Timing of SOX9 downregulation and female sex determination in gonads of the sea turtle Lepidochelys olivacea

The SRY-related gene SOX9 is involved in the differentiation of Sertoli cel ls in male gonads of vertebrates with different kinds of sex determination. In the olive ridley Lepidochelys olivacea, a species with temperature sex determination (TSD), the SOX9 protein is expressed at stages 21-24 in medul lary cells in gonads of embryos incubated at both male-(MPT) or female-prom oting temperatures (FPT). However, at FPT the expression of SOX9 protein de creases at stage 25 and disappears at stage 26, suggesting this as the crit ical period for SOX9 regulation by temperature. Here, we used reverse trans criptase polymerase chain reaction (RT-PCR) to detect SOX9 transcripts in g onads of embryos switched from MPT to FPT at stage 23 and sampled at days 6 -14. Simultaneously, groups of embryos were switched back to M-PT and gonad al sex was established. SOX9 transcripts were detected at days 6-12 of swit ching, when embryos reached stage 25 and were no longer detected at day 14, when the embryos were at stage 26. Embryos switched back to MPT at days 6 or 8 formed testes, whereas embryos switched at days 10 or 14 developed ova ries. Results suggest that at MPT the male sex-determining pathway that mai ntains SOX9 expression in male gonads is established at stage 24. In contra st, at FPT, the female sex-determining pathway involved in downregulation o f SOX9 in female gonads occurs within two days at stage 25. J. Exp. Zool. 2 90:498-503, 2001. (C) 2001 Wiley-Liss, Inc.