Are reptiles predisposed to temperature-dependent sex determination?

Vertebrates show an astonishing array of sex determining mechanisms, including male and female heterogamety, multiple sex chromosome systems, environmental sex determination, parthenogenesis and hermaphroditism. Sex determination in mammals and birds is extraordinarily conservative compared to that of reptiles, amphibians and fish. In this paper, we explore possible explanations for the diversity of sex determining modes in reptiles, and in particular, address the prevalence of reptilian temperature-dependent sex determination (TSD) and its almost haphazard distribution across the reptile phylogeny. We suggest that reptiles are predisposed to evolving TSD from genotypic sex determination (GSD) by virtue of the uniquely variable thermal environment experienced by their embryos during the critical period in which sex is determined. Explicit mechanisms for canalization of sexual phenotype in the face of high thermal variation during development provide a context for thermolability in sex determination at extremes and the raw material for natural selection to move this thermolability into the developmental mainstream when there is a selective advantage to do so. Release of cryptic variation when canalization is challenged and fails at extremes may accelerate evolutionary transitions between GSD and TSD.