Elucidating the molecular network underlying temperature-dependent sex determination in the red-eared slider turtle, Trachemys scripta

Components of the molecular pathway underlying gonadogenesis in organisms with temperature-dependent sex determination (TSD) have been retained from genetic sex determination. Furthermore, although much of this network has been conserved, new functions for these genes have evolved in this different mode of sex determination. We find that the transcription factors Sox9 and Dmrt1 and the hormone Mis are involved in the formation of a testis and/or the repression of an ovary at a male-producing temperature. While Mis expression may be maintained by Sox9, the initial upregulation of Mis in the developing testis is most likely modulated by some other upstream factor. Dmrt1 appears to play an upstream role in testis sex determination. We provide evidence that the transcription factor Dax1 and the signaling molecule Wnt4, cloned for the first time in an organism with TSD, play roles in gonadogenesis in both sexes. Finally, we show that the transcription factor FoxL2 and the signaling molecule Rspo1 are involved in the formation of an ovary and/or the repression of a testis at a female-producing temperature. In the first investigation of Rspo1 in any organism exhibiting TSD, we demonstrate it is involved upstream in ovarian sex determination. Complementary to descriptive studies, we optimize a whole organ culture system in which gonad explants develop in vitro for up to three weeks. We show that expression of the sex-determining network in isolated gonads mimics in ovo patterns, revealing an endogenous temperature-sensing mechanism that does not require other embryonic tissues. Ectopic expression of Sox9 reveals a possible positive feedback regulation of Dmrt1. The use of this culture system opens the door to functional manipulation of the gonad at the molecular level and is suitable for a myriad of future studies. This work makes strides in elucidating the molecular network underlying gonadogenesis in an organism exhibiting TSD, and invites investigation of the evolution of gene function. The data lend insight into the changing roles of molecules in sex determination across diverse taxa, and into the evolution of developmental pathways in general.