Project Summary Animals of many species develop as two anatomic sexes that perform distinct and complex social behaviors. Across clades, master regulator transcription factors downstream of sex determination programs “switch” distinct brain regions and neurons involved in sex-typical behaviors by altering neuron number, anatomy, and connectivity. In insects, the male-specific transcription factor Fruitless (FruM) is required during development for mating behaviors, including innate courtship behavior towards conspecific female virgins. FruM changes the number, anatomy and connectivity of neurons which comprise the circuit. However, how FruM executes these disparate genomic programs during development, when these programs hard-wire mate choice, and how the developmental landscape constrains the function of FruM is still poorly understood. To address these questions, we propose to investigate how FruM establishes and maintains changes in neurons of the courtship circuit by altering chromatin accessibility of regulatory elements across constituent neurons. We will investigate 1) the repressive activities of FruM on neurodevelopmental genes in circuit connectivity and function, and 2) determine how and when FruM acts on the chromatin landscape to hard-wire courtship behavior. In this proposal, we will thereby elucidate the molecular and circuit function of FruM that hard-wires mate choice and courtship behavior during development of the circuit. This F31 proposal describes a comprehensive training and mentorship program for the applicant, a Ph.D. candidate in the Cellular and Molecular Biology program at the University of Michigan. The applicant will participate in a rigorous didactic and laboratory training curriculum, supervised by her sponsor, Dr. E. Josie Clowney, and co-sponsor, Dr. Scott Barolo. She will receive additional support from a multi-disciplinary thesis committee and the extended neuroscience community at the University of Michigan. This training plan includes extensive and rigorous molecular genetics and systems neuroscience laboratory training, as well as mentored opportunities to engage in scientific writing, presentations, and grant applications. The ultimate goal of this proposal is to best position the applicant for an independent and productive scientific career.