Research Article

Epigenetic (re)programming of caste-specific behavior in the ant Camponotus floridanus

Science  01 Jan 2016:
Vol. 351, Issue 6268,
DOI: 10.1126/science.aac6633

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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

Structured Abstract

INTRODUCTION

Eusocial insects, such as ants, live a communal lifestyle within colonies of close genetic relatives. Colony members are organized into castes defined by behavioral and, in some species, morphological traits. This caste system allows for colonial division of labor and is a key adaptation of eusocial insects. However, there is limited understanding of the molecular regulation of caste-specific behavior and the principles underlying division of labor. In the carpenter ant Camponotus floridanus, morphologically distinct worker castes called minors and majors exhibit unique patterns of histone posttranslational modifications, including lysine acetylation regulated by CBP [cyclic adenosine monophosphate response element–binding protein (CREB) binding protein], a conserved histone acetyltransferase (HAT).

RATIONALE

Because chromatin regulators such as CBP have been associated with caste-specific traits, we tested whether caste-specific behavioral states in eusocial insects are functionally regulated via epigenetic mechanisms. We assessed innate differences in foraging and scouting (by the lead forager), classic altruistic behaviors of eusocial systems, between minor and major worker castes in C. floridanus. We examined whether CBP and histone deacetylases (HDACs) functionally regulate caste-specific foraging and scouting behaviors. Further, we tested whether caste-specific behavioral states may be reprogrammed through epigenomic manipulations.

RESULTS

C. floridanus minors and majors exhibited innate differences in foraging and scouting behaviors, with minors performing the bulk of both activities. Treatments with small-molecule inhibitors of class I and II HDAC activity (HDACi) enhanced foraging and scouting. This gain of function was suppressed by treatment with a small-molecule HAT inhibitor of CBP (HATi). Transcriptome and chromatin analyses in the brains of minors treated with HATi and HDACi revealed changes in genes linked to regions of hyperacetylated histone H3 Lys27 (a lysine targeted by CBP) near CBP binding sites. Although untreated majors rarely foraged, suppression of histone deacetylation by injection of HDACi or small interfering RNAs (siRNAs) against the HDAC-encoding gene Rpd3 into young major brains was sufficient to induce and sustain minor-like foraging and scouting for up to 50 days. Strikingly, coinjection of CBP HATi suppressed HDACi-induced foraging and scouting in majors.

CONCLUSION

Caste-specific foraging and scouting behaviors are tightly linked to morphology and are likely regulated epigenetically by the balance between CBP-mediated acetylation and HDAC-mediated deacetylation of histones in the ant central brain. Thus, behavioral plasticity can be manipulated in the ant C. floridanus by pharmacological and genetic tools that target chromatin regulatory enzymes to stimulate, inhibit, and reprogram behavior. These findings reveal the epigenome as a likely substrate underlying caste-based division of labor in eusocial insects. Furthermore, in light of the conserved role of CBP in learning and memory in both invertebrates and mammals, these data suggest that CBP-mediated histone acetylation may similarly facilitate the complex social interactions found in vertebrate species.

An epigenetic model for division of labor.

Left: Workers were injected at eclosion and tested for foraging activity. HDAC inhibition (HDACi) with chromatin drugs or siRNA enhanced foraging; HATi suppressed foraging. Right: Minor and major workers express distinct behavioral ontogenies. Minors forage earlier in life and with greater intensity than majors. HDACi in majors stimulated minor-like foraging behavior, a gain of function suppressed by HATi treatment.

Abstract

Eusocial insects organize themselves into behavioral castes whose regulation has been proposed to involve epigenetic processes, including histone modification. In the carpenter ant Camponotus floridanus, morphologically distinct worker castes called minors and majors exhibit pronounced differences in foraging and scouting behaviors. We found that these behaviors are regulated by histone acetylation likely catalyzed by the conserved acetyltransferase CBP. Transcriptome and chromatin analysis in brains of scouting minors fed pharmacological inhibitors of CBP and histone deacetylases (HDACs) revealed hundreds of genes linked to hyperacetylated regions targeted by CBP. Majors rarely forage, but injection of a HDAC inhibitor or small interfering RNAs against the HDAC Rpd3 into young major brains induced and sustained foraging in a CBP-dependent manner. Our results suggest that behavioral plasticity in animals may be regulated in an epigenetic manner via histone modification.

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