Reproductive constraints, direct fitness and indirect fitness benefits explain helping behaviour in the primitively eusocial wasp, Polistes canadensis

Nepotism mediates enforced cooperation in asymmetric negotiations

In cooperative societies, group members typically exchange different commodities among each other, which involves an incessant negotiation process. How is the conflict of fitness interests resolved in this continual bargaining process between unequal partners, so that maintaining the cooperative interaction is the best option for all parties involved? Theory predicts that relatedness between group members may alleviate the conflict of fitness interests, thereby promoting the evolution of cooperation. To evaluate the relative importance of relatedness and direct fitness effects in the negotiation process, we experimentally manipulated both the relatedness and mutual behavioral responses of dominant breeders and subordinate helpers in the cooperatively breeding cichlid fish Neolamprologus pulcher. Results show that coercion by breeders is crucial for the performance of alloparental egg care by helpers, but that kinship significantly decreases the need for coercion as predicted by theory. This illustrates the relative importance of kinship and enforcement in the bargaining process.

The evolution of cooperative breeding by direct and indirect fitness effects

The evolution of cooperative breeding has been traditionally attributed to the effect of kin selection. While there is increasing empirical evidence that direct fitness benefits are relevant, the relative importance of alternative selection mechanisms is largely obscure. Here, we model the coevolution of the cornerstones of cooperative breeding, delayed dispersal, and alloparental care, across different ecological scenarios while allowing individuals to adjust philopatry and helping levels. Our results suggest that (i) direct fitness benefits from grouping are the main driver for the evolution of philopatry; (ii) kin selection is mainly responsible for the emergence of alloparental care, but group augmentation can be a sufficient promoter in harsh environments; (iii) the coevolution of philopatry and alloparental care is subject to positive feedback; and (iv) age-dependent dispersal is triggered by both group benefits and relatedness. Model predictions are supported by empirical data and provide good opportunities for comparative analyses and experimental tests of causality.

Multi-level social organization and nest-drifting behaviour in a eusocial insect

Stable social groups usually consist of families. However, recent studies have revealed higher level social structure, with interactions between family groups across different levels of social organization in multiple species. The explanations for why this apparently paradoxical behaviour arises appear to be varied and remain untested. Here, we use automated radio-tagging data from over 1000 wasps from 93 nests and social network analyses of over 30 000 nest visitation records to describe and explain interactions across levels of social organization in the eusocial paper wasp Polistes canadensis. We detected three levels of social organization (nest, aggregation and community) which exchange 'drifter' individuals within and between levels. The highest level (community) may be influenced by the patchiness of high-quality nesting habitats in which these insects exist. Networks of drifter movements were explained by the distance between nests, the group size of donor nests and the worker-to-brood ratios on donor and recipient nests. These findings provide some explanation for the multi-level social interactions, which may otherwise seem paradoxical. Fitness benefits across multiple levels of social organization should be considered when trying to understand animal societies.

Diminishing returns drive altruists to help extended family

Altruism between close relatives can be easily explained. However, paradoxes arise when organisms divert altruism towards more distantly related recipients. In some social insects, workers drift extensively between colonies and help raise less related foreign brood, seemingly reducing inclusive fitness. Since being highlighted by W. D. Hamilton, three hypotheses (bet hedging, indirect reciprocity and diminishing returns to cooperation) have been proposed for this surprising behaviour. Here, using inclusive fitness theory, we show that bet hedging and indirect reciprocity could only drive cooperative drifting under improbable conditions. However, diminishing returns to cooperation create a simple context in which sharing workers is adaptive. Using a longitudinal dataset comprising over a quarter of a million nest cell observations, we quantify cooperative payoffs in the Neotropical wasp Polistes canadensis, for which drifting occurs at high levels. As the worker-to-brood ratio rises in a worker's home colony, the predicted marginal benefit of a worker for expected colony productivity diminishes. Helping related colonies can allow effort to be focused on related brood that are more in need of care. Finally, we use simulations to show that cooperative drifting evolves under diminishing returns when dispersal is local, allowing altruists to focus their efforts on related recipients. Our results indicate the power of nonlinear fitness effects to shape social organization, and suggest that models of eusocial evolution should be extended to include neglected social interactions within colony networks.

Elder Barn Owl Nestlings Flexibly Redistribute Parental Food according to Siblings' Need or in Return for Allopreening

Kin selection and reciprocation of biological services are distinct theories invoked to explain the origin and evolutionary maintenance of altruistic and cooperative behaviors. Although these behaviors are not considered to be mutually exclusive, the cost-benefit balance of behaving altruistically or cooperating reciprocally and the conditions promoting a switch between such different strategies have rarely been tested. Here, we examine the association between allofeeding, allopreening, and vocal solicitations in wild barn owl (Tyto alba) broods under different food abundance conditions: natural food provisioning and after an experimental food supplementation. Allofeeding was performed mainly by elder nestlings (hatching is asynchronous) in prime condition, especially when the cost of forgoing a prey was small (when parents allocated more prey to the food donor and after food supplementation). Nestlings preferentially shared food with the siblings that emitted very intense calls, thus potentially increasing indirect fitness benefits, or with the siblings that provided extensive allopreening to the donor, thus possibly promoting direct benefits from reciprocation. Finally, allopreening was mainly directed toward older siblings, perhaps to maximize the probability of being fed in return. Helping behavior among relatives can therefore be driven by both kin selection and direct cooperation, although it is dependent on the contingent environmental conditions.

Patterns of reproductive differentiation and reproductive plasticity in the major evolutionary transition to superorganismality

Major evolutionary transitions in individuality are characterised by the formation of new levels of biological complexity from the cooperation of previously independent lower-level units. The evolution of superorganismality in insects is one such major transition, and is characterised by an extreme division of reproductive labour between ancestrally autonomous units, in the form of queen and worker castes. Here, we discuss the nature of plasticity in the emergence of castes across the major transition to superorganismality in insects. We identify key changes in plasticity which act at different levels of selection: a loss of reproductivity plasticity at the individual level is matched by a gain in plasticity at the colony level. Taking multi-level selection into consideration has important implications for formulating testable hypotheses regarding the nature of plasticity in a major transition from a lower to a higher level of biological complexity.

Current indirect fitness and future direct fitness are not incompatible

In primitively eusocial insects, many individuals function as workers despite being capable of independent reproduction. Such altruistic behaviour is usually explained by the argument that workers gain indirect fitness by helping close genetic relatives. The focus on indirect fitness has left open the question of whether workers are also capable of getting direct fitness in the future in spite of working towards indirect fitness in the present. To investigate this question, we recorded behavioural profiles of all wasps on six naturally occurring nests of Ropalidia marginata, and then isolated all wasps in individual plastic boxes, giving them an opportunity to initiate nests and lay eggs. We found that 41% of the wasps successfully did so. Compared to those that failed to initiate nests, those that did were significantly younger, had significantly higher frequency of self-feeding behaviour on their parent nests but were not different in the levels of work performed in the parent nests. Thus ageing and poor feeding, rather than working for their colonies, constrain individuals for future independent reproduction. Hence, future direct fitness and present work towards gaining indirect fitness are not incompatible, making it easier for worker behaviour to be selected by kin selection or multilevel selection.

Phylogenomic Evidence Overturns Current Conceptions of Social Evolution in Wasps (Vespidae)

The hypothesis that eusociality originated once in Vespidae has shaped interpretation of social evolution for decades and has driven the supposition that preimaginal morphophysiological differences between castes were absent at the outset of eusociality. Many researchers also consider casteless nest-sharing an antecedent to eusociality. Together, these ideas endorse a stepwise progression of social evolution in wasps (solitary → casteless nest-sharing → eusociality with rudimentary behavioral castes → eusociality with preimaginal caste-biasing (PCB) → morphologically differentiated castes). Here, we infer the phylogeny of Vespidae using sequence data generated via anchored hybrid enrichment from 378 loci across 136 vespid species and perform ancestral state reconstructions to test whether rudimentary and monomorphic castes characterized the initial stages of eusocial evolution. Our results reject the single origin of eusociality hypothesis, contest the supposition that eusociality emerged from a casteless nest-sharing ancestor, and suggest that eusociality in Polistinae + Vespinae began with castes having morphological differences. An abrupt appearance of castes with ontogenetically established morphophysiological differences conflicts with the current conception of stepwise social evolution and suggests that the climb up the ladder of sociality does not occur through sequential mutation. Phenotypic plasticity and standing genetic variation could explain how cooperative brood care evolved in concert with nest-sharing and how morphologically dissimilar castes arose without a rudimentary intermediate. Furthermore, PCB at the outset of eusociality implicates a subsocial route to eusociality in Polistinae + Vespinae, emphasizing the role of mother-daughter interactions and subfertility (i.e. the cost component of kin selection) in the origin of workers.

Social wasps as models to study the major evolutionary transition to superorganismality

The major evolutionary transition to superorganismality has taken place several times in the insects. Although there has been much consideration of the ultimate evolutionary explanations for superorganismality, we know relatively little about what proximate mechanisms constrain or promote this major transition. Here, we propose that Vespid wasps represent an understudied, but potentially very useful, model system for studying the mechanisms underpinning superorganismality. We highlight how there is an abundance of behavioural data for many wasp species, confirming their utility in studies of social evolution; however, there is a sparsity of genomic data from which we can test proximate and ultimate hypotheses on this major evolutionary transition.

Do social insects support Haig's kin theory for the evolution of genomic imprinting?

Although numerous imprinted genes have been described in several lineages, the phenomenon of genomic imprinting presents a peculiar evolutionary problem. Several hypotheses have been proposed to explain gene imprinting, the most supported being Haig's kinship theory. This theory explains the observed pattern of imprinting and the resulting phenotypes as a competition for resources between related individuals, but despite its relevance it has not been independently tested. Haig's theory predicts that gene imprinting should be present in eusocial insects in many social scenarios. These lineages are therefore ideal for testing both the theory's predictions and the mechanism of gene imprinting. Here we review the behavioral evidence of genomic imprinting in eusocial insects, the evidence of a mechanism for genomic imprinting and finally we evaluate recent results showing parent of origin allele specific expression in honeybees in the light of Haig's theory.

Delayed dispersal and prolonged brood care in a family-living beetle

Delayed juvenile dispersal is an important prerequisite for the evolution of family-based social systems, such as cooperative breeding and eusociality. In general, young adults forego dispersal if there are substantial benefits to remaining in the natal nest and/or the likelihood of dispersing and breeding successfully is low. We investigate some general factors thought to drive delayed juvenile dispersal in the horned passalus beetle, a family-living beetle in which young adults remain with their families in their natal nest for several months before dispersing. Fine-scale population genetic structure indicated high gene flow between nest sites, suggesting that constraints on mobility are unlikely to explain philopatry. Young adults do not breed in their natal log and likely disperse before reaching breeding age, suggesting that they do not gain direct reproductive benefits from delayed dispersal. We also examined several ways in which parents might incentivize delayed dispersal by providing prolonged care to adult offspring. Although adult beetles inhibit fungal growth in the colony by manipulating both the nest site and deceased conspecifics, this is unlikely to be a major explanation for family living as both parents and adult offspring seem capable of controlling fungal growth. Adult offspring that stayed with their family groups also neither gained more mass nor experienced faster exoskeleton development than those experimentally removed from their families. The results of these experiments suggest that our current understanding of the factors underlying prolonged family living may be insufficient to explain delayed dispersal in at least some taxa, particularly insects.

Molecular signatures of plastic phenotypes in two eusocial insect species with simple societies

Phenotypic plasticity is important in adaptation and shapes the evolution of organisms. However, we understand little about what aspects of the genome are important in facilitating plasticity. Eusocial insect societies produce plastic phenotypes from the same genome, as reproductives (queens) and nonreproductives (workers). The greatest plasticity is found in the simple eusocial insect societies in which individuals retain the ability to switch between reproductive and nonreproductive phenotypes as adults. We lack comprehensive data on the molecular basis of plastic phenotypes. Here, we sequenced genomes, microRNAs (miRNAs), and multiple transcriptomes and methylomes from individual brains in a wasp (Polistes canadensis) and an ant (Dinoponera quadriceps) that live in simple eusocial societies. In both species, we found few differences between phenotypes at the transcriptional level, with little functional specialization, and no evidence that phenotype-specific gene expression is driven by DNA methylation or miRNAs. Instead, phenotypic differentiation was defined more subtly by nonrandom transcriptional network organization, with roles in these networks for both conserved and taxon-restricted genes. The general lack of highly methylated regions or methylome patterning in both species may be an important mechanism for achieving plasticity among phenotypes during adulthood. These findings define previously unidentified hypotheses on the genomic processes that facilitate plasticity and suggest that the molecular hallmarks of social behavior are likely to differ with the level of social complexity.

Group augmentation and the evolution of cooperation

The group augmentation (GA) hypothesis states that if helpers in cooperatively breeding animals raise the reproductive success of the group, the benefits of living in a resulting larger group--improved survival or future reproductive success--favour the evolution of seemingly altruistic helping behaviour. The applicability of the GA hypothesis remains debatable, however, partly owing to the lack of a clear conceptual framework and a shortage of appropriate empirical studies. We conceptualise here the GA hypothesis and illustrate that benefits of GA can accrue via different evolutionary mechanisms that relate closely to well-supported general concepts of group living and cooperation. These benefits reflect several plausible explanations for the evolutionary maintenance of helping behaviour in cooperatively breeding animals.

Transcriptome analyses of primitively eusocial wasps reveal novel insights into the evolution of sociality and the origin of alternative phenotypes

BACKGROUND

Understanding how alternative phenotypes arise from the same genome is a major challenge in modern biology. Eusociality in insects requires the evolution of two alternative phenotypes - workers, who sacrifice personal reproduction, and queens, who realize that reproduction. Extensive work on honeybees and ants has revealed the molecular basis of derived queen and worker phenotypes in highly eusocial lineages, but we lack equivalent deep-level analyses of wasps and of primitively eusocial species, the latter of which can reveal how phenotypic decoupling first occurs in the early stages of eusocial evolution.

RESULTS

We sequenced 20 Gbp of transcriptomes derived from brains of different behavioral castes of the primitively eusocial tropical paper wasp Polistes canadensis. Surprisingly, 75% of the 2,442 genes differentially expressed between phenotypes were novel, having no significant homology with described sequences. Moreover, 90% of these novel genes were significantly upregulated in workers relative to queens. Differential expression of novel genes in the early stages of sociality may be important in facilitating the evolution of worker behavioral complexity in eusocial evolution. We also found surprisingly low correlation in the identity and direction of expression of differentially expressed genes across similar phenotypes in different social lineages, supporting the idea that social evolution in different lineages requires substantial de novo rewiring of molecular pathways.

CONCLUSIONS

These genomic resources for aculeate wasps and first transcriptome-wide insights into the origin of castes bring us closer to a more general understanding of eusocial evolution and how phenotypic diversity arises from the same genome.

Shifting behaviour: epigenetic reprogramming in eusocial insects

Epigenetic modifications are ancient and widely utilised mechanisms that have been recruited across fungi, plants and animals for diverse but fundamental biological functions, such as cell differentiation. Recently, a functional DNA methylation system was identified in the honeybee, where it appears to underlie queen and worker caste differentiation. This discovery, along with other insights into the epigenetics of social insects, allows provocative analogies to be drawn between insect caste differentiation and cellular differentiation, particularly in mammals. Developing larvae in social insect colonies are totipotent: they retain the ability to specialise as queens or workers, in a similar way to the totipotent cells of early embryos before they differentiate into specific cell lineages. Further, both differentiating cells and insect castes lose phenotypic plasticity by committing to their lineage, losing the ability to be readily reprogrammed. Hence, a comparison of the epigenetic mechanisms underlying lineage differentiation (and reprogramming) between cells and social insects is worthwhile. Here we develop a conceptual model of how loss and regain of phenotypic plasticity might be conserved for individual specialisation in both cells and societies. This framework forges a novel link between two fields of biological research, providing predictions for a unified approach to understanding the molecular mechanisms underlying biological complexity.

Workers influence royal reproduction

Understanding which parties regulate reproduction is fundamental to understanding conflict resolution in animal societies. In social insects, workers can influence male production and sex ratio. Surprisingly, few studies have investigated worker influence over which queen(s) reproduce(s) in multiple queen (MQ) colonies (skew), despite skew determining worker-brood relatedness and so worker fitness. We provide evidence for worker influence over skew in a functionally monogynous population of the ant Leptothorax acervorum. Observations of MQ colonies leading up to egg laying showed worker aggressive and non-aggressive behaviour towards queens and predicted which queen monopolized reproduction. In contrast, among-queen interactions were rare and did not predict queen reproduction. Furthermore, parentage analysis showed workers favoured their mother when present, ensuring closely related fullsibs (average r = 0.5) were reared instead of less related offspring of other resident queens (r ≤ 0.375). Discrimination among queens using relatedness-based cues, however, seems unlikely as workers also biased their behaviour in colonies without a mother queen. In other polygynous populations of this species, workers are not aggressive towards queens and MQs reproduce, showing the outcome of social conflicts varies within species. In conclusion, this study supports non-reproductive parties having the power and information to influence skew within cooperative breeding groups.

Differential gene expression and protein abundance evince ontogenetic bias toward castes in a primitively eusocial wasp

Polistes paper wasps are models for understanding conditions that may have characterized the origin of worker and queen castes and, therefore, the origin of paper wasp sociality. Polistes is “primitively eusocial” by virtue of having context-dependent caste determination and no morphological differences between castes. Even so, Polistes colonies have a temporal pattern in which most female larvae reared by the foundress become workers, and most reared by workers become future-reproductive gynes. This pattern is hypothesized to reflect development onto two pathways, which may utilize mechanisms that regulate diapause in other insects. Using expressed sequence tags (ESTs) for Polistes metricus we selected candidate genes differentially expressed in other insects in three categories: 1) diapause vs. non-diapause phenotypes and/or worker vs. queen differentiation, 2) behavioral subcastes of worker honey bees, and 3) no a priori expectation of a role in worker/gyne development. We also used a non-targeted proteomics screen to test for peptide/protein abundance differences that could reflect larval developmental divergence. We found that foundress-reared larvae (putative worker-destined) and worker-reared larvae (putative gyne-destined) differed in quantitative expression of sixteen genes, twelve of which were associated with caste and/or diapause in other insects, and they also differed in abundance of nine peptides/proteins. Some differentially-expressed genes are involved in diapause regulation in other insects, and other differentially-expressed genes and proteins are involved in the insulin signaling pathway, nutrient metabolism, and caste determination in highly social bees. Differential expression of a gene and a peptide encoding hexameric storage proteins is especially noteworthy. Although not conclusive, our results support hypotheses of 1) larval developmental pathway divergence that can lead to caste bias in adults and 2) nutritional differences as the foundation of the pathway divergence. Finally, the differential expression in Polistes larvae of genes and proteins also differentially expressed during queen vs. worker caste development in honey bees may indicate that regulatory mechanisms of caste outcomes share similarities between primitively eusocial and advanced eusocial Hymenoptera.