The gonad, which arises as a bipotential primordium, is an ideal model system for studying the processes of cell fate determination and organogenesis. The primary step in sex determination is the decision that occurs within the bipotential gonad to differentiate as a testis or ovary. This decision guides subsequent sexual differentiation throughout the embryo. Underlying this decision is a complex transcriptional network composed of male and female sub-networks that act in an antagonistic manner to transiently balance the gonad between its alternate fates. In XX gonads, the female sub-network dominates and represses the male pathway. However, expression of the Y-linked male sex-determining gene, Sry, tips the balance in XY gonads in favor of the male pathway, which leads to the organization and differentiation of gonadal cells into a testis. We have employed a genetic systems level approach to investigate the dynamics of the transcriptome underlying the cell fate decision that occurs in gonadal cells. We have determined the origin of several key cell types of the testis, and identified a number of pathways that lead to the architectural patterning of the testis and ovary. We are also interested in comparing sex determination and gonadogenesis in the red-eared slider turtle, where sex is determined by theincubation temperature of the egg rather than by a sex chromosome difference. Finally, we are very interested in how interactions with the somatic cells of the gonad regulate the latent totipotency of germ cells and control their differentiation as oogonia or spermatogonia. Experimental approaches include the use of molecular and biochemical techniques, advanced imaging methods, null mutant mice, transgenic reporter mice, organ culture/tissue recombination assays, mouse genetics and genomics, and systems biology.