Differential expression of SOX9 in gonads of the sea turtle Lepidochelys olivacea at male- or female-promoting temperatures

In mouse and chick embryos, the SOX9 gene is down-regulated in genetic fema les whereas in genetic males it remains in the Sertoli cells. We studied th e distribution of SOX9 protein in developing genital ridges of embryos of t he sea turtle Lepidochelys olivacea incubated at male- or female-promoting temperatures, using the antibody for detection. At stages 22-24, cells in m edullary cords show SOX9 positive nuclei, while coelomic epithelial cells a ppear negative. At stage 25 however, most medullary cells are SOX9 negative and at the female-promoting temperature, and from stage 26 onwards, SOX9 p rotein is not detected. At the male-promoting temperature, medullary cords remain SOX9-positive at all stages. These results suggest that SOX9 is up-r egulated in Sertoli cells irrespective of primary sex-determining switch. S ex is irreversibly determined at stage 24 or 26 at the male- or female-prom oting temperature, respectively (Merchant-Larios et al.,'97). The present r esults suggest that there is a correlation between SOX9 expression and sex determination in the olive ridley. At the male-promoting temperature, Serto li cells expressing SOX9 become committed at stage 24 and. male sex is dete rmined, whereas at the female-promoting temperature, SOX9 is down-regulated at stage 26 and female sex is determined. J. Exp. Zool. 284:705-710, 1999. (C)1999 Wiley-Liss, Inc.