A ZZ/ZW-type sex determination in Xenopus laevis

Genetic sex-determining systems in vertebrates include two basic types of heterogamety, which are represented by the XX/XY and ZZ/ZW types. Both types occur among amphibian species. Little is known, however, about the molecular mechanisms underlying amphibian sex determination. Recently, a W-linked gene, DM-W, was isolated as a paralog of DMRT1 in the African clawed frog Xenopus laevis, which has a female heterogametic ZZ/ZW-type sex-determining system. The DNA-binding domain of DM-W shows high sequence identity with that of DMRT1, but DM-W does not contain a domain with homology to DMRT1's transactivation domain. Importantly, phenotypic analysis of transgenic individuals bearing a DM-W-expression or -knockdown vector strongly suggested that DM-W acts as a female sex-determining gene in this species. In this minireview, we briefly describe the sex-determining systems in amphibians, discuss recent findings from the discovery of the DM-W gene in terms of its molecular evolution and its function in sex determination and ovary formation, and introduce a new model for the ZZ/ZW-type sex determination elicited by DM-W and DMRT1 in X. laevis. Finally, we discuss sex-determining systems and germ-cell development during vertebrate evolution, especially in view of a conserved role of DMRT1 in gonadal masculinization.