Presence of soldier larvae determines the outcome of competition in a polyembryonic wasp

Soldier‐producing polyembryonic waSPS are the only social animals that develop as parasites inside the bodies of other insects. Characterizing the kin composition of broods is central to understanding the evolution of the soldier caste in these unique social insects. Here we studied the role of soldiers in mediating the outcome of competition among clones of the polyembryonic wasp Copidosoma floridanum. Soldier‐producing female clones usually monopolized host resources, whereas soldierless male clones usually coexisted in hosts. Behavioural experiments further indicated that early‐emerging soldiers are specialized to combat intraspecific competitors and later‐emerging soldiers are specialized for defence against interspecific competitors. Taken together, our results point to intraspecific competition as a major selective force in the evolution of the soldier caste. Our data also present an evolutionary conundrum: given the benefit of soldiers, why are male clones functionally soldierless?     

Social organization in a flatworm: trematode parasites form soldier and reproductive castes

In some of the most complex animal societies, individuals exhibit a cooperative division of labour to form castes. The most pronounced types of caste formation involve reproductive and non-reproductive forms that are morphologically distinct. In colonies comprising separate or mobile individuals, this type of caste formation has been recognized only among the arthropods, sea anemones and mole-rats. Here, we document physical and behavioural caste formation in a flatworm. Trematode flatworm parasites undergo repeated clonal reproduction of ‘parthenitae’ within their molluscan hosts forming colonies. We present experimental and observational data demonstrating specialization among trematode parthenitae to form distinct soldier and reproductive castes. Soldiers do not reproduce, have relatively large mouthparts, and are much smaller and thinner than reproductives. Soldiers are also more active, and are disproportionally common in areas of the host where invasions occur. Further, only soldiers readily and consistently attack heterospecifics and conspecifics from other colonies. The division of labour described here for trematodes is strongly analogous to that characterizing other social systems with a soldier caste. The parallel caste formation in these systems, despite varying reproductive mode and taxonomic affiliation, indicates the general importance of ecological factors in influencing the evolution of social behaviour. Further, the ‘recognition of self’ and the defence of the infected host body from invading parasites are comparable to aspects of immune defence. A division of labour is probably widespread among trematodes and trematode species encompass considerable taxonomic, life history and environmental diversity. Trematodes should therefore provide new, fruitful systems to investigate the ecology and evolution of sociality.     

The Soldiers in Societies: Defense,Regulation, and Evolution

The presence of reproductively altruistic castes is one of the primary traits of the eusocial societies. Adaptation and regulation of the sterile caste, to a certain extent, drives the evolution of eusociality. Depending on adaptive functions of the first evolved sterile caste, eusocial societies can be categorized into the worker-first and soldier-first lineages, respectively. The former is marked by a worker caste as the first evolved altruistic caste, whose primary function is housekeeping, and the latter is highlighted by a sterile soldier caste as the first evolved altruistic caste, whose task is predominantly colony defense. The apparent functional differences between these two fundamentally important castes suggest worker-first and soldier-first eusociality are potentially driven by a suite of distinctively different factors. Current studies of eusocial evolution have been focused largely on the worker-first Hymenoptera, whereas understanding of soldier-first lineages including termites, eusocial aphids, gall-dwelling thrips, and snapping shrimp, is greatly lacking. In this review, we summarize the current state of knowledge on biology, morphology, adaptive functions, and caste regulation of the soldier caste. In addition, we discuss the biological, ecological and genetic factors that might contribute to the evolution of distinct caste systems within eusocial lineages.     

Differential gene expression and phenotypic plasticity in behavioural castes of the primitively eusocial wasp, Polistes canadensis

Understanding how a single genome can produce a variety of different phenotypes is of fundamental importance in evolutionary and developmental biology. One of the most striking examples of phenotypic plasticity is the female caste system found in eusocial insects, where variation in reproductive (queens) and non-reproductive (workers) phenotypes results in a broad spectrum of caste types, ranging from behavioural through to morphological castes. Recent advances in genomic techniques allow novel comparisons on the nature of caste phenotypes to be made at the level of the genes in organisms for which there is little genome information, facilitating new approaches in studying social evolution and behaviour. Using the paper wasp Polistes canadensis as a model system, we investigated for the first time how behavioural castes in primitively eusocial insect societies are associated with differential expression of shared genes. We found that queens and newly emerged females express gene expression patterns that are distinct from each other whilst workers generally expressed intermediate patterns, as predicted by Polistes biology. We compared caste-associated genes in P. canadensis with those expressed in adult queens and workers of more advanced eusocial societies, which represent four independent origins of eusociality. Nine genes were conserved across the four taxa, although their patterns of expression and putative functions varied. Thus, we identify several genes that are putatively of evolutionary importance in the molecular biology that underlies a number of caste systems of independent evolutionary origin.     

Both endogenous and environmental factors affect embryo proliferation in the polyembryonic wasp Copidosoma floridanum

Copidosoma floridanum is a polyembryonic, parasitic wasp of the moth Trichoplusia ni. Following oviposition into a host, the C. floridanum egg initially undergoes complete (holoblastic) cleavage to form a single morula stage embryo. This embryo then undergoes a proliferation phase in which multiple, secondary morulae develop. C. floridanum has also evolved a caste system whereby some secondary morulae develop into soldier larvae whose function is defense whereas others develop into reproductive larvae that become adult wasps. In the current study, we conducted manipulative and candidate gene studies to identify factors affecting the proliferation phase of C. floridanum development. Transplantation of morulae of different ages into different host stages indicated that both embryo age and host environment affected the total number of offspring produced per morula. Morula age and brood size also significantly affected whether offspring of one or both castes were produced in a brood. In contrast, the host environment did not significantly affect caste formation. A putative homolog of the gene hedgehog (Cf-hh) was partially cloned from C. floridanum. In situ hybridization studies indicated that Cf-hh was expressed in secondary morulae during the proliferation phase of development, suggesting a possible role for the Hh signaling pathway in the evolution of polyembryony.     

Selection on defensive traits in a sterile caste – caste evolution: a mechanism to overcome life‐history trade‐offs?

SUMMARY During development and evolution individuals generally face a trade-off between the development of weapons and gonads. In termites, characterized by reproductive division of labor, a caste evolved—the soldiers—which is completely sterile and which might be released from developmental trade-offs between weapons and testes. These soldiers are exclusively dedicated to defense. First, we investigated whether defensive traits are under selection in sterile termite soldiers using allometric analyses. In soldiers of the genus Cryptotermes phragmotic traits such as a sculptured and foreshortened head evolve rapidly but were also lost twice. Second, we compared the scaling relationships of these weapons with those in solitary insects facing a trade-off between weapons and gonads. Defensive traits consistently had lower slopes than nondefensive traits which supports the existence of stabilizing selection on soldier phragmotic traits in order to plug galleries. Moreover, soldier head widths were colony specific and correlated with the minimum gallery diameter of a colony. This can proximately be explained by soldiers developing from different instars. The scaling relationships of these termite soldiers contrast strikingly with those of weapons of solitary insects, which are generally exaggerated (i.e., overscaling) male traits. These differences may provide important insights into trait evolution. Trade-offs constraining the development of individuals may have been uncoupled in termites by evolving different castes, each specialized for one function. When individuals in social insect are "released" from developmental constraints through the evolution of castes, this certainly contributed to the ecological and evolutionary success of social insects.     

Developmental regulation of caste-specific characters in social-insect polyphenism

Phenotypes of organisms are not determined completely genetically, but vary according to environmental factors (phenotypic plasticity). Some organisms express several discrete adaptive phenotypes (polyphenism). Social insects possess a few types of individuals (castes) in their colonies, to which specific tasks are allocated. Here, I review studies on caste polyphenism in ants and termites, in terms of the developmental mechanisms of caste-specific characters, such as alate wings and soldier mandibles. In ants, the developmental fate of caste is probably determined by the pattern-formation genes in the early stage of postembryonic development, but apoptotic degeneration occurs in the wing primordia of future workers. As apoptotic wing degeneration has been observed in two phylogenetically distant groups of ants, this phenomenon is suggested to be conserved in many ant species. On the other hand, all termite species possess distinct sterile soldiers with specific morphologies suitable for defense. Recent studies using molecular techniques isolated genes related to soldier differentiation and analyzed the expression profiles of those genes in order to understand the mechanism of caste differentiation and the link between molecular and social evolution. In this review, I focus on these studies, in terms of the alteration of body plan in response to environmental signals, and discuss the evolutionary process of the interaction between ontogeny and environment.     

Male soldier caste larvae are non-aggressive in the polyembryonic wasp Copidosoma floridanum

Polyembryonic wasps are the only parasitoids in which sociality has evolved. Theory implicates both competition and sex ratio manipulation in the evolution of a sterile soldier caste. However, investment in soldiers by males and females is predicted to differ depending upon how offspring are allocated to hosts and the mating system. Here, we compared male and female soldiers in the polyembryonic wasp Copidosoma floridanum. We found that male and female soldiers are morphologically identical. Unlike females, however, male soldiers were non-aggressive towards all competitors. We discuss these results in relation to theory and polyembryonic wasp biology.     

Disruptive selection as a driver of evolutionary branching and caste evolution in social insects

Theory suggests that evolutionary branching via disruptive selection may be a relatively common and powerful force driving phenotypic divergence. Here, we extend this theory to social insects, which have novel social axes of phenotypic diversification. Our model, built around turtle ant (Cephalotes) biology, is used to explore whether disruptive selection can drive the evolutionary branching of divergent colony phenotypes that include a novel soldier caste. Soldier evolution is a recurrent theme in social insect diversification that is exemplified in the turtle ants. We show that phenotypic mutants can gain competitive advantages that induce disruptive selection and subsequent branching. A soldier caste does not generally appear before branching, but can evolve from subsequent competition. The soldier caste then evolves in association with specialized resource preferences that maximize defensive performance. Overall, our model indicates that resource specialization may occur in the absence of morphological specialization, but that when morphological specialization evolves, it is always in association with resource specialization. This evolutionary coupling of ecological and morphological specialization is consistent with recent empirical evidence, but contrary to predictions of classical caste theory. Our model provides a new theoretical understanding of the ecology of caste evolution that explicitly considers the process of adaptive phenotypic divergence and diversification.     

Colony size,social complexity and reproductive conflict in social insects

The broad limits of mature colony size in social insect species are likely to be set by ecological factors. However, any change in colony size has a number of important social consequences. The most fundamental is a change in the expected reproductive potential of workers. If colony size rises, workers experience a fall in their chances of becoming replacement reproductives and, it is shown, increasing selection for mutual inhibition of one another's reproduction (worker policing). As workers’ reproductive potential falls, the degree of dimorphism between reproductive and worker castes (morphological skew) can rise. This helps explain why small societies have low morphological skew and tend to be simple in organization, whereas large societies have high morphological skew and tend to be complex. The social consequences of change in colony size may also alter colony size itself in a process of positive feedback. For these reasons, small societies should be characterized by intense, direct conflict over reproduction and caste determination. By contrast, conflict in large societies should predominantly be over brood composition, and members of these societies should be relatively compliant to manipulation of their caste. Colony size therefore deserves fuller recognition as a key determinant, along with kin structure, of social complexity, the reproductive potential of helpers, the degree of caste differentiation, and the nature of within-group conflict.     

The evolution of eusociality in allodapine bees: workers began by waiting

Understanding how sterile worker castes in social insects first evolved is one of the supreme puzzles in social evolution. Here, we show that in the bee tribe Allodapini, the earliest societies did not entail a foraging worker caste, but instead comprised females sharing a nest with supersedure of dominance. Subordinates delayed foraging until they became reproductively active, whereupon they provided food for their own brood as well as for those of previously dominant females. The earliest allodapine societies are, therefore, not consistent with an 'evo-devo' paradigm, where decoupling of foraging and reproductive tasks is proposed as a key early step in social evolution. Important features of these ancestral societies were insurance benefits for dominants, headstart benefits for subordinates and direct reproduction for both. The two lineages where morphologically distinct foraging worker castes evolved both occur in ecosystems with severe constraints on independent nesting and where brood rearing periods are very seasonally restricted. These conditions would have strongly curtailed dispersal options and increased the likelihood that dominance supersedure occurred after brood rearing opportunities were largely degraded. The origins of foraging castes, therefore, represented a shift towards assured fitness gains by subordinates, mediated by the dual constraints of social hierarchies and environmental harshness.     

Life history and development--a framework for understanding developmental plasticity in lower termites

Termites (Isoptera) are the phylogenetically oldest social insects, but in scientific research they have always stood in the shadow of the social Hymenoptera. Both groups of social insects evolved complex societies independently and hence, their different ancestry provided them with different life-history preadaptations for social evolution. Termites, the 'social cockroaches', have a hemimetabolous mode of development and both sexes are diploid, while the social Hymenoptera belong to the holometabolous insects and have a haplodiploid mode of sex determination. Despite this apparent disparity it is interesting to ask whether termites and social Hymenoptera share common principles in their individual and social ontogenies and how these are related to the evolution of their respective social life histories. Such a comparison has, however, been much hampered by the developmental complexity of the termite caste system, as well as by an idiosyncratic terminology, which makes it difficult for non-termitologists to access the literature.
Here, we provide a conceptual guide to termite terminology based on the highly flexible caste system of the "lower termites". We summarise what is known about ultimate causes and underlying proximate mechanisms in the evolution and maintenance of termite sociality, and we try to embed the results and their discussion into general evolutionary theory and developmental biology. Finally, we speculate about fundamental factors that might have facilitated the unique evolution of complex societies in a diploid hemimetabolous insect taxon. This review also aims at a better integration of termites into general discussions on evolutionary and developmental biology, and it shows that the ecology of termites and their astounding phenotypic plasticity have a large yet still little explored potential to provide insights into elementary evo-devo questions.     

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

Polyembryonic encyrtid wasps are parasitoids that have evolved a clonal form of embryogenesis and a caste system where some progeny become reproducing wasps whereas others develop into a sterile soldier caste. Theory based on the biology of Copidosoma floridanum predicts that the primary role of soldier larvae is to mediate conflict over sex ratio, which also favours female‐biased soldier production. Other data, however, suggest that 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 Copidosoma bakeri, a species that produces clutch sizes similar to C. floridanum yet rarely produces broods associated with sex ratio conflict. Our results indicate that the oviposition behaviour of adults, development of progeny and function of soldier larvae differ 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 that the primary function of the soldier caste in some species is defence of host resources from competitors whereas in others it is the resolution of sex ratio conflict.     

The effects of kin selection on rates of molecular evolution in social insects

The evolution of sociality represented a major transition point in biological history. The most advanced societies, such as those displayed by social insects, consist of reproductive and nonreproductive castes. The caste system fundamentally affects the way natural selection operates. Specifically, selection acts directly on reproductive castes, such as queens, but only indirectly through the process of kin selection on nonreproductive castes, such as workers. In this study, we present theoretical analyses to determine the rate of substitution at loci expressed exclusively in the queen or worker castes. We show that the rate of substitution is the same for queen- and worker-selected loci when the queen is singly mated. In contrast, when a queen is multiply mated, queen-selected loci show higher rates of substitution for adaptive alleles and lower rates of substitution for deleterious alleles than worker-selected loci. We compare our theoretical expectations to previously obtained genomic data from the honeybee, Apis mellifera, where queens mate multiply and the fire ant, Solenopsis invicta, where queens mate singly and find that rates of evolution of queen- and worker-selected loci are consistent with our predictions. Overall, our research tests theoretical expectations using empirically obtained genomic data to better understand the evolution of advanced societies.     

Reproductive skew and the origin of sterile castes

Reproductive skew theory has not heretofore formally addressed one of the most important questions in evolutionary biology: How can whole-life sterile castes evolve? We construct a transactional skew model investigating under what conditions a subordinate in a multimember group is favored to develop into a morphologically specialized worker caste. Our model demonstrates that, contrary to former expectations, the ecological and genetic conditions favoring caste differentiation are far more restrictive than those favoring high skew. Caste differentiation cannot be selected in saturated, symmetrical relatedness groups unless the genetic relatedness among group members is extremely high. In contrast, it can be selected in the saturated, asymmetrical relatedness (parent-offspring) groups with complete skew. If we also consider the future reproduction of subordinates, caste differentiation is possible only after the group size reaches a certain critical point. Most importantly, caste differentiation in a parent-offspring group increases its saturated group size. The positive feedback between group size and the degree of caste differentiation can continue in principle until completely sterile worker castes emerge. Thus, at least in the case of parent-offspring groups, group size but not the degree of reproductive skew may be a better index of the level of social complexity. A scheme for the evolution of sterile worker castes that integrates the role of group size into the framework of reproductive skew theory is proposed.     

Building a new research framework for social evolution: intralocus caste antagonism

The breeding and non‐breeding ‘castes’ of eusocial insects provide a striking example of role‐specific selection, where each caste maximises fitness through different morphological, behavioural and physiological trait values. Typically, queens are long‐lived egg‐layers, while workers are short‐lived, largely sterile foragers. Remarkably, the two castes are nevertheless produced by the same genome. The existence of inter‐caste genetic correlations is a neglected consequence of this shared genome, potentially hindering the evolution of caste dimorphism: alleles that increase the productivity of queens may decrease the productivity of workers and vice versa, such that each caste is prevented from reaching optimal trait values. A likely consequence of this ‘intralocus caste antagonism’ should be the maintenance of genetic variation for fitness and maladaptation within castes (termed ‘caste load’), analogous to the result of intralocus sexual antagonism. The aim of this review is to create a research framework for understanding caste antagonism, drawing in part upon conceptual similarities with sexual antagonism. By reviewing both the social insect and sexual antagonism literature, we highlight the current empirical evidence for caste antagonism, discuss social systems of interest, how antagonism might be resolved, and challenges for future research. We also introduce the idea that sexual and caste antagonism could interact, creating a three‐way antagonism over gene expression. This includes unpacking the implications of haplodiploidy for the outcome of this complex interaction.     

Evolutionary constraints on polyembryony in parasitic wasps: a simulation model

Polyembryony involves the production of several genetically identical progeny from a single egg through clonal division. Although polyembryonic development allows highly efficient reproduction, especially in some parasitoid wasps, it is far less common than monoembryony (development of one embryo per egg). To understand what might constrain the evolutionary success of polyembryony in parasitoids, we developed Monte Carlo models that simulate the competition between polyembryonic females and their monoembryonic counterparts. We investigated which simulated life‐history traits of the females allow the monoembryonic mode of development to succeed. Published empirical studies were surveyed to explore whether these traits indeed differ between polyembryonic parasitoids and related monoembryonic species. The simulations predict an advantage to monoembryony in parasitoids whose reproduction is limited by host availability rather than by egg supply, and that parasitize small‐bodied hosts. Comparative data on the parasitoid families Encyrtidae and (to a lesser extent) Braconidae, but not the data from Platygastridae, circumstantially support these predictions. The model also predicts monoembryony to outcompete polyembryony when: 1) hosts vary considerably in quality, 2) polyembryonic development carries high physiological costs, and 3) monoembryonic females make optimal clutch size decisions upon attacking hosts. These multiple constraints may account for the rarity of polyembryony among parasitoid species.     

A theory of natural selection incorporating interaction among individuate. X. Use of groups consisting of a mating pair together with haploid and diploid caste members

This paper and the previous member of the series, deal with genetical mechanisms responsible for the evolution of eusociality (a level of social organization that includes differentiated sterile castes) among the “social” insects. Eusociality has evolved in a number of different species. Two different types of genetic systems are represented among these species: diplodiploidy (both sexes diploid) and haplodiploidy (haploid males and diploid females). The previous paper examined the evolution of a sterile caste system in the context of diplodiploidy, and the present paper considers the evolution of eusociality in the context of haplodiploidy.The present study demonstrates that selection operating with regard to random groups within the haplodiploid inheritance system cannot result in the evolution of a sterile caste system. Thus haplodiploidy, in itself, is not sufficient for the evolution of eusociality. However, if the sterile caste members are related to the reproductive members of the group, the appropriate caste associate gene effects are included in the function determining gene frequency change (i.e. Δp_i), and therefore, eusociality can evolve. This is true for both haploid and diploid castes.In comparing the two modes of inheritance, it is demonstrated that haplodiploidy provides up to 37·5% increased selection efficiency relative to diplodiploidy in evolving a social caste system in the absence of inbreeding.     

The adaptive significance of inquiline parasite workers

Social parasites exploit the socially managed resources of their host's society. Inquiline social parasites are dependent on their host throughout their life cycle, and so many of the traits inherited from their free-living ancestor are removed by natural selection. One trait that is commonly lost is the worker caste, the functions of which are adequately fulfilled by host workers. The few inquiline parasites that have retained a worker caste are thought to be at a transitional stage in the evolution of social parasitism, and their worker castes are considered vestigial and non-adaptive. However, this idea has not been tested. Furthermore, whether inquiline workers have an adaptive role outside the usual worker repertoire of foraging, brood care and colony maintenance has not been examined. In this paper, we present data that suggest that workers of the inquiline ant Acromyrmex insinuator play a vital role in ensuring the parasite's fitness. We show that the presence of these parasite workers has a positive effect on the production of parasite sexuals and a negative effect on the production of host sexuals. This suggests that inquiline workers play a vital role in suppressing host queen reproduction, thus promoting the rearing of parasite sexuals. To our knowledge, these are the first experiments on inquiline workers and the first to provide evidence that inquiline workers have an adaptive role.