PhD Candidate B.S. (2016) Presbyterian College Vertebrate sex determination is commonly regulated by a master sex determination gene that initiates differentiation into a male or female. A wide variety of genes have been co-opted for this function, but how genes become master regulators of sex determination is not well understood. My research aims to identify the master sex determining gene in threespine stickleback (Gasterosteus aculeatus) and determine how this gene regulates conserved downstream elements in the sex differentiation pathway. My findings will define the mechanism of sex determination in threespine stickleback and clarify how sex determination genes evolve.