Integrating Natural Variation in Thermal Conditions and Maternal Steroids into Temperature-Dependent Sex Determination

Offspring phenotype is strongly influenced by the environmental conditions experienced early in ontogeny when embryos are rapidly organizing physiological systems. Numerous evolutionary theories posit that processes that match offspring phenotype to their environment can enhance survival and fitness into adulthood. Importantly, the phenotype that is most beneficial for a given environment often differs between sexes. Turtles with temperature-dependent sex determination are a powerful system in which to investigate how early environmental conditions impact offspring phenotype and sex because these traits are affected by both maternally derived steroids and incubation temperatures. The following research focuses on how natural variation in maternal steroids and fluctuating incubation temperatures affect offspring behavior and sex. In Trachemys scripta hatchlings, behavioral types consisting of repeatable and correlated behaviors exist, and are robust to maternal steroids and incubation temperatures. Conversely, offspring sex is affected by both maternal steroid levels and fluctuating incubation temperatures. Late season clutches have higher concentrations of maternally derived estrogens and are more likely to produce female hatchlings. This seasonal increase in female biased sex ratios is exacerbated by subtle increases in diurnally fluctuating incubation temperatures, as would occur across the nesting season. Embryos are sensitive to brief exposures to warm incubation temperatures, which mimic heat waves, with a seasonal pattern in the sensitivity where late season eggs produce more females at any given heat wave duration. Embryos in three climatically disparate populations of T. scripta respond similarly to heat waves during incubation. Models derived from the empirical studies, which were designed to predict sex ratios based on thermal variability, perform better than models based on temperature aggregation methods (e.g. average). Taken together, these data underscore the importance of considering biologically meaningful variation and provide insight into how phenotype is affected by early developmental conditions. The results shed light onto how sex is determined in nature in species with TSD, ultimately informing their conservation and management in rapidly changing environments