Changing paradigms in insect social evolution: insights from halictine and allodapine bees

Maternal manipulation of offspring size can trigger the evolution of eusociality in promiscuous species

Eusocial organisms typically live in colonies with one reproductive queen supported by thousands of sterile workers. It is widely believed that monogamous mating is a precondition for the evolution of eusociality. Here, we present a theoretical model that simulates a realistic scenario for the evolution of eusociality. In the model, mothers can evolve control over resource allocation to offspring, affecting offspring's body size. The offspring can evolve body-size-dependent dispersal, by which they disperse to breed or stay at the nest as helpers. We demonstrate that eusociality can evolve even if mothers are not strictly monogamous, provided that they can constrain their offspring's reproduction through manipulation. We also observe the evolution of social polymorphism with small individuals that help and larger individuals that disperse to breed. Our model unifies the traditional kin selection and maternal manipulation explanations for the evolution of eusociality and demonstrates that-contrary to current consensus belief-eusociality can evolve despite highly promiscuous mating.

The evolution of morphological castes under decoupled control

Eusociality, where units that previously reproduced independently function as one entity, is of major interest in evolutionary biology. Obligate eusociality is characterized by morphologically differentiated castes and reduced conflict. We explore conditions under which morphological castes may arise in the Hymenoptera and factors constraining their evolution. Control over offspring morphology and behaviour seems likely to be decoupled. Provisioners (queens and workers) can influence offspring morphology directly through the nutrition they provide, while adult offspring control their own behaviour. Provisioners may, however, influence worker behaviour indirectly if offspring modify their behaviour in response to their morphology. If manipulation underlies helping, we should not see helping evolve before specialized worker morphology, yet empirical observations suggest that behavioural castes precede morphological castes. We use evolutionary invasion analyses to show how the evolution of a morphologically differentiated worker caste depends on the prior presence of a behavioural caste: specialist worker morphology will be mismatched with behaviour unless some offspring already choose to work. A mother's certainty about her offspring's behaviour is also critical-less certainty results in greater mismatch. We show how baseline worker productivity can affect the likelihood of a morphological trait being favoured by natural selection. We then show how under a decoupled control scenario, morphologically differentiated castes should be less and less likely to be lost as they become more specialized. We also suggest that for eusociality to be evolutionarily irreversible, workers must be unable to functionally replace reproductives and reproductives must be unable to reproduce without help from workers.

Variation in season length and development time is sufficient to drive the emergence and coexistence of social and solitary behavioral strategies

Season length and its associated variables can influence the expression of social behaviors, including the occurrence of eusociality in insects. Eusociality can vary widely across environmental gradients, both within and between different species. Numerous theoretical models have been developed to examine the life history traits that underlie the emergence and maintenance of eusociality, yet the impact of seasonality on this process is largely uncharacterized. Here, we present a theoretical model that incorporates season length and offspring development time into a single, individual-focused model to examine how these factors can shape the costs and benefits of social living. We find that longer season lengths and faster brood development times are sufficient to favor the emergence and maintenance of a social strategy, while shorter seasons favor a solitary one. We also identify a range of season lengths where social and solitary strategies can coexist. Moreover, our theoretical predictions are well-matched to the natural history and behavior of two flexibly-eusocial bee species, suggesting our model can make realistic predictions about the evolution of different social strategies. Broadly, this work reveals the crucial role that environmental conditions can have in shaping social behavior and its evolution and underscores the need for further models that explicitly incorporate such variation to study evolutionary trajectories of eusociality.

Test of the negative feedback hypothesis of colony size sensing in social insects

Social insects can sense colony size-even without visual information in a dark environment. How they achieve this is yet largely unknown. We empirically tested a hypothesis on the proximate mechanism using ant colonies. In Diacamma colonies, the monogynous queen is known to increase the effort devoted to queen pheromone transmission behaviour (patrolling) as the colony grows, as if she perceives colony size. The negative feedback hypothesis assumes that, through repeated physical contact with workers, the queen monitors the physiological state (fertility) of workers and increases her patrolling effort when she encounters more fertile workers. Supporting this hypothesis, we found that the queen increased her patrolling effort in response to a higher ratio of fertile workers under the experimental condition of constant colony size. Furthermore, chemical analyses and bioassays suggested that cuticular hydrocarbons have queen pheromone activity and can mediate the observed queen-worker communication of fertility state. Such a self-organizing mechanism of sensing colony size may also operate in other social insects living in small colonies.

Developmental transcriptomes predict adult social behaviours in the socially flexible sweat bee, Lasioglossum baleicum

Natural variation can provide important insights into the genetic and environmental factors that shape social behaviour and its evolution. The sweat bee, Lasioglossum baleicum, is a socially flexible bee capable of producing both solitary and eusocial nests. We demonstrate that within a single nesting aggregation, soil temperatures are a strong predictor of the social structure of nests. Sites with warmer temperatures in the spring have a higher frequency of social nests than cooler sites, perhaps because warmer temperatures provide a longer reproductive window for those nests. To identify the molecular correlates of this behavioural variation, we generated a de novo genome assembly for L. baleicum, and we used transcriptomic profiling to compare adults and developing offspring from eusocial and solitary nests. We find that adult, reproductive females have similar expression profiles regardless of social structure in the nest, but that there are strong differences between reproductive females and workers from social nests. We also find substantial differences in the transcriptomic profiles of stage-matched pupae from warmer, social-biased sites compared to cooler, solitary-biased sites. These transcriptional differences are strongly predictive of adult reproductive state, suggesting that the developmental environment may set the stage for adult behaviours in L. baleicum. Together, our results help to characterize the molecular mechanisms shaping variation in social behaviour and highlight a potential role of environmental tuning during development as a factor shaping adult behaviour and physiology in this socially flexible bee.

Developmental transcriptomes predict adult social behaviors in the socially flexible sweat bee, Lasioglossum baleicum

Natural variation can provide important insights into the genetic and environmental factors that shape social behavior and its evolution. The sweat bee, Lasioglossum baleicum , is a socially flexible bee capable of producing both solitary and eusocial nests. We demonstrate that within a single nesting aggregation, soil temperatures are a strong predictor of the social structure of nests. Sites with warmer temperatures in the spring have a higher frequency of social nests than cooler sites, perhaps because warmer temperatures provide a longer reproductive window for those nests. To identify the molecular correlates of this behavioral variation, we generated a de novo genome assembly for L. baleicum , and we used transcriptomic profiling to compare adults and developing offspring from eusocial and solitary nests. We find that adult, reproductive females have similar expression profiles regardless of social structure in the nest, but that there are strong differences between reproductive females and workers from social nests. We also find substantial differences in the transcriptomic profiles of stage-matched pupae from warmer, social-biased sites compared to cooler, solitary-biased sites. These transcriptional differences are strongly predictive of adult reproductive state, suggesting that the developmental environment may set the stage for adult behaviors in L. baleicum . Together, our results help to characterize the molecular mechanisms shaping variation in social behavior and highlight a potential role of environmental tuning during development as a factor shaping adult behavior and physiology in this socially flexible bee.

Phylogeny of the carpenter bees (Apidae: Xylocopinae) highlights repeated evolution of sociality

Many groups of animals have evolved social behaviours in different forms, from intimate familial associations to the complex eusocial colonies of some insects. The subfamily Xylocopinae, including carpenter bees and their relatives, is a diverse clade exhibiting a wide range of social behaviours, from solitary to obligate eusociality with distinct morphological castes, making them ideal focal taxa in studying the evolution of sociality. We used ultraconserved element data to generate a broadly sampled phylogeny of the Xylocopinae, including several newly sequenced species. We then conducted ancestral state reconstructions on the evolutionary history of sociality in this group under multiple coding models. Our results indicate solitary origins for the Xylocopinae with multiple transitions to sociality across the tree and subsequent reversals to solitary life, demonstrating the lability and dynamic nature of social evolution in carpenter bees. Ultimately, this work clarifies the evolutionary history of the Xylocopinae, and expands our understanding of independent origins and gains and losses of social complexity.

Cryptic kin discrimination during communal lactation in mice favours cooperation between relatives

Breeding females can cooperate by rearing their offspring communally, sharing synergistic benefits of offspring care but risking exploitation by partners. In lactating mammals, communal rearing occurs mostly among close relatives. Inclusive fitness theory predicts enhanced cooperation between related partners and greater willingness to compensate for any partner under-investment, while females are less likely to bias investment towards own offspring. We use a dual isotopic tracer approach to track individual milk allocation when familiar pairs of sisters or unrelated house mice reared offspring communally. Closely related pairs show lower energy demand and pups experience better access to non-maternal milk. Lactational investment is more skewed between sister partners but females pay greater energetic costs per own offspring reared with an unrelated partner. The choice of close kin as cooperative partners is strongly favoured by these direct as well as indirect benefits, providing a driver to maintain female kin groups for communal breeding.

Bee pollination services and the burden of biogeography

Native bees augment pollination services in the Northern Hemisphere, especially cultivated apple crops, yet Southern Hemisphere contexts are poorly known. We observed the foraging behaviour of 69 354 invertebrate flower visitors in Australian orchards (two regions, 3 years) to assess the efficacy of pollination service (Peff). Native stingless bees and introduced honey bees were the most abundant visitors and most efficacious pollinators (Tetragonula Peff = 6.16; Apis Peff = 13.02), with Tetragonula becoming important service providers above 22°C. However, visits by tree-nesting stingless bees decreased with distance from native forest (less than 200 m) and their tropical/subtropical distribution precludes pollination service in other major Australian apple-producing regions. More broadly distributed native allodapine and halictine bees transferred the most pollen per-visit, but their low abundances reduce efficacies (Exoneura Peff = 0.03; Lasioglossum Peff = 0.06), resulting in a general dependence on honey bees. This reliance is a burden of biogeography, since key Northern Hemisphere pollinators of apple (Andrena, Apis, Bombus, Osmia) do not naturally occur in Australasia-where there is only 15% generic overlap with Central Asian bees sympatric with wild apple distributions (cf. Palaearctic 66% and Nearctic 46% generic overlaps). The historical biogeography of bees therefore drives an extreme dependence on one introduced species for apple pollination in Australia.

Convergent and complementary selection shaped gains and losses of eusociality in sweat bees

Sweat bees have repeatedly gained and lost eusociality, a transition from individual to group reproduction. Here we generate chromosome-length genome assemblies for 17 species and identify genomic signatures of evolutionary trade-offs associated with transitions between social and solitary living. Both young genes and regulatory regions show enrichment for these molecular patterns. We also identify loci that show evidence of complementary signals of positive and relaxed selection linked specifically to the convergent gains and losses of eusociality in sweat bees. This includes two pleiotropic proteins that bind and transport juvenile hormone (JH)-a key regulator of insect development and reproduction. We find that one of these proteins is primarily expressed in subperineurial glial cells that form the insect blood-brain barrier and that brain levels of JH vary by sociality. Our findings are consistent with a role of JH in modulating social behaviour and suggest that eusocial evolution was facilitated by alteration of the proteins that bind and transport JH, revealing how an ancestral developmental hormone may have been co-opted during one of life's major transitions. More broadly, our results highlight how evolutionary trade-offs have structured the molecular basis of eusociality in these bees and demonstrate how both directional selection and release from constraint can shape trait evolution.

Gene dynamics of haplodiploidy favor eusociality in the Hymenoptera

The problem of whether haplodiploidy is responsible for the frequent evolution of eusociality in the Hymenoptera remains unresolved. The little-known "protected invasion hypothesis" posits that because a male will transmit a new allele for alloparental care to all his daughters under haplodiploidy, such an allele has a higher probability of spreading to fixation under haplodiploidy than under diploidy. This mechanism is investigated using the mating system and lifecycles ancestral to eusocial lineages. It is shown that although haplodiploidy increases the probability of fixation of a new allele, the effect is cancelled by a higher probability of the allele arising in a diploid population. However, the same effect of male haploidy results in a 30% lower threshold amount of reproductive help by a worker necessary to favor eusociality if the sex ratio of dispersing first-brood offspring remains even. This occurs because when first-brood daughters become workers, the sex ratio of dispersing first-brood offspring becomes male-biased, selecting for an overall female-biased first-brood sex ratio. Through this mechanism, haplodiploidy may favor eusociality in the absence of a female-biased sex ratio in dispersing reproductive offspring. The gene-centric approach used here reveals the critical role of male haploidy in structuring the social group.

Sisters doing it for themselves: extensive reproductive plasticity in workers of a primitively eusocial bee

Abstract

Plasticity is a key trait when an individual’s role in the social environment, and hence its optimum phenotype, fluctuates unpredictably. Plasticity is especially important in primitively eusocial insects where small colony sizes and little morphological caste differentiation mean that individuals may find themselves switching from non-reproductive to reproductive roles. To understand the scope of this plasticity, workers of the primitively eusocial sweat bee Lasioglossum malachurum were experimentally promoted to the reproductive role (worker-queens) and their performance compared with foundress-queens. We focussed on how their developmental trajectory as workers influenced three key traits: group productivity, monopolisation of reproduction, and social control of foraging nest-mates. No significant difference was found between the number of offspring produced by worker-queens and foundress-queens. Genotyping of larvae showed that worker-queens monopolised reproduction in their nests to the same extent as foundress queens. However, non-reproductives foraged less and produced a smaller total offspring biomass when the reproductive was a promoted worker: offspring of worker-queens were all males, which are the cheaper sex to produce. Greater investment in each offspring as the number of foragers increased suggests a limit to both worker-queen and foundress-queen offspring production when a greater quantity of pollen arrives at the nest. The data presented here suggest a remarkable level of plasticity and represent one of the first quantitative studies of worker reproductive plasticity in a non-model primitively eusocial species.

Significance statement

The ability of workers to take on a reproductive role and produce offspring is expected to relate strongly to the size of their colony. Workers in species with smaller colony sizes should have greater reproductive potential to insure against the death of the queen. We quantified the reproductive plasticity of workers in small colonies of sweat bees by removing the queen and allowing the workers to control the reproductive output of the nest. A single worker then took on the reproductive role and hence prevented her fellow workers from producing offspring of their own. These worker-queens produced as many offspring as control queens, demonstrating remarkable worker plasticity in a primitively eusocial species.

Does effective population size affect rates of molecular evolution: Mitochondrial data for host/parasite species pairs in bees suggests not

Adaptive evolutionary theory argues that organisms with larger effective population size (N e) should have higher rates of adaptive evolution and therefore greater capacity to win evolutionary arm races. However, in some certain cases, species with much smaller N e may be able to survive besides their opponents for an extensive evolutionary time. Neutral theory predicts that accelerated rates of molecular evolution in organisms with exceedingly small N e are due to the effects of genetic drift and fixation of slightly deleterious mutations. We test this prediction in two obligate social parasite species and their respective host species from the bee tribe Allodapini. The parasites (genus Inquilina) have been locked into tight coevolutionary arm races with their exclusive hosts (genus Exoneura) for ~15 million years, even though Inquilina exhibit N e that are an order of magnitude smaller than their host. In this study, we compared rates of molecular evolution between host and parasite using nonsynonymous to synonymous substitution rate ratios (dN/dS) of eleven mitochondrial protein-coding genes sequenced from transcriptomes. Tests of selection on mitochondrial genes indicated no significant differences between host and parasite dN/dS, with evidence for purifying selection acting on all mitochondrial genes of host and parasite species. Several potential factors which could weaken the inverse relationship between N e and rate of molecular evolution are discussed.

Foraging gene expression patterns in queens, workers, and males in a eusocial insect

Reproductive division of labour is based on biased expression of complementary parental behaviours, brood production (egg-laying) by queens and brood care (in particular, brood-provisioning) by workers. In many social insect species, queens provision brood when establishing colonies at the beginning of a breeding season and reproductive division of labour begins with the emergence of workers. In many social insect species, the expression of foraging (for) mRNA is associated with the intensity of foraging behaviour and therefore brood-provisioning. However, only two studies have compared queen and worker for expression levels and neither accounted for transcript splice variation. In this study, we compare the expression level of the for-α transcript variant across four life stages of the queen caste, two behavioural groups of workers, and males of a eusocial sweat bee Lasioglossum laevissimum (Smith, 1853). Foundresses collected prior to the onset of the foraging season and males had the highest for-α expression levels. All active (post-hibernatory) queens and workers had similar for-α expression levels independent of behaviour. These results suggest that the for gene in L. laevissimum acts as a primer before foraging activity and that caste-specific expression patterns correlate with the timing of foraging activity in queens and workers.

Life History and the Transitions to Eusociality in the Hymenoptera

Although indirect selection through relatives (kin selection) can explain the evolution of effectively sterile offspring that act as helpers at the nest (eusociality) in the ants, bees, and stinging wasps (aculeate Hymenoptera), the genetic, ecological, and life history conditions that favor transitions to eusociality are poorly understood. In this study, ancestral state reconstruction on recently published phylogenies was used to identify the independent transitions to eusociality in each of the taxonomic families that exhibit eusociality. Semisociality, in which a single nest co-foundress monopolizes reproduction, often precedes eusociality outside the vespid wasps. Such a route to eusociality, which is consistent with groups consisting of a mother and her daughters (subsocial) at some stage and ancestral monogamy, is favored by the haplodiploid genetic sex determination of the Hymenoptera (diploid females and haploid males) and thus may explain why eusociality is common in the Hymenoptera. Ancestral states were also reconstructed for life history characters that have been implicated in the origins of eusociality. A loss of larval diapause during unfavorable seasons or conditions precedes, or coincides with, all but one transition to eusociality. This pattern is confirmed using phylogenetic tests of associations between state transition rates for sweat bees and apid bees. A loss of larval diapause may simply reflect the subsocial route to eusociality since subsociality is defined as females interacting with their adult daughters. A loss of larval diapause and a gain of subsociality may be associated with an extended breeding season that permits the production of at least two broods, which is necessary for helpers to evolve. Adult diapause may also lower the selective barrier to a first-brood daughter becoming a helper. Obligate eusociality meets the definition of a major evolutionary transition, and such transitions have occurred five times in the Hymenoptera.

Competition Drives Group Formation and Reduces Within Nest Relatedness in a Facultatively Social Carpenter Bee

Animals respond to competition among kin for critical breeding resources in two ways: avoidance of direct fitness costs via dispersal of siblings to breed separately, and formation of kin-based societies in which subordinates offset direct fitness costs of breeding competition via altruism and increased indirect fitness. In the facultatively social eastern carpenter bee, nests are a critical breeding resource in perpetually short supply, leading to strong competition among females. Observations of individually marked and genotyped females in conditions of high and low resource competition demonstrate that competition leads to resource sharing and group nesting. However, in contrast to almost all known animal societies, females avoid nesting with relatives, and disperse from their natal nests to join social groups of non-relatives. This is the first example of a structured insect society with cooperation nestmates, the majority of which are unrelated; thus cooperation is more likely based on selection for direct, rather than indirect fitness. By forming social groups of non-kin, females avoid the indirect fitness costs of kin competition among sisters, yet increase their chances of successful reproduction, and thus direct fitness, when forming colonies of non-relatives.

Spontaneous choices for insect-pollinated flower shapes by wild non-eusocial halictid bees

The majority of angiosperms require animal pollination for reproduction, and insects are the dominant group of animal pollinators. Bees are considered one of the most important and abundant insect pollinators. Research into bee behaviour and foraging decisions has typically centred on managed eusocial bee species, including Apis mellifera and Bombus terrestris. Non-eusocial bees are understudied with respect to foraging strategies and decision making, such as flower preferences. Understanding whether there are fundamental foraging strategies and preferences that are features of insect groups can provide key insights into the evolution of flower-pollinator co-evolution. In the current study, Lasioglossum (Chilalictus) lanarium and Lasioglossum (Parasphecodes) sp., two native Australian generalist halictid bees, were tested for flower shape preferences between native insect-pollinated and bird-pollinated flowers. Each bee was presented with achromatic images of either insect-pollinated or bird-pollinated flowers in a circular arena. Both native bee species demonstrated a significant preference for images of insect-pollinated flowers. These preferences are similar to those found in A. mellifera, suggesting that flower shape preference may be a deep-rooted evolutionary occurrence within bees. With growing interest in the sensory capabilities of non-eusocial bees as alternative pollinators, the current study also provides a valuable framework for further behavioural testing of such species.

Cuticular and Dufour's Gland Chemistry Reflect Reproductive and Social State in the Facultatively Eusocial Sweat Bee Megalopta genalis (Hymenoptera: Halictidae)

Queen pheromones evolved independently in multiple eusocial insect lineages, in which they mediate reproductive conflict by inhibiting worker ovarian development. Although fundamentally important for reproductive division of labor - the hallmark of eusociality - their evolutionary origins are enigmatic. Here, we analyze cuticular and Dufour's gland chemistries across alternative social and reproductive phenotypes in Megalopta genalis bees (tribe Augochlorini, family Halictidae) that facultatively express simple eusociality. Reproductive bees have distinct overall glandular and cuticular chemical phenotypes compared with non-reproductive workers. On the cuticle, a likely site of signal transmission, reproductives are enriched for certain alkenes, most linear alkanes, and are heavily enriched for all methyl-branched alkanes. Chemicals belonging to these compound classes are known to function as fertility signals in other eusocial insect taxa. Some macrocyclic lactones, compounds that serve as queen pheromones in the other eusocial halictid tribe (Halictini), are also enriched among reproductives relative to workers. The intra-population facultative eusociality of M. genalis permits direct comparisons between individuals expressing alternative reproductive phenotypes - females that reproduce alone (solitary reproductives) and social queens - to highlight traits in the latter that may be important mediators of eusociality. Compared with solitary reproductives, the cuticular chemistries of queens are more strongly differentiated from those of workers, and furthermore are especially enriched for methyl-branched alkanes. Determining the pheromonal function(s) and information content of the candidate signaling compounds we identify will help illuminate the early evolutionary history of queen pheromones, chemical signals central to the organization of insect eusocial behavior.

Sociality sculpts similar patterns of molecular evolution in two independently evolved lineages of eusocial bees

While it is well known that the genome can affect social behavior, recent models posit that social lifestyles can, in turn, influence genome evolution. Here, we perform the most phylogenetically comprehensive comparative analysis of 16 bee genomes to date: incorporating two published and four new carpenter bee genomes (Apidae: Xylocopinae) for a first-ever genomic comparison with a monophyletic clade containing solitary through advanced eusocial taxa. We find that eusocial lineages have undergone more gene family expansions, feature more signatures of positive selection, and have higher counts of taxonomically restricted genes than solitary and weakly social lineages. Transcriptomic data reveal that caste-affiliated genes are deeply-conserved; gene regulatory and functional elements are more closely tied to social phenotype than phylogenetic lineage; and regulatory complexity increases steadily with social complexity. Overall, our study provides robust empirical evidence that social evolution can act as a major and surprisingly consistent driver of macroevolutionary genomic change.

Split sex ratios and genetic relatedness in a primitively eusocial sweat bee

Abstract

In eusocial Hymenoptera, queens and their helper offspring should favour different sex investment ratios. Queens should prefer a 1:1 investment ratio, as they are equally related to offspring of both sexes (r = 0.5). In contrast, helpers should favour an investment ratio of 3:1 towards the production of female brood. This conflict arises because helpers are more closely related to full sisters (r = 0.75) than brothers (r = 0.25). However, helpers should invest relatively more in male brood if relatedness asymmetry within their colony is reduced. This can occur due to queen replacement after colony orphaning, multiple paternity and the presence of unrelated alien helpers. We analysed an unprecedentedly large number of colonies (n = 109) from a UK population of Lasioglossum malachurum, an obligate eusocial sweat bee, to tease apart the effects of these factors on colony-level investment ratios. We found that multiple paternity, unrelated alien helpers and colony orphaning were all common. Queen-right colonies invested relatively more in females than did orphaned colonies, producing a split sex ratio. However, investment ratios did not change due to multiple paternity or the presence of alien helpers, reducing inclusive fitness pay-offs for helpers. Queen control may also have been important: helpers rarely laid male eggs, and investment in female brood was lower when queens were large relative to their helpers. Genetic relatedness between helpers and the brood that they rear was 0.43 in one year and 0.37 in another year, suggesting that ecological benefits, as well as relatedness benefits, are necessary for the maintenance of helping behaviour.

Significance statement

How helping behaviour is maintained in eusocial species is a key topic in evolutionary biology. Colony-level sex investment ratio changes in response to relatedness asymmetries can dramatically influence inclusive fitness benefits for helpers in eusocial Hymenoptera. The extent to which helpers in primitively eusocial colonies can respond adaptively to different sources of variation in relatedness asymmetry is unclear. Using data from 109 colonies of the sweat bee Lasioglossum malachurum, we found that queen loss, but not multiple paternity or the presence of alien helpers, was correlated with colony sex investment ratios. Moreover, we quantified average helper-brood genetic relatedness to test whether it is higher than that predicted under solitary reproduction (r = 0.5). Values equal to and below r = 0.5 suggest that relatedness benefits alone cannot explain the maintenance of helping behaviour. Ecological benefits of group living and/or coercion must also contribute.

Vitellogenin expression corresponds with reproductive status and caste in a primitively eusocial bee

Vitellogenin (vg) expression is consistently associated with variation in insect phenotypes, particularly egg-laying. Primitively eusocial species, such as eusocial sweat bees, have behaviourally totipotent castes, in which each female is capable of high levels of ovarian development. Few studies have investigated vg expression patterns in primitively eusocial insects, and only one study has focused on a primitively eusocial bee. Here we use a primitively eusocial sweat bee, Lasioglossum laevissimum, and Real Time quantitative PCR (RT-qPCR) to investigate the relationship between vg expression, castes, and variation in phenotypes associated with castes differences. These assays showed that females with high ovarian development had the highest levels of vg expression, and that vg expression levels reflected the reproductive status of females first and caste second. This is in contrast to vg expression patterns observed in advanced eusocial queens and workers, which differ in vg expression based on caste and have caste-specific vg expression patterns. Furthermore, future queens (gynes) do not have ovarian development and had similar vg expression levels to early spring foundresses, which do have ovarian development, supporting Vg's function as a transporter of lipids and amino acids before diapause.

Macrocyclic Lactones Act as a Queen Pheromone in a Primitively Eusocial Sweat Bee

Eusociality is characterized by the reproductive division of labor between two castes: fertile queens and largely sterile workers. Queen pheromones are known to influence worker behavior and reproductive physiology and are therefore key components in regulating complex eusocial behavior [1]. Recent studies indicate that cuticular hydrocarbons (CHCs) act as queen pheromones in various eusocial hymenopteran species [2-8]. However, almost all species investigated to date are highly eusocial and do not include extant transitory stages from solitary to eusocial behavior [9]. Indeed, primitively eusocial species, which largely lack morphologically distinct castes, are thought to control worker reproduction through the physical aggression of the queen rather than via pheromones [10-12]. Halictid or sweat bees exhibit a high variability of eusociality including solitary and facultatively eusocial species [9, 13-16]. However, the mechanisms controlling worker reproduction in these transitory species are unknown. The results of a recent correlative study based on caste-specific chemical profiles in various halictid bees of different social levels have revealed an overproduction of macrocyclic lactones in queens compared with workers [17]. Using chemical analyses and behavioral experiments in which we simulated below-ground nests of the primitively eusocial sweat bee Lasioglossum malachurum, we identified a queen pheromone and found that macrocyclic lactones, not CHCs, influence worker behavior and decrease ovarian activation in this species. Our data suggest that the evolution of queen pheromones is more complex than previously inferred from highly eusocial species and shed new light on the complexity of the evolution of queen pheromones.

Operationalizing evolutionary transitions in individuality

Evolutionary transitions in individuality (hereafter, ETIs), such as the transition to multi-cellularity and the transition to social colonies, have been at the centre of evolutionary research, but only few attempts were made to systematically operationalize this concept. Here, we devise a set of four indicators intended to assess the change in complexity during ETIs: system size, inseparability, reproductive specialization and non-reproductive specialization. We then conduct a quantitative comparison across multiple taxa and ETIs. Our analysis reveals that inseparability has a crucial role in the process; it seems irreversible and may mark the point where a group of individuals becomes a new individual at a higher hierarchical level. Interestingly, we find that disparate groups demonstrate a similar pattern of progression along ETIs.

Re-thinking the social ladder approach for elucidating the evolution and molecular basis of insect societies

The evolution of large insect societies is a major evolutionary transition that occurred in the long-extinct ancestors of termites, ants, corbiculate bees, and vespid wasps. Researchers have long used 'social ladder thinking': assuming progressive stepwise phenotypic evolution and asserting that extant species with simple societies (e.g. some halictid bees) represent the ancestors of species with complex societies, and thus provide insight into general early steps of eusocial evolution. We discuss how this is inconsistent with data and modern evolutionary 'tree thinking'. Phylogenetic comparative methods with broad sampling provide the best means to make rigorous inferences about ancestral traits and evolutionary transitions that occurred within each lineage, and to determine whether consistent phenotypic and genomic changes occurred across independent lineages.

Genetic accommodation and the role of ancestral plasticity in the evolution of insect eusociality

For over a century, biologists have proposed a role for phenotypic plasticity in evolution, providing an avenue for adaptation in addition to 'mutation-first' models of evolutionary change. According to the various versions of this idea, the ability of organisms to respond adaptively to their environment through phenotypic plasticity may lead to novel phenotypes that can be screened by natural selection. If these initially environmentally induced phenotypes increase fitness, then genetic accommodation can lead to allele frequency change, influencing the expression of those phenotypes. Despite the long history of 'plasticity-first' models, the importance of genetic accommodation in shaping evolutionary change has remained controversial - it is neither fully embraced nor completely discarded by most evolutionary biologists. We suggest that the lack of acceptance of genetic accommodation in some cases is related to a lack of information on its molecular mechanisms. However, recent reports of epigenetic transgenerational inheritance now provide a plausible mechanism through which genetic accommodation may act, and we review this research here. We also discuss current evidence supporting a role for genetic accommodation in the evolution of eusociality in social insects, which have long been models for studying the influence of the environment on phenotypic variation, and may be particularly good models for testing hypotheses related to genetic accommodation. Finally, we introduce 'eusocial engineering', a method by which novel social phenotypes are first induced by environmental modification and then studied mechanistically to understand how environmentally induced plasticity may lead to heritable changes in social behavior. We believe the time is right to incorporate genetic accommodation into models of the evolution of complex traits, armed with new molecular tools and a better understanding of non-genetic heritable elements.

Recent Advances in Behavioral (Epi)Genetics in Eusocial Insects

Eusocial insects live in societies in which distinct family members serve specific roles in maintaining the colony and advancing the reproductive ability of a few select individuals. Given the genetic similarity of all colony members, the diversity of morphologies and behaviors is surprising. Social communication relies on pheromones and olfaction, as shown by mutants of orco, the universal odorant receptor coreceptor, and through electrophysiological analysis of neuronal responses to pheromones. Additionally, neurohormonal factors and epigenetic regulators play a key role in caste-specific behavior, such as foraging and caste switching. These studies start to allow an understanding of the molecular mechanisms underlying social behavior and provide a technological foundation for future studies of eusocial insects. In this review, we highlight recent findings in eusocial insects that advance our understanding of genetic and epigenetic regulations of social behavior and provide perspectives on future studies using cutting-edge technologies.

Evolution of Caste-Specific Chemical Profiles in Halictid Bees

Chemical communication is crucial for the maintenance of colony organization in eusocial insects and chemical signals are known to mediate important aspects of their social life, including the regulation of reproduction. Sociality is therefore hypothesized to be accompanied by an increase in the complexity of chemical communication. However, little is known about the evolution of odor signals at the transition from solitary living to eusociality. Halictid bees are especially suitable models to study this question as they exhibit considerable variability in social behavior. Here we investigated whether the dissimilarities in cuticle chemical signals in females of different castes and life stages reflect the level of social complexity across halictid bee species. Our hypothesis was that species with a higher social behavior ergo obligate eusocial species possess a more distinct chemical profile between castes or female life stages. We analyzed cuticular chemical profiles of foundresses, breeding females and workers of ancestrally solitary species, facultative and obligate eusocial halictid species. We also tested whether social complexity was associated with a higher investment in chemical signals. Our results revealed higher chemical dissimilarity between castes in obligate than in facultative eusocial species, especially regarding macrocyclic lactones, which were the single common compound class overproduced in queens compared with workers. Chemical dissimilarities were independent of differences in ovarian status in obligate eusocial species but were dependent on ovarian status in facultative eusocial species, which we discuss in an evolutionary framework.

Environmental barriers to sociality in an obligate eusocial sweat bee

Understanding the ecological and environmental contexts in which eusociality can evolve is fundamental to elucidating its evolutionary origins. A sufficiently long active season is postulated to have been a key factor facilitating the transition to eusociality. Many primitively eusocial species exhibit an annual life cycle, which is thought to preclude the expression of eusociality where the active season is too short to produce successive worker and reproductive broods. However, few studies have attempted to test this idea experimentally. We investigated environmental constraints on the expression of eusociality in the obligate primitively eusocial sweat bee Lasioglossum malachurum, by transplanting nest foundresses from the south to the far north of the United Kingdom, far beyond the natural range of L. malachurum. We show that transplanted bees can exhibit eusociality, but that the short length of the season and harsher environmental conditions could preclude its successful expression. In one year, when foundresses were transplanted only after provisioning first brood (B1) offspring, workers emerged in the north and provisioned a second brood (B2) of reproductives. In another year, when foundresses were transplanted prior to B1 being provisioned, they were just as likely to initiate nesting and provisioned just as many B1 cells as foundresses in the south. However, the life cycle was delayed by approximately 7 weeks and nests suffered 100% B1 mortality. Our results suggest that short season length together with poor weather conditions represent an environmental barrier to the evolution and expression of eusociality in sweat bees.

Limited social plasticity in the socially polymorphic sweat bee Lasioglossum calceatum

Abstract

Eusociality is characterised by a reproductive division of labour, where some individuals forgo direct reproduction to instead help raise kin. Socially polymorphic sweat bees are ideal models for addressing the mechanisms underlying the transition from solitary living to eusociality, because different individuals in the same species can express either eusocial or solitary behaviour. A key question is whether alternative social phenotypes represent environmentally induced plasticity or predominantly genetic differentiation between populations. In this paper, we focus on the sweat bee Lasioglossum calceatum, in which northern or high-altitude populations are solitary, whereas more southern or low-altitude populations are typically eusocial. To test whether social phenotype responds to local environmental cues, we transplanted adult females from a solitary, northern population, to a southern site where native bees are typically eusocial. Nearly all native nests were eusocial, with foundresses producing small first brood (B1) females that became workers. In contrast, nine out of ten nests initiated by transplanted bees were solitary, producing female offspring that were the same size as the foundress and entered directly into hibernation. Only one of these ten nests became eusocial. Social phenotype was unlikely to be related to temperature experienced by nest foundresses when provisioning B1 offspring, or by B1 emergence time, both previously implicated in social plasticity seen in two other socially polymorphic sweat bees. Our results suggest that social polymorphism in L. calceatum predominantly reflects genetic differentiation between populations, and that plasticity is in the process of being lost by bees in northern populations.

Significance statement

Phenotypic plasticity is thought to play a key role in the early stages of the transition from solitary to eusocial behaviour, but may then be lost if environmental conditions become less variable. Socially polymorphic sweat bees exhibit either solitary or eusocial behaviour in different geographic populations, depending on the length of the nesting season. We tested for plasticity in the socially polymorphic sweat bee Lasioglossum calceatum by transplanting nest foundresses from a northern, non-eusocial population to a southern, eusocial population. Plasticity would be detected if transplanted bees exhibited eusocial behaviour. We found that while native bees were eusocial, 90% of transplanted bees and their offspring did not exhibit traits associated with eusociality. Environmental variables such as time of offspring emergence or temperatures experienced by foundresses during provisioning could not explain these differences. Our results suggest that the ability of transplanted bees to express eusociality is being lost, and that social polymorphism predominantly reflects genetic differences between populations.

Electronic supplementary material

The online version of this article (10.1007/s00265-018-2475-9) contains supplementary material, which is available to authorized users.

Identification of 24 new microsatellite loci in the sweat bee Lasioglossum malachurum (Hymenoptera: Halictidae)

Objective

The objective here is to identify highly polymorphic microsatellite loci for the Palaearctic sweat bee Lasioglossum malachurum. Sweat bees (Hymenoptera: Halictidae) are widespread pollinators that exhibit an unusually large range of social behaviours from non-social, where each female nests alone, to eusocial, where a single queen reproduces while the other members of the colony help to rear her offspring. They thus represent excellent models for understanding social evolution.

Results

24 new microsatellite loci were successfully optimized. When amplified across 23–40 unrelated females, the number of alleles per locus ranged from 3 to 17 and the observed heterozygosities 0.45 to 0.95. Only one locus showed evidence of significant deviation from Hardy–Weinberg equilibrium. No evidence of linkage disequilibrium was found. These 24 loci will enable researchers to gain greater understanding of colony relationships within this species, an important model for the study of eusociality. Furthermore, 22 of the same loci were also successfully amplified in L. calceatum, suggesting that these loci may be useful for investigating the ecology and evolution of sweat bees in general.

The bee family Halictidae (Hymenoptera, Apoidea) from Central Asia collected by the Kyushu and Shimane Universities Expeditions

Background

Central Asia is one of the important centers of bee diversity in the Palearctic Region. However, there is insufficient information for many taxa in the central Asian bee fauna. The Kyushu and Shimane Universities (Japan) Expeditions to Kazakhstan, Kyrgyzstan, Uzbekistan, and Xinjiang Uyghur of China were conducted in the years 2000 to 2004 and 2012 to 2014.

New information

Eighty-eight species of the bee family Halictidae Thomson, 1869 are enumerated including new localities in central Asia. Halictus tibialis Walker, 1871, H. persephone Ebmer, 1976, Lasioglossum denislucum (Strand, 1909), L. griseolum (Morawitz, 1872), L. melanopus (Dalla Torre, 1896), L. nitidiusculum (Kirby, 1802), L. sexnotatulum (Nylander, 1852), L. subequestre (Blüthgen, 1931), L. sublaterale (Blüthgen, 1931), and L. zonulum (Smith, 1848) are recorded from central Asia for the first time. Thirty-two species are newly recorded from Kazakhstan, 19 spp. from Kyrgyzstan, 2 spp. from Uzbekistan, and 11 spp. from Xinjiang Uyghur of China. The genus Lasioglossum dominated the number of species and individuals in the collection. The halictid fauna mostly composed of western to central Asian elements in our surveyed area.

Nutritional Regulation of Phenotypic Plasticity in a Solitary Bee (Hymenoptera: Megachilidae)

Phenotypic plasticity involves adaptive responses to predictable environmental fluctuations and may promote evolutionary change. We studied the regulation of phenotypic plasticity in an important agricultural pollinator, the solitary alfalfa leafcutting bee (Megachile rotundata F.). Specifically, we investigated how larval nutrition affects M. rotundata diapause plasticity and how diapause plasticity affects adult female reproductive behavior. Field surveys and laboratory manipulations of aspects of larval diet demonstrated nutritional regulation of M. rotundata diapause plasticity. Manipulation of larval diet quality through the addition of royal jelly, the caste-determining substance of the honey bee Apis mellifera L., increased the probability of diapause in M. rotundata. We also found that larval nutrition and diapause status affected M. rotundata adult female reproductive behavior. Nutritional effects on larval diapause that also impact adult fitness have intriguing implications for the evolution of developmental plasticity in bees. In particular, as the solitary lifestyle of M. rotundata is considered to be the ancestral condition in bees, nutritionally regulated plasticity may have been an ancestral condition in all bees that facilitated the evolution of other forms of phenotypic plasticity, such as the castes of social bees.

Juvenile hormone, but not nutrition or social cues, affects reproductive maturation in solitary alkali bees (Nomia melanderi)

Eusocial insect colonies are defined by extreme variation in reproductive activity among castes, but the ancestral conditions from which this variation arose are unknown. Investigating the factors that contribute to variation in reproductive physiology among solitary insects that are closely related to social species can help to fill this gap. We experimentally tested the role of nutrition, juvenile hormone (JH) and social cues on reproductive maturation in solitary alkali bees (Halictidae: Nomia melanderi). We found that alkali bee females emerge from overwintering with small Dufour's glands and small ovaries, containing oocytes in the early stages of development. Oocyte maturation occurs rapidly, and is staggered between the two ovaries. Lab-reared females reached reproductive maturity without access to mates or nesting opportunities, and many had resorbed oocytes. Initial activation of these reproductive structures does not depend on pollen consumption, though dietary protein or lipids may be necessary for long-term reproductive activity. JH is likely to be a limiting factor in alkali bee reproductive activation, as females treated with JH were more likely to develop mature oocytes and Dufour's glands. Unlike for related social bees, the effects of JH were not suppressed by the presence of older, reproductive females. These results provide valuable insight into the factors that influence reproductive activity in an important native pollinator, and those that may have been particularly influential in the evolution of reproductive castes.

A unified model of Hymenopteran preadaptations that trigger the evolutionary transition to eusociality

Explaining the origin of eusociality, with strict division of labour between workers and reproductives, remains one of evolutionary biology's greatest challenges. Specific combinations of genetic, behavioural and demographic traits in Hymenoptera are thought to explain their relatively high frequency of eusociality, but quantitative models integrating such preadaptations are lacking. Here we use mathematical models to show that the joint evolution of helping behaviour and maternal sex ratio adjustment can synergistically trigger both a behavioural change from solitary to eusocial breeding, and a demographic change from a life cycle with two reproductive broods to a life cycle in which an unmated cohort of female workers precedes a final generation of dispersing reproductives. Specific suits of preadaptations are particularly favourable to the evolution of eusociality: lifetime monogamy, bivoltinism with male generation overlap, hibernation of mated females and haplodiploidy with maternal sex ratio adjustment. The joint effects of these preadaptations may explain the abundance of eusociality in the Hymenoptera and its virtual absence in other haplodiploid lineages.

Caste-biased gene expression in a facultatively eusocial bee suggests a role for genetic accommodation in the evolution of eusociality

Developmental plasticity may accelerate the evolution of phenotypic novelty through genetic accommodation, but studies of genetic accommodation often lack knowledge of the ancestral state to place selected traits in an evolutionary context. A promising approach for assessing genetic accommodation involves using a comparative framework to ask whether ancestral plasticity is related to the evolution of a particular trait. Bees are an excellent group for such comparisons because caste-based societies (eusociality) have evolved multiple times independently and extant species exhibit different modes of eusociality. We measured brain and abdominal gene expression in a facultatively eusocial bee, Megalopta genalis, and assessed whether plasticity in this species is functionally linked to eusocial traits in other bee lineages. Caste-biased abdominal genes in M. genalis overlapped significantly with caste-biased genes in obligately eusocial bees. Moreover, caste-biased genes in M. genalis overlapped significantly with genes shown to be rapidly evolving in multiple studies of 10 bee species, particularly for genes in the glycolysis pathway and other genes involved in metabolism. These results provide support for the idea that eusociality can evolve via genetic accommodation, with plasticity in facultatively eusocial species like M. genalis providing a substrate for selection during the evolution of caste in obligately eusocial lineages.