Molecular Evolution of Sex-determining Genes during Vertebrate Evolution

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In the XX/XY sex-determining system, the Y-linked SRY genes of most mammals and the DMY/DMRT1Y gene of the teleost fish medaka have been characterized as sex-determining genes that initiate testis formation, leading to male sexual development. In contrast, the molecular mechanism for the ZZ/ZW sex-determining system remains unclear, because no sex-determining genes have been isolated. The South African clawed frog Xenopus laevis uses the ZZ/ZW system. A DM-domain gene DMRT1 is implicated in testis formation of various vertebrate species. Then, we first showed that X laevis DMRT1 is expressed during embryogenesis, and is then restricted to the primordial gonads. Next, we isolated a W-linked paralogue of DMRT1, DM-W, in X laevis. Although the DNA-binding domains of DM-W and DMRT1 shared high sequence identity (89%), their C-terminal regions had no significant sequence similarity. A comparative analysis of the DM-W and DMRT1 mRNA expression patterns showed that DM-W was expressed predominantly in the primordial ZW gonads during early sex determination. These findings and phenotypic analyses of transgenic animals carrying DM-W expression vectors indicated that the W-linked gene, DM-W, is most likely to be a sex (female)-determining gene in X. laevis.