To investigate the role of SOX9 gene in amphibian gonadogenesis, we analyzed its expression during male and female gonadogenesis in Xenopus tropicalis. The results showed that in both sexes SOX9 mRNA and protein were first detectable after metamorphosis when the gonads were well differentiated and remained present until the adult stage. In the testis, SOX9 expression was restricted to the nucleus of Sertoli-like cells, similarly to what has been observed in other vertebrates suggesting a conserved role in vertebrate testicular differentiation. In the ovary, in sharp contrast with what has been observed in all vertebrates examined so far, the SOX9 protein was localized in the cytoplasm of previtellogenic oocytes before being translocated into the nucleus of vitellogenic oocytes suggesting an unexpected role during oogenesis. These results suggest that the SOX9 gene may not be a sex-determining gene in X. tropicalis and may play different functions in testicular and ovarian differentiation. Developmental Dynamics 237:2996 -3005, 2008.

In an effort to contribute to the development of Xenopus tropicalis as an amphibian model system, we carried out a detailed histological analysis of the process of gonadal sex differentiation and were able to find evidence that gonadal differentiation in X. tropicalis follows an antero-posterior gradient. Although the main reason for the presence of a gradient of sex differentiation is still unknown, this gradient enabled us to define the early events that signal ovarian and testicular differentiation and to identify the undifferentiated gonad structure. Given the various advantages of this emerging model, our work paves the way for experiments that should contribute to our understanding of the dynamics and mechanisms of gonadal sex differentiation in amphibians.