PROJECT SUMMARY / ABSTRACT Societies are organized into hierarchies, which fundamentally involve social conflict. As a result, there are winners and losers, and social defeat can profoundly impact animals, with the potential to induce abnormal behavior and disease. In this research interest statement, I propose the social ant Harpegnathos saltator as a novel model system to study how epigenetic factors and gene regulation modulate neuronal circuitry resulting in specific social behaviors. The social ant Harpegnathos saltator is an emerging model to study epigenetics of the social behavior because the distinct caste phenotypes of workers and gamergates are specified by the same genome. Unlike most ant species, Harpegnathos individuals can change caste through adulthood: when the queen dies or is removed from a colony, workers enter a dominance tournament until a few become reproductive individuals, called gamergates. Along with changes to behavior and physiology, gamergate status is accompanied by a six-fold lifespan extension and changes in neuronal gene expression and chromatin organization. This transition to gamergate can also be completely reversed by inducing social defeat in gamergates. Our research will address two major topics of epigenetic regulation in the social brain. First, how do social interactions like social bonding, aggression and social defeat induce epigenetic changes responsible for long-lasting behavioral phenotypes? Second, we plan to develop ant models of autism spectrum disorders by generating mutants of ant homologs to human ASD risk genes. We will study the impact of these mutations on chromatin organization, gene expression and behavior in both castes. Harpegnathos ants can provide mechanistic insights into connection between epigenetic pathways, behavior and neurodevelopmental disorders.