Gene regulation supporting sociality shared across lineages and variation in complexity

Across evolutionary lineages, insects vary in social complexity, from those that exhibit extended parental care to those with elaborate divisions of labor. Here, we synthesize the sociogenomic resources from hundreds of species to describe common gene regulatory mechanisms in insects that regulate social organization across phylogeny and levels of social complexity. Different social phenotypes expressed by insects can be linked to the organization of co-expressing gene networks and features of the epigenetic landscape. Insect sociality also stems from processes like the emergence of parental care and the decoupling of ancestral genetic programs. One underexplored avenue is how variation in a group's social environment affects the gene expression of individuals. Additionally, an experimental reduction of gene expression would demonstrate how the activity of specific genes contributes to insect social phenotypes. While tissue specificity provides greater localization of the gene expression underlying social complexity, emerging transcriptomic analysis of insect brains at the cellular level provides even greater resolution to understand the molecular basis of social insect evolution.

The effects of maternal care on the developmental transcriptome and metatranscriptome of a wild bee

Maternal care acts as a strong environmental stimulus that can induce phenotypic plasticity in animals and may also alter their microbial communities through development. Here, we characterize the developmental metatranscriptome of the small carpenter bee, Ceratina calcarata, across developmental stages and in the presence or absence of mothers. Maternal care had the most influence during early development, with the greatest number and magnitude of differentially expressed genes between maternal care treatments, and enrichment for transcription factors regulating immune response in motherless early larvae. Metatranscriptomic data revealed fungi to be the most abundant group in the microbiome, with Aspergillus the most abundant in early larvae raised without mothers. Finally, integrative analysis between host transcriptome and metatranscriptome highlights several fungi correlating with developmental and immunity genes. Our results provide characterizations of the influence of maternal care on gene expression and the microbiome through development in a wild bee.

Brain Gene Expression of Foraging Behavior and Social Environment in Ceratina calcarata

Rudimentary social systems have the potential to both advance our understanding of how complex sociality may have evolved and our understanding of how changes in social environment may influence gene expression and cooperation. Recently, studies of primitively social Hymenoptera have greatly expanded empirical evidence for the role of social environment in shaping behavior and gene expression. Here, we compare brain gene expression profiles of foragers across social contexts in the small carpenter bee, Ceratina calcarata. We conducted experimental manipulations of field colonies to examine gene expression profiles among social contexts including foraging mothers, regular daughters, and worker-like dwarf eldest daughters in the presence and absence of mother. Our analysis found significant differences in gene expression associated with female age, reproductive status, and social environment, including circadian clock gene dyw, hexamerin, and genes involved in the regulation of juvenile hormone and chemical communication. We also found that candidate genes differentially expressed in our study were also associated with division of labor, including foraging, in other primitively and advanced eusocial insects. Our results offer evidence for the role of the regulation of key developmental hormones and circadian rhythms in producing cooperative behavior in rudimentary insect societies.

Differential Gene Expression Correlates with Behavioural Polymorphism during Collective Behaviour in Cockroaches

Simple Summary

It is currently well accepted that animals differ from one another in their behaviour and tendency to perform actions, a property we refer to as animal personality. In group-living animals, variation in animal personality can be important to determine group survival, as it determines how individuals interact with each other and with their environment. However, we have little knowledge of the proximal mechanisms underlying personality, particularly in group-living organisms. Here, we investigate the relationship between gene expression and two behavioural types (bold and shy) in a gregarious species: the American cockroach. Our results show that bold individuals have upregulated genes with functions associated with sensory activity (phototaxis and odour detection) and aggressive/dominant behaviour, and suggest that social context can modulate gene expression related to bold/shy characteristics. This work could help identify genes important in the earliest stages of group living and social life, and provides a first step toward establishing cockroaches as a focal group for the study of the evolution of sociality.

Abstract

Consistent inter-individual variation in the propensity to perform different tasks (animal personality) can contribute significantly to the success of group-living organisms. The distribution of different personalities in a group influences collective actions and therefore how these organisms interact with their environment. However, we have little understanding of the proximate mechanisms underlying animal personality in animal groups, and research on this theme has often been biased towards organisms with advanced social systems. The goal of this study is to investigate the mechanistic basis for personality variation during collective behaviour in a species with rudimentary societies: the American cockroach. We thus use an approach which combines experimental classification of individuals into behavioural phenotypes (‘bold’ and ‘shy’ individuals) with comparative gene expression. Our analyses reveal differences in gene expression between behavioural phenotypes and suggest that social context may modulate gene expression related to bold/shy characteristics. We also discuss how cockroaches could be a valuable model for the study of genetic mechanisms underlying the early steps in the evolution of social behaviour and social complexity. This study provides a first step towards a better understanding of the molecular mechanisms associated with differences in boldness and behavioural plasticity in these organisms.

Reduced neural investment in post-reproductive females of the bee Ceratina calcarta

Many insects show plasticity in the area of the brain called the mushroom bodies (MB) with foraging and social experience. MBs are paired neuropils associated with learning and memory. MB volume is typically greater in mature foragers relative to young and/or inexperienced individuals. Long-term studies show that extended experience may further increase MB volume, but long-term studies have only been performed on non-reproductive social insect workers. Here we use the subsocial bee Ceratina calcarata to test the effect of extended foraging experience on MB volume among reproductive females. Ceratina calcarata females forage to provision their immature offspring in the spring, and then again to provision their adult daughters in the late summer. We measured the volume of the MB calyces and peduncle, antennal lobes (AL), optic lobes (OL), central complex (CX), and whole brains of three groups of bees: newly emerged females, reproductive females in spring (foundresses), and post-reproductive mothers feeding their adult daughters in late summer. Post-reproductive late summer mothers had smaller MB calyces and ALs than foundresses. Moreover, among late mothers (but not other bees), wing wear, which is a measure of foraging experience, negatively correlated with both MB and OL volume. This is contrary to previously studied non-reproductive social insect workers in which foraging experience correlates postiviely with MB volume, and suggests that post-reproductive bees may reduce neural investment near the end of their lives.

Social divergence: molecular pathways underlying castes and longevity in a facultatively eusocial small carpenter bee

Unravelling the evolutionary origins of eusocial life is a longstanding endeavour in the field of evolutionary-developmental biology. Descended from solitary ancestors, eusocial insects such as honeybees have evolved ontogenetic division of labour in which short-lived workers perform age-associated tasks, while a long-lived queen produces brood. It is hypothesized that (i) eusocial caste systems evolved through the co-option of deeply conserved genes and (ii) longevity may be tied to oxidative damage mitigation capacity. To date, however, these hypotheses have been examined primarily among only obligately eusocial corbiculate bees. We present brain transcriptomic data from a Japanese small carpenter bee, Ceratina japonica (Apidae: Xylocopinae), which demonstrates both solitary and eusocial nesting in sympatry and lives 2 or more years in the wild. Our dataset captures gene expression patterns underlying first- and second-year solitary females, queens and workers, providing an unprecedented opportunity to explore the molecular mechanisms underlying caste-antecedent phenotypes in a long-lived and facultatively eusocial bee. We find that C. japonica's queens and workers are underpinned by divergent gene regulatory pathways, involving many differentially expressed genes well-conserved among other primitively eusocial bee lineages. We also find support for oxidative damage reduction as a proximate mechanism of longevity in C. japonica.

Krüppel-homologue 1 Mediates Hormonally Regulated Dominance Rank in a Social Bee

Dominance hierarchies are ubiquitous in invertebrates and vertebrates, but little is known on how genes influence dominance rank. Our gaps in knowledge are specifically significant concerning female hierarchies, particularly in insects. To start filling these gaps, we studied the social bumble bee Bombus terrestris, in which social hierarchies among females are common and functionally significant. Dominance rank in this bee is influenced by multiple factors, including juvenile hormone (JH) that is a major gonadotropin in this species. We tested the hypothesis that the JH responsive transcription factor Krüppel homologue 1 (Kr-h1) mediates hormonal influences on dominance behavior. We first developed and validated a perfluorocarbon nanoparticles-based RNA interference protocol for knocking down Kr-h1 expression. We then used this procedure to show that Kr-h1 mediates the influence of JH, not only on oogenesis and wax production, but also on aggression and dominance rank. To the best of our knowledge, this is the first study causally linking a gene to dominance rank in social insects, and one of only a few such studies on insects or on female hierarchies. These findings are important for determining whether there are general molecular principles governing dominance rank across gender and taxa.

Dynamic neurogenomic responses to social interactions and dominance outcomes in female paper wasps

Social interactions have large effects on individual physiology and fitness. In the immediate sense, social stimuli are often highly salient and engaging. Over longer time scales, competitive interactions often lead to distinct social ranks and differences in physiology and behavior. Understanding how initial responses lead to longer-term effects of social interactions requires examining the changes in responses over time. Here we examined the effects of social interactions on transcriptomic signatures at two times, at the end of a 45-minute interaction and 4 hours later, in female Polistes fuscatus paper wasp foundresses. Female P. fuscatus have variable facial patterns that are used for visual individual recognition, so we separately examined the transcriptional dynamics in the optic lobe and the non-visual brain. Results demonstrate much stronger transcriptional responses to social interactions in the non-visual brain compared to the optic lobe. Differentially regulated genes in response to social interactions are enriched for memory-related transcripts. Comparisons between winners and losers of the encounters revealed similar overall transcriptional profiles at the end of an interaction, which significantly diverged over the course of 4 hours, with losers showing changes in expression levels of genes associated with aggression and reproduction in paper wasps. On nests, subordinate foundresses are less aggressive, do more foraging and lay fewer eggs compared to dominant foundresses and we find losers shift expression of many genes in the non-visual brain, including vitellogenin, related to aggression, worker behavior, and reproduction within hours of losing an encounter. These results highlight the early neurogenomic changes that likely contribute to behavioral and physiological effects of social status changes in a social insect.

Social isolation and group size are associated with divergent gene expression in the brain of ant queens

Social life and isolation pose a complex suite of challenges to organisms prompting significant changes in neural state. However, plasticity in how brains respond to social challenges remains largely unexplored. The fire ants Solenopsis invicta provide an ideal scenario for examining this. Fire ant queens may found colonies individually or in groups of up to 30 queens, depending on key factors such as density of newly mated queens and availability of nesting sites. We artificially manipulated availability of nesting sites to test how the brain responds to social versus solitary colony founding at two key timepoints (early vs. late colony founding) and to group size (large vs. small groups). We adopted a powerful neurogenomic approach to identify even subtle differences of gene expression between treatment groups, and we built a global gene co-expression network of the fire ant brain to identify gene modules specifically associated with the different components of the social environment. The difference between group and single founding queens involves only one gene when founding behavior is still plastic and queens can switch from one modality to another, while hundreds of genes are involved later in the process, when behaviors have lost the initial plasticity and are more canalized. Furthermore, we find that large groups are associated with greater changes in gene expression than small groups, showing that even potentially subtle differences in the social environment can be linked to different neurogenomic states.

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.

Female spider aggression is associated with genetic underpinnings of the nervous system and immune response to pathogens

Identifying the genetic architecture underlying phenotypic variation in natural populations and assessing the consequences of polymorphisms for individual fitness are fundamental goals in evolutionary and molecular ecology. Consistent between-individual differences in behaviour have been documented for a variety of taxa. Dissecting the genetic basis of such behavioural differences is however a challenging endeavour. The molecular underpinnings of natural variation in aggression remain elusive. Here, we used comparative gene expression (transcriptome analysis and RT-PCR), genetic association analysis and pharmacological experiments to gain insight into the genetic basis of aggression in wild-caught jumping spiders (Portia labiata). We show that spider aggression is associated with a putative viral infection response gene, BTB/POZ domain-containing protein 17 (BTBDH), in addition to a putative serotonin receptor 1A (5-HT1A) gene. Spider aggression varies with virus loads, and BTBDH is upregulated in docile spiders and exhibits a genetic variant associated with aggression. We also identify a putative serotonin receptor 5-HT1A gene upregulated in docile P. labiata. Individuals that have been treated with serotonin become less aggressive, but individuals treated with a nonselective serotonin receptor antagonist (methiothepin) also reduce aggression. Further, we identify the genetic variants in the 5-HT1A gene that are associated with individual variation in aggression. We therefore conclude that co-evolution of the immune and nervous systems may have shaped the between-individual variation in aggression in natural populations of jumping spiders.

Developmental plasticity shapes social traits and selection in a facultatively eusocial bee

Developmental processes are an important source of phenotypic variation, but the extent to which this variation contributes to evolutionary change is unknown. We used integrative genomic analyses to explore the relationship between developmental and social plasticity in a bee species that can adopt either a social or solitary lifestyle. We find genes regulating this social flexibility also regulate development, and positive selection on these genes is influenced by their function during development. This suggests that developmental plasticity may influence the evolution of sociality. Our additional finding of genetic variants linked to differences in social behavior sheds light on how phenotypic variation derived from development may become encoded into the genome, and thus contribute to evolutionary change.

Developmental plasticity generates phenotypic variation, but how it contributes to evolutionary change is unclear. Phenotypes of individuals in caste-based (eusocial) societies are particularly sensitive to developmental processes, and the evolutionary origins of eusociality may be rooted in developmental plasticity of ancestral forms. We used an integrative genomics approach to evaluate the relationships among developmental plasticity, molecular evolution, and social behavior in a bee species (Megalopta genalis) that expresses flexible sociality, and thus provides a window into the factors that may have been important at the evolutionary origins of eusociality. We find that differences in social behavior are derived from genes that also regulate sex differentiation and metamorphosis. Positive selection on social traits is influenced by the function of these genes in development. We further identify evidence that social polyphenisms may become encoded in the genome via genetic changes in regulatory regions, specifically in transcription factor binding sites. Taken together, our results provide evidence that developmental plasticity provides the substrate for evolutionary novelty and shapes the selective landscape for molecular evolution in a major evolutionary innovation: Eusociality.

Social modularity: conserved genes and regulatory elements underlie caste-antecedent behavioural states in an incipiently social bee

The evolutionary origins of advanced eusociality, one of the most complex forms of phenotypic plasticity in nature, have long been a focus within the field of sociobiology. Although eusocial insects are known to have evolved from solitary ancestors, sociogenomic research among incipiently social taxa has only recently provided empirical evidence supporting theories that modular regulation and deeply conserved genes may play important roles in both the evolutionary emergence and elaboration of insect sociality. There remains, however, a paucity of data to further test the biological reality of these and other evolutionary theories among taxa in the earliest stages of social evolution. Here, we present brain transcriptomic data from the incipiently social small carpenter bee, Ceratina calcarata, which captures patterns of cis-regulation and gene expression associated with female maturation, and underlying two well-defined behavioural states, foraging and guarding, concurrently demonstrated by mothers and daughters during early autumn. We find that an incipiently social nest environment may dramatically affect gene expression. We further reveal foraging and guarding behaviours to be putatively caste-antecedent states in C. calcarata, and offer strong empirical support for the operation of modular regulation, involving deeply conserved and differentially expressed genes in the expression of early social forms.

Loss of developmental diapause as prerequisite for social evolution in bees

Diapause is a physiological arrest of development ahead of adverse environmental conditions and is a critical phase of the life cycle of many insects. In bees, diapause has been reported in species from all seven taxonomic families. However, they exhibit a variety of diapause strategies. These different strategies are of particular interest since shifts in the phase of the insect life cycle in which diapause occurs have been hypothesized to promote the evolution of sociality. Here we provide a comprehensive evaluation of this hypothesis with phylogenetic analysis and ancestral state reconstruction (ASR) of the ecological and evolutionary factors associated with diapause phase. We find that social lifestyle, latitude and voltinism are significant predictors of the life stage in which diapause occurs. ASR revealed that the most recent common ancestor of all bees likely exhibited developmental diapause and shifts to adult, reproductive, or no diapause have occurred in the ancestors of lineages in which social behaviour has evolved. These results provide fresh insight regarding the role of diapause as a prerequisite for the evolution of sociality in bees.

Genetic signatures of dominance hierarchies reveal conserved cis-regulatory and brain gene expression underlying aggression in a facultatively social bee

Agonistic interactions among individuals can result in the formation of dominance hierarches that can reinforce individual behavior and social status. Such dominance hierarches precede the establishment of reproductive dominance, division of labor and caste formation in highly social insect taxa. As such, deciphering the molecular basis of aggression is fundamental in understanding the mechanisms of social evolution. Assessing the proximate mechanisms of aggression in incipiently social bees can provide insights into the foundations of genomic mechanisms of social behavior. Here, we measured the effects of aggression on brain gene expression in the incipiently social bee, Ceratina australensis. We examine the brain transcriptomic differences between individuals who have experienced recurrent winning, losing, or a change in rank during repeated encounters. Using comparative analyses across taxa, we identify deeply conserved candidate genes, pathways, and regulatory networks for the formation of social hierarchies.

Conserved Genes Underlie Phenotypic Plasticity in an Incipiently Social Bee

Despite a strong history of theoretical work on the mechanisms of social evolution, relatively little is known of the molecular genetic changes that accompany transitions from solitary to eusocial forms. Here, we provide the first genome of an incipiently social bee that shows both solitary and social colony organization in sympatry, the Australian carpenter bee Ceratina australensis. Through comparative analysis, we provide support for the role of conserved genes and cis-regulation of gene expression in the phenotypic plasticity observed in nest-sharing, a rudimentary form of sociality. Additionally, we find that these conserved genes are associated with caste differences in advanced eusocial species, suggesting these types of mechanisms could pave the molecular pathway from solitary to eusocial living. Genes associated with social nesting in this species show signatures of being deeply conserved, in contrast to previous studies in other bees showing novel and faster-evolving genes are associated with derived sociality. Our data provide support for the idea that the earliest social transitions are driven by changes in gene regulation of deeply conserved genes.

Candidate genes for cooperation and aggression in the social wasp Polistes dominula

Cooperation and aggression are ubiquitous in social groups, and the genetic mechanisms underlying these behaviours are of great interest for understanding how social group formation is regulated and how it evolves. In this study, we used a candidate gene approach to investigate the patterns of expression of key genes for cooperation and aggression in the brain of a primitively eusocial wasp, Polistes dominula, during colony founding, when multiple foundresses can join the same nest and establish subtle hierarchies of dominance. We used a comparative approach to select candidate genes for cooperation and aggression looking at two previously published studies on global gene expression in wasps and ants. We tested the expression of these genes in P. dominula wasps that were either displaying aggressive behaviour (dominant and single foundresses) or cooperation (subordinate foundresses and workers) towards nestmates. One gene in particular, the egg yolk protein vitellogenin, known for its reproductive role in insects, displayed patterns of expression that strongly matched wasp social rank. We characterize the genomic context of vitellogenin by building a head co-expression gene network for P. dominula, and we discuss a potential role for vitellogenin as a mediator of social interactions in wasps.

Effects of nutritional deprivation on development and behavior in the subsocial bee Ceratina calcarata (Hymenoptera: Xylocopinae)

By manipulating resources or dispersal opportunities, mothers can force offspring to remain at the nest to help raise siblings, creating a division of labor. In the subsocial bee Ceratina calcarata, mothers manipulate the quantity and quality of pollen provided to the first female offspring, producing a dwarf eldest daughter that is physically smaller and behaviorally subordinate. This daughter forages for her siblings and forgoes her own reproduction. To understand how the mother's manipulation of pollen affects the physiology and behavior of her offspring, we manipulated the amount of pollen provided to offspring and measured the effects of pollen quantity on offspring development, adult body size and behavior. We found that by experimentally manipulating pollen quantities we could recreate the dwarf eldest daughter phenotype, demonstrating how nutrient deficiency alone can lead to the development of a worker-like daughter. Specifically, by reducing the pollen and nutrition to offspring, we significantly reduced adult body size and lipid stores, creating significantly less aggressive, subordinate individuals. Worker behavior in an otherwise solitary bee begins to explain how maternal manipulation of resources could lead to the development of social organization and reproductive hierarchies, a major step in the transition to highly social behaviors.

Introduction to Symposium: The Developmental and Proximate Mechanisms Causing Individual Variation in Cooperative Behavior

Nearly all animals interact with members of their own species at some point during their lives. These behavioral interactions range from courtship, mating, and parental care to the complex cooperative behavior among related or unrelated individuals in group-living species. A number of theoretical models have attempted to explain how cooperation can evolve through natural selection. Although tremendously influential in animal behavior research, these traditional models have largely ignored individual variation in cooperative behavior and its underlying developmental and proximate mechanisms. However, a set of emerging models suggest that the evolution of cooperation can be heavily influenced by the degree of individual variation in cooperative behavior, as well as the complexity of the underlying mechanisms. Yet, while theoreticians argue the importance of studying individual variation in cooperation and the mechanisms underlying it, empiricists have not focused upon these aspects. The main objectives of our symposium at the 2017 meeting of the Society for Integrative and Comparative Biology is to establish new research avenues to study variation in cooperative behavior using both proximate and ultimate explanations and to produce a road map to study the developmental and proximate mechanisms in generating individual variation in cooperative behavior. This symposium brought together empiricists and theoreticians investigating cooperative behavior in diverse taxa and across multiple levels of analysis. Here we briefly describe the rationale for this symposium and why we thought it was needed as well as provide a brief overview of the contributions.