Mechanisms of behavioral sex plasticity in sex-changing wrasses The biological processes that underlie behavioral sex phenotype are incompletely understood. Organisms that naturally undergo sex transformations in response to changes in their social environment provide excellent systems in which to investigate mechanisms of behavioral sex specification. Bluehead wrasses in particular are well suited for addressing this problem. Adult females can switch sex in response to changes in social structure. Further, behavioral sex transformation can occur rapidly, thereby providing an opportunity to investigate the neurobiological basis of this plasticity. We propose to further develop this system as a model for investigating mechanisms of plasticity of behavioral sex phenotype. The proposed work builds on our ‘priming/gating’ hypothesis of behavioral sex transformation; this hypothesis accounts for the relation between social rank and competency for transformation. Our first objective is to compare the expression of the immediate-early genes, c-Fos and p-S6, in courting and non- courting fish. We expect that results will further elucidate the neural circuits that control the expression of male-typical courtship behavior. To test for causal roles of these circuits in courtship behavior, we plan to iontophorese glutamate to discretely activate particular brain regions and monitor courtship-related changes in skin coloration. Our goal is to identify properties of neural circuits that underlie competency to undergo behavioral sex transformation.