Project Summary Successful reproduction of all vertebrate species depends on the differentiation of germ cells into two types of functional gametes: sperm and eggs. Thirty-seven percent of infertility cases are due to female-factor infertility. While the underlying causes of human infertility are often unknown, defects in germ cell sexual identity can lead to infertility and germ cell tumors. Despite its importance to human health, the mechanisms regulating sex-specific germ cell identity and differentiation, and therefore proper oocyte development, are unknown. Oogenesis is a precisely controlled process characterized by conserved periods of meiotic activity, extended arrest, and oocyte growth. Before the first meiotic division the oocyte stops producing new RNAs, therefore, oocytes rely heavily on post-transcriptional regulation of RNA. RNA-binding proteins are the predominant post-transcriptional regulators that coordinate the activation, silencing, and localization of RNAs. Therefore, identifying the targets of RNA binding proteins, and determining their role in oocyte development will lead to a better understanding of the mechanisms and molecular pathways that establish and maintain sex-specific germ cell identity. We utilize zebrafish to identify genes crucial for proper oocyte development, female gonad differentiation and ovary maintenance, and examine the mechanisms underlying establishment and maintenance of germ cell identity. Identifying genes essential to these processes may define new molecular pathways that, when defective, can result in infertility, diminished ovarian reserve, premature ovarian insufficiency, polycystic ovary syndrome, or reproductive cancers in humans. In this proposal, we use a combination of genetic loss of function and overexpression approaches to investigate the mechanisms regulating germ cell identity, sex- specific differentiation, and fertility.