Abstract
Oestradiol application during embryonic development reverses the sex of male embryos and results in normal female differentiation in reptiles lacking heteromorphic sex chromosomes, but fails to do so in birds and mammals with heteromorphic sex chromosomes. It is not clear whether the evolution of heteromorphic sex chromosomes in amniotes is accompanied by insensitivity to oestradiol, or if the association between oestradiol insensitivity and heteromorphic sex chromosomes can be attributable to phylogenetic constraints in these taxa. Turtles provide an ideal system to examine the potential relationship between oestradiol insensitivity and sex chromosome heteromorphy, since there are species with heteromorphic sex chromosomes that are closely related to species lacking heteromorphic sex chromosomes. We investigated this relationship by examining the long-term effects of oestradiol-17beta application on sex determination in Staurotypus triporcatus and Staurotypus salvinii, two turtle species with male heterogamety. After raising the turtles in the lab for 3 years, we found follicular and Müllerian duct morphology in oestradiol-treated turtles that was identical to that of untreated females. The lasting sex reversal suggests that the evolutionary transition between systems lacking heteromorphic sex chromosomes and those with heteromorphic sex chromosomes is not constrained by a fundamental mechanistic difference.
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