Endogenous Yolk Steroid Hormones in Turtles with Different Sex-Determining Mechanisms

Abstract

Maternal transfer of nutrients, including steroid hormones, to embryos during gestation in viviparous amniotes is well known, but the concordant process in oviparous amniotes is poorly understood. Recent evidence suggests that steroid hormones are present in freshly laid eggs of archosaurs and that their concentrations may influence offspring phenotypes. This process might be especially important in reptiles with temperature-dependent sex determination (TSD), because embryonic sex determination as well as other traits is thought to be influenced by temperature-dependent steroid metabolism. To evaluate the potential importance of endogenous steroid hormones to developing reptilian embryos, we (1) measured testosterone and estradiol-17β levels in fresh eggs from natural populations of three species of turtles with TSD and two with genotypic sex determination (GSD), (2) evaluated testosterone concentrations within and among clutches of two turtle species with TSD, and (3) correlated clutch testosterone concentrations with incubation length, body mass, and sex ratio of offspring of two turtle species with TSD from eggs incubated at pivotal sex-determining temperatures. Turtles with TSD had higher levels of yolk testosterone (up to ≈300 ng/g) than turtles with GSD (up to ≈15 ng/g), suggesting a potential functional dichotomy between these two classes of sex-determining mechanisms; concentrations of yolk estradiol-17β were low and fairly uniform among all five species (≈1 ng/g). Yolk testosterone varied substantially among, and relatively little within, clutches, indicating considerable potential as a mechanism behind clutch effects on offspring phenotypes. Steroid concentrations were unrelated to order of oviposition, unlike in birds, but yolk testosterone levels were correlated with incubation length and possibly with offspring sex ratio. Embryos ofChelydra serpentina serpentinafrom clutches with higher yolk testosterone hatched soonest at 21.8°C; the opposite was true forTrachemys scripta elegansembryos incubated at 28.2°C. At 27.6°C,C. s. serpentinaclutches with higher yolk testosterone produced more male-biased offspring sex ratios. Taken together, these results are strikingly consistent with published data on other oviparous amniotes and with the known physiology of follicular development and embryonic sexual differentiation in these disparate taxa. The findings of these experiments suggest that yolk testosterone in turtle eggs may be biologically significant.