbeards. Science. 2010;327:1341–1344. doi: 10.1126/science.1178332. [DOI] [PubMed] [Google Scholar]
  42. White JA, Andow DA. Benefits of self-superparasitism in a polyembryonic parasitoid. Biol. Control. 2008;46:133–139. doi: 10.1016/j.biocontrol.2008.04.005. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Yamamoto D, Henderson R, Corley LS, Iwabuchi K. Intrinsic, inter-specific competition between egg, egg-larval, and larval parasitoids of plusiine loopers. Ecol. Entomol. 2007;32:221–228. [Google Scholar]
  44. Zolnerowich G. PhD thesis. College Station, TX: Texas A&M University; 1995. Revision of the genus Copidosoma. [Google Scholar]
  45. Zhurov V, Terzin T, Grbic M. Early blastomere determines embryo proliferation and caste fate in a polyembryonic wasp. Nature. 2004;43:764–769. doi: 10.1038/nature03171. [DOI] [PubMed] [Google Scholar]

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