Tracing the Emergence of a Novel Sex-Determining Gene in Medaka, Oryzias luzonensis

Three sex-determining (SD) genes, SRY (mammals), Dmy (medaka), and DM-W (Xenopus laevis), have been identified to date in vertebrates. However, how and why a new sex-determining gene appears remains unknown, as do the switching mechanisms of the master sex-determining gene. Here, we used positional cloning to search for the sex-determining gene in Oryzias luzonensis and found that Gsdf(Y) (gonadal soma derived growth factor on the Y chromosome) has replaced Dmy as the master sex-determining gene in this species. We found that Gsdf(Y) showed high expression specifically in males during sex differentiation. Furthermore, the presence of a genomic fragment that included Gsdf(Y) converts XX individuals into fertile XX males. Luciferase assays demonstrated that the upstream sequence of Gsdf(Y) contributes to the male-specific high expression. Gsdf is downstream of Dmy in the sex-determining cascade of O. latipes, suggesting that emergence of the Dmy-independent Gsdf allele led to the appearance of this novel sexdetermining gene in O. luzonensis.