Sex determination and gonadal development in the common toad (Bufo bufo)

Amphibians have been traditionally used as animal models in embryology. Nevertheless, the knowledge about the mechanisms involved in sex determination and differentiation are very scarce in this group. In the species Bufo bufo there are no sex chromosomes morphologically distinguishable. However, by analysing the sex ratio of the offspring of sex reversed animals it is possible to conclude that the females are the heterogametic sex in this species. That is, the species Bufo bufo has a ZZ/ZW chromosomes system for sex determination. Regarding to gonadal development, the species of the genus Bufo, have the particularity of developing undifferentiated ovaries (Bidder's organs) in the cranial portion of male and female gonads. To study gonadal development in B. Bufo, we are cloning in this species orthologous genes to those involved in gonadal development in other vertebrate groups. The study of the expression pattern of these genes during gonadal differentiation, and its relation with the morphological changes that take place, will enable us to establish their role in this process. SOX9 gene, involved in Sertoli cell differentiation, is among the genes we are studying. This gene codes for a transcription factor closely related to SRY gene, is male-specific and necessary for testis differentiation. In B. bufo this gene is expressed only in developing male gonads, starting at the stage where morphological differences start to be observed between developing testes and ovaries.