ABSTRACT The goal of this proposal is to generate genetic reporter lines for neuronal circuits that regulate social behavior in Harpegnathos saltator ants and to utilize these lines to visualize brain remodeling as adult workers reprogram themselves to behave as ant queens. Social insects are an emerging model system to study the epigenetic regulation of brain function and behavior because the same genome specifies alternative behavioral states in different castes. Whether and how brains of workers and queens are differently wired to implement distinct sets of caste-specific behaviors is not known. In Harpegnathos ants, adult workers can become queens via a caste transition that involves dramatic phenotypic changes in reproduction, physiology, and behavior. This feature of Harpegnathos ants allows for easy propagation of engineered alleles, as any individual can become reproductive. The ability of adult Harpegnathos workers to become queens offers unique opportunities for the mechanistic dissection of adult brain plasticity. Single-cell RNA-seq analyses revealed major changes in cellular composition affecting both neurons and glia, indicating that a structural remodeling of the brain accompanies the caste transition. The molecular signals that direct this brain remodeling remain unknown. We previously showed that the neuropeptide corazonin stimulates hunting, a worker-specific behavior, and is downregulated as workers become queens. We have since obtained new preliminary evidence that the ant homolog of human vasopressin is expressed in a caste-specific manner and likely also regulates a subset of social behaviors in Harpegnathos ants. We hypothesize that signaling by these neuropeptides directs brain remodeling events that underpin the switch in social behaviors during the caste transitions. In Aim 1, we will utilize a transgenic approach to label corazonin- and vasopressin-producing neurons and their projection with membrane-bound GFP and to observe their remodeling during the natural caste transition. In Aim 2, we will utilize CRISPR/Cas9 to generate knock-in driver and reporter lines to label neurons that respond to corazonin or vasopressin and visualize their plasticity at the cellular and synaptic level in response to the neuropeptides. These experiments will reveal crucial information on adult brain plasticity in Harpegnathos ants and provide sophisticated genetic tools for the further dissection of the epigenetic regulation of social behavior in these ants. Given that corazonin and vasopressin have mammalian counterparts, our results are expected to have a broad impact on our understanding of how neuropeptides regulate brain plasticity and social behavior.