© ALEX WILD
Eusocial insects are among the most successful living creatures on Earth. Found in terrestrial ecosystems across the globe (on every continent except Antarctica), the world’s ants alone weigh more than all vertebrates put together. Bees are key pollinators of major crops as well as many other ecologically important plants. Termites construct thermoregulating homes that can dominate the landscape, and that are inspiring new energy-efficient skyscraper designs. The organization and collective decision making of eusocial insects is even yielding new insights into human behavior and what it means to be part of a society. But one of the biggest unanswered questions in our understanding of these complex insect groups is how a single genome can produce such diverse and contrasting physical and behavioral forms, from egg layers, provisioners, and caretakers to soldiers.
In a eusocial colony, reproduction is dominated by one or a few individuals adapted to egg laying, while their offspring—colony workers—display physical and behavioral adaptations that help them perform their subordinate roles. These phenotypic adaptations can be extreme. A leafcutter ant queen is 10 times larger than her smallest workers, for example. (See photograph below.) And some carpenter ant species have evolved a “kamikaze” caste, born with a self-destruct button that causes the insect to explode upon colony attack, killing itself and covering the invading animals in toxic chemicals. Remarkably, differences in the behavior and morphology of insect castes are usually generated through differences in the expression of identical sets of genes. (There are a few cases of genetically determined castes, but this is the exception, not the rule.)
The extreme altruism exhibited by eusocial insects was one of the most perplexing traits that Darwin encountered when developing his theory of natural selection.
We are now entering a new era of research into eusocial insects. For the first time, scientists are investigating the molecules that underlie eusocial behavior at a depth that was previously unimaginable. New, affordable sequencing technologies enable scientists to examine how genes across the entire genome are regulated to generate different caste phenotypes, the roles of DNA methylation and microRNAs in this differential expression, and what proteins are synthesized as a result. This burgeoning area of research, dubbed “sociogenomics” in 2005 by Gene E. Robinson,1 is revolutionizing our understanding of the evolution of eusociality from a solitary wasp-like ancestor to the million-strong colonies we see today. New work is yielding insights into how genomes interact dynamically with the physical and social environment to produce highly adapted, specialized castes with remarkable phenotypic innovations. These findings are, in turn, illuminating the importance of gene regulation and epigenetics in controlling behavioral plasticity across the animal kingdom.
The birth of eusociality
© ALEX WILDThe extreme altruism exhibited by eusocial insects was one of the most perplexing traits that Darwin encountered when developing his theory of natural selection. How can nature select for a worker phenotype, which exists solely to help others reproduce, when it does not have any offspring of its own? We now understand that worker behavior can evolve because workers still pass on their genes through the related offspring they help raise. This has allowed eusociality to evolve multiple times throughout biological history: 10 times in the Hymenoptera (the ants, bees, and wasps), and an additional 22 times in termites, aphids, thrips, and snapping shrimps. It has even appeared a couple of times in mammals, with independent origins in two species of mole rat. (See “Underground Supermodels,” The Scientist, June 2012.)
Each eusocial lineage evolved from a solitary ancestor—a species in which a single genome produced a single adult phenotype, as is the case for the majority of insects alive today. Based on the morphology of both extant and extinct species, it was long believed that bees represented the most ancestral of the hymenopteran lineages. However, recent high-throughput sequencing of transcriptomes indicates that wasps may in fact be the more ancient group, with bees and ants having diverged from the wasp lineage around 145 million years ago.2,3 The first eusocial societies were simple, much like some of today’s halictid bees and Polistes paper wasps, whose behavioral castes look identical. Since then the order Hymenoptera has diverged into more than 14,000 eusocial species spanning almost every level of social organization, including the much more complex societies of honeybees, ants, and others. Collectively, these insects provide glimpses into the evolution of eusociality. (See illustration.)
So how did we get from a solitary ancestor to a species with diverse specialized phenotypes? A long-standing hypothesis, proposed by the eminent social insect biologist Mary Jane West-Eberhard, goes like this: the solitary ancestor lived as a single mother; she laid eggs and foraged alone to provide food for her growing brood. Once mature, her offspring would leave the nest to forage and reproduce, also on their own. This is how most insects still make a living. One of the first steps on the road to eusociality was for these offspring to stay behind at the nest for some time into adulthood, where they helped their mother raise their younger siblings. As these helpers evolved to specialize in particular roles, characteristics and behaviors that were once enacted sequentially by the solitary female slowly became decoupled. Reproductive traits were the exclusive responsibility of a newly evolved phenotype, the “queen,” and behaviors such as foraging were now performed by another new phenotype—the “worker.”
The hypothesis that social castes arose from the decoupling of once-solitary behaviors is compelling in its simplicity and its conformity with a well-established theory on the molecular mechanisms of evolution. Like the HOX cluster, a relatively small set of genes that underpins multicellular development in almost all life on Earth, a genetic toolkit for social behavior could have enabled the evolution of eusocial systems via an uncoupling of the genes regulating different solitary behaviors. If so, we expect to find suites of the same “toolkit” genes regulating caste-specific morphology and behavior across multiple independent evolutionary iterations of eusocial life. These genes may have been predisposed to a role in eusocial behavior, perhaps because of their key role in provisioning or in physiological activity.
Although this mechanism is supported by behavioral data, we have previously lacked the molecular tools to help us test the importance of phenotypic uncoupling in caste evolution. The genome sequences of 11 eusocial hymenopteran species have now been published, and these data are further accompanied by caste-specific transcriptomic and proteomic analyses. Together, these resources are unveiling the gene-level dynamics that underlie eusocial behavior. Methylome and microRNA sequencing have also begun to reveal the regulatory factors involved in mediating caste-biased differential gene expression.
Make new genes, but keep the old
© CATHERINE DELPHIATo some extent, recent sociogenomic studies have confirmed the existence of common genes underlying queen and worker phenotypes across social species. For example, a gene associated with roving behaviors in fruit flies and nematodes, in which the animals go looking for food, is also associated with foraging behavior in honeybees, ants, and bumblebees, which represent multiple independent origins of eusociality. Moreover, recent investigations of division of labor in eusocial insects with simpler societies have highlighted many of the same toolkit genes associated with castes found in the highly eusocial honeybee.
Some of these “old” genes have adopted new functions in certain species. The ancestral function of juvenile hormone (JH), for example, was to produce yolk for egg development. And in all eusocial insects studied to date, JH is upregulated in queens, suggesting they retain the gene’s ancestral function. However, JH has also evolved a new function—regulating foraging behavior in workers of several eusocial species. In honeybee workers, JH regulates the fine-scale, age-based transition from nursing (as a young worker) to foraging (as an older worker). Recent studies have also shown that the hormone forms a functional link between insulin signaling pathways and the insect neuroendocrine system, which allows foraging and brood-rearing behavior to be modulated by the nutritional and energetic needs of both the individual and the colony.
Sociogenomic analyses are also unearthing surprises. Across three independent origins of eusociality in bees, two-thirds of genes that show recent rapid evolution were linked to the level of eusociality—complex or simple.4 Such genes include novel, or taxonomically restricted, genes—those that have evolved uniquely in a single taxonomic group, and so, to date, lack any sequence similarity with any known organism outside the sequenced group. In the honeybee gene set, for example, more than 250 genes are “orphans,” meaning they are unique to honeybees, or are restricted to the Hymenoptera; of these, 58 percent are expressed differently in queens and workers or in different worker castes.5 More than 40 percent of worker-biased genes in the rock ant Temnothorax longispinosus,6 and 75 percent of caste-biased genes in the paper wasp Polistes canadensis,2 are novel.
Thus, a core sociality toolkit appears to have been augmented by the de novo birth of novel genes and gene families and rapid evolution of ancestral genes to generate queen and worker phenotypes in eusocial insects. But our understanding of the role of orphan genes is largely dependent on the available sequence data. As more species are sequenced over the coming years, our picture of the importance of new, old, and modified genes in eusocial evolution will become clearer.
From genotype to phenotype
© ANDREY PAVLOV/SHUTTERSTOCKDifferential expression of shared genes is just one small step in the link from genes to physical form. Sociogenomics research is now starting to focus on dissecting the mechanisms that regulate gene expression and determine the resulting proteome, and ultimately, the phenotype.
The role of microRNAs and epigenetic processes, such as DNA methylation and posttranslational histone modification, in suppressing or activating genes during development has long been recognized in model organisms such as Drosophila and mice. Such mechanisms may also regulate caste differentiation and behavioral plasticity in eusocial insects.7 A functional DNA methylation system appears to operate in eusocial bees, wasps, ants, and termites, whose genomes encode the key DNA methyltransferases DNMT1 and DNMT3.8 These methyltransferases tag specific genes with methyl groups, resulting in their reduced transcription.
Researchers first suspected a role for DNA methylation in eusocial insects in 2008, when Robert Kucharski of Australian National University and colleagues used RNA interference (RNAi) to knock down DNMT3 in honeybee worker larvae, which as adults went on to develop ovaries, like a queen.9 A more recent study found that honeybee DNA methylation levels changed with gene expression during the transition from nursing to foraging, and back again.10 A similar role for DNA methylation in regulating caste fate has since been suggested for bumblebees, where chemical inhibition of DNMT3 promotes reproduction by workers in colonies with no queens.11 These findings suggest that the role of DNA methylation may be much more dynamic and unstable in insects than in mammals, changing with age, developmental stage, and social environment.
The methods, applicability, and affordability of omics technologies are improving at breakneck speed, giving us the tools we need to uncover the molecular secrets behind the complex lives of eusocial insects.
Several lines of evidence now suggest that histones, the proteins responsible for the tight packaging of DNA into chromatin, also play important roles in regulating caste-biased gene expression in eusocial insects. One recent study by Astrid Spannhoff and colleagues at the University of Texas in Austin identified a histone-regulating protein as a key ingredient in royal jelly, which worker bees secrete to nourish hive larvae and to trigger the switch from worker to queen in select larvae as needed.12
Understanding the regulation of gene transcription is a major piece of the puzzle. But a lot can happen between transcription and protein production, and a new challenge in sociogenomics is to connect the dots among transcription, translation, and protein products. Regulatory elements called microRNAs are known to mediate cell fate and posttranscriptional gene regulation, and have been found to show caste-specific expression in honeybees and ants.13,14 However, we still lack a deep understanding of exactly how microRNAs influence caste and behavior in eusocial insects.
Natural selection acts on the phenotype, not directly on genes; the proteome is the closest molecular representation of the phenotype, and perhaps the key to understanding the evolution of eusociality. So far, proteomics studies on eusocial insects are few and far between, but recent large-scale mapping of the proteome of the honeybee worker brain has revealed proteins that are differentially expressed in nursing and foraging individuals.15 Researchers must now begin to embrace cutting-edge bioinformatics methods that allow dual analysis of transcriptomes and proteomes in the same individuals. A coordinated analysis of transcription, gene regulation, and protein production, alongside carefully assayed behavioral repertoires, will bring us closer to understanding the emergence of social diversity from a single genome.
The future of sociogenomics
FABIO BRAMBILLA/WIKIMEDIA COMMONSSociogenomics is young, but the field is exploding. The methods, applicability, and affordability of omics technologies are improving at breakneck speed, giving us the tools we need to uncover the molecular secrets behind the complex lives of eusocial insects. It is now possible to study any species, and most importantly, to study them in their natural habitat. This is especially important for studying simple societies, such as those of stenogastrine hover wasps and allodapine bees, where worker behavior depends so much on the ecological constraints of the environment.
Understanding the molecular basis of queen and worker caste formation and maintenance is only the start. The next steps will focus on what, if any, molecular changes accompanied major transitions in eusocial evolution, such as workers’ loss of the ability to mate, and the honeybee queen’s loss of the ability to found a nest on her own. The next few years will also see the scientific community studying a broader taxonomic spread, to capture the extent to which molecular processes vary within different eusocial lineages and across different levels of societal complexity. Sociogenomics provides an exciting common ground for ecologists, evolutionary genomicists, and developmental biologists to study broad-scale macroevolutionary patterns and behaviors in the fine-scale detail of gene regulation. When disparate disciplines of biology are united, new ideas, new hypotheses, and a deeper understanding of the natural world invariably emerge.
Claire Asher works in knowledge transfer at the Centre for Biodiversity and Environment Research, University College London, and is also a freelance science writer who writes the Curious Meerkat blog (www.curiousmeerkat.co.uk). She recently completed a PhD studying the social and sociogenomic controls of behavior in simple ant societies. Seirian Sumner is a senior lecturer in behavioral biology at the School of Biological Sciences, University of Bristol. Her work specializes in exploiting molecular tools to address questions of how and why eusocial behavior evolves.
- G.E. Robinson et al., “Sociogenomics: social life in molecular terms,” Nat Rev Genet, 6:257-70, 2005.
- P.G. Ferreira et al., “Transcriptome analyses of primitively eusocial wasps reveal novel insights into the evolution of sociality and the origin of alternative phenotypes.” Genome Biol, 14:R20, 2013.
- B.R. Johnson et al., “Phylogenomics resolves evolutionary relationships among ants, bees, and wasps,” Curr Biol, 23:2058-62, 2013.
- S.H. Woodard et al., “Genes involved in convergent evolution of eusociality in bees,” PNAS, 108:7472-77 2011.
- B.R. Johnson, N.D. Tsutsui, “Taxonomically restricted genes are associated with the evolution of sociality in the honey bee,” BMC Genomics, 12:164, 2011.
- B. Feldmeyer et al., “Gene expression patterns associated with caste and reproductive status in ants: worker-specific genes are more derived than queen-specific ones,” Mol Ecol, 23:151-61, 2014.
- S. Patalano et al., “Shifting behaviour: epigenetic reprogramming in eusocial insects,” Curr Opin Cell Biol, 24:367-73, 2012.
- H. Yan et al., “Eusocial insects as emerging models for behavioural epigenetics,” Nat Rev Genet, 15:677-88, 2014.
- R. Kucharski et al., “Nutritional control of reproductive status in honeybees via DNA methylation,” Science, 319:1827-30, 2008.
- B.R. Herb et al., “Reversible switching between epigenetic states in honeybee behavioral subcastes,” Nat Neurosci, 15:1371-73, 2012.
- H.E. Amarasinghe et al., “Methylation and worker reproduction in the bumble-bee (Bombus terrestris),” Proc Roy Soc B, 281:20132502, 2014.
- A. Spannhoff et al., “Histone deacetylase inhibitor activity in royal jelly might facilitate caste switching in bees,” EMBO Rep, 12:238-43, 2011.
- D.F. Simola et al., “Social insect genomes exhibit dramatic evolution in gene composition and regulation while preserving regulatory features linked to sociality,” Genome Res, 23:1235-47, 2013.
- J.K. Greenberg et al., “Behavioral plasticity in honey bees is associated with differences in brain microRNA transcriptome,” Genes Brain Behav, 11:660-70, 2012.
- L.G. Hernández et al., “Worker honeybee brain proteome,” J Proteome Res, 11:1485-93, 2012.