Temperature-dependent sex determination: Upregulation of SOX9 expression after commitment to male development

In mammals, birds and reptiles the morphological development of the gonads appear to be conserved. This conservation is evident despite the different sex determining switches employed by these vertebrate groups. Mammals exhib it chromosomal sex determination (CSD) where the key sex determining switch is the Y-linked gene, SRY, Although SRY is the trigger for testis determin ation in mammals, it is not conserved in other vertebrate groups. However, a gene closely related to SRY, the highly conserved transcription factor, S OX9, plays an important role in the testis pathway of mammals and birds. In contrast to the CSD mechanism evident in mammals and birds, many reptiles exhibit temperature dependent sex determination (TSD) where the egg incubat ion temperature triggers sex determination, Here we examine the expression of SOX9 during gonadogenesis in the American alligator, (Alligator mississi ppiensis), a reptile that exhibits TSD, Alligator SOX9 is expressed in the embryonic testis but not in the ovary. However, the timing of SOX9 upregula tion in the developing testis is not consistent with a role for this gene i n the early stages of alligator sex determination. Since SOX9 upregulation in male embryos coincides with the structural organisation of the testis, S OX9 may operate farther downstream in the vertebrate sex differentiation pa thway than previously postulated, Dev Dyn 1999; 214:171-177. (C) 1999 Wiley -Liss, Inc.