Ant Variation: Ants of the same genotype can exhibit numerous phenotypic forms and develop multiple functional castes within a colony. Bonasio et al. (p. 1068 ) sequenced the genomes of two ant species exhibiting differences in caste development— Camponotus floridanus and Harpegnathos saltator —and used the sequences to compare gene expression and identify differences in epigenetic gene regulation that lead to the phenotypic differences. Ants may offer a model system for studying the role of epigenetics in behavior and development.

Although eusociality evolved independently within several orders of insects, research into the molecular underpinnings of the transition towards social complexity has been confined primarily to Hymenoptera (for example, ants and bees). Here we sequence the genome and stage-specific transcriptomes of the dampwood termite Zootermopsis nevadensis (Blattodea) and compare them with similar data for eusocial Hymenoptera, to better identify commonalities and differences in achieving this significant transition. We show an expansion of genes related to male fertility, with upregulated gene expression in male reproductive individuals reflecting the profound differences in mating biology relative to the Hymenoptera. For several chemoreceptor families, we show divergent numbers of genes, which may correspond to the more claustral lifestyle of these termites. We also show similarities in the number and expression of genes related to caste determination mechanisms. Finally, patterns of DNA methylation and alternative splicing support a hypothesized epigenetic regulation of caste differentiation.

Epigenetic control of caste-specific foraging: In carpenter ants, separate behavioral classes, known as castes, are determined by the epigenetic regulation of genes. Simola et al. treated ants of different castes with drugs that affected histone acetylation. Reducing histone acetylation stimulated scouting and foraging behavior. The foraging and scouting behaviors of young ants were permanently changed by directly injecting their brains with histone acetylation inhibitors. Science , this issue p. 10.1126/science.aac6633

In many ant species, sibling larvae follow alternative ontogenetic trajectories that generate striking variation in morphology and behavior among adults. These organism-level outcomes are often determined by environmental rather than genetic factors. Therefore, epigenetic mechanisms may mediate the expression of adult polyphenisms. We produced the first genome-wide maps of chromatin structure in a eusocial insect and found that gene-proximal changes in histone modifications, notably H3K27 acetylation, discriminate two female worker and male castes in Camponotus floridanus ants and partially explain differential gene expression between castes. Genes showing coordinated changes in H3K27ac and RNA implicate muscle development, neuronal regulation, and sensory responses in modulating caste identity. Binding sites of the acetyltransferase CBP harbor the greatest caste variation in H3K27ac, are enriched with motifs for conserved transcription factors, and show evolutionary expansion near developmental and neuronal genes. These results suggest that environmental effects on caste identity may be mediated by differential recruitment of CBP to chromatin. We propose that epigenetic mechanisms that modify chromatin structure may help orchestrate the generation and maintenance of polyphenic caste morphology and social behavior in ants.