Temperature-Dependent Sex Determination in the Snapping Turtle: Manipulation of the Embryonic Sex Steroid Environment

Abstract

In certain reptiles with temperature-dependent sex determination (TSD), estrogens act as a signal for female differentiation. Because aromatase produces estrogens from androgens, this enzyme plays a pivotal role in TSD. Whether androgens act as the signal for male differentiation in TSD species is not yet clear. We manipulated the hormonal environment in eggs of the common snapping turtle (Chelydra serpentina) to determine the effects of an estrogen (estradiol 17-β), an aromatase inhibitor (fadrozole; CGS 16949A), and androgens (testosterone and dihydrotestosterone) on sex determination in this TSD species. Test solutions were applied topically to representative eggs (total tested = 1054 from 27 clutches) and incubated at two male-producing temperatures (24 and 26.5°) and at a predominantly female-producing temperature (29°). In this species, application of an estrogen induced female development at all temperatures tested. In contrast, the aromatase inhibitor had no effect at the male-producing temperatures, but induced male development at the predominantly female-producing temperature. At this temperature, aromatase inhibitor plus testosterone had a similar male-producing effect, but when applied alone, testosterone failed to augment male production. Dihydrotestosterone had a similar effect, in contrast to its reported androgenic effects in other TSD species. In the snapping turtle, male differentiation may not be androgen dependent; rather, it may proceed in the absence of female differentiation. In this species, female development is clearly estrogen-dependent and is altered by aromatase inhibition at female-producing temperatures. Our results not only provide additional evidence that sex steroids mediate gonadal differentiation in TSD species, but also suggest caution with respect to generalizations about the proximal mechanisms of TSD in reptiles.