A role of the Sox 9 gene in amphibian sex determination

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Sox is a family of SRY-related testis-determining genes. We have isolated two different mRNA isoforms of the frog Sox9 gene from adult frog testis cDNA library. One form (Sox9 α) encodes a 482-amino acid protein containing the HMG box, whereas the other (Sox9 β), which completely lacks the HMG box, is a N-terminal truncated 265-amino acid protein of Sox9 α. Sox9 α is 82 % similar to mouse, 86 % to chicken, and 77 % to trout Sox9s at the amino acid level. In the phylogenetic tree, frog Sox9 was clearly diverged from trout Sox9, and then from chicken and turtle Sox9s. The Sox9 expression was up-regulated in embryos after stage 16, and was seen in both differentiating testis and ovary. The size of Sox9 transcripts was determined to be 7.8 knt by Northern blot analysis. In addition, The expression of Sox9 α was dominant in the testis and brain among various tissues of adult frogs examined, and was considerably higher than that of Sox9 β. Sox9 is expressed at early stages of gonadal development in both sexes, but declined earlier in females. This is dissimilar to the pattern in birds and mammals, in which Sox9 expression is male-specific. In addition, Sox9 α and 9 β expressions were up-regulated in the gonad of female-to-male sex-reversed tadpoles. Taken together, the results suggest that the Sox9 is involved in testicular differentiation of amphibians. Since the Sox9 expression is found in both the testis and ovary of frogs at early stages of gonadal development, its role is probably different from that in birds and mammals.