Data from: Thyroid hormone modulates offspring sex ratio in a turtle with temperature-dependent sex determination

The adaptive significance of temperature-dependent sex determination (TSD) has attracted a great deal of research, but the underlying mechanisms by which temperature determines the sex of a developing embryo remain poorly understood. Here we manipulated the level of a thyroid hormone, triiodothyronine (T3), during embryonic development (by adding excess T3 to the eggs of the red-eared slider turtle Trachemys scripta, a reptile with TSD), to test two competing hypotheses on the proximate basis for TSD: the developmental rate hypothesis vs the hormone hypothesis. Exogenous thyroid hormone accelerated embryonic heart rate (and hence metabolic rate), developmental rate, and rates of early post-hatching growth. More importantly, hyperthyroid conditions depressed expression of Cyp19a1 (the gene encoding for aromatase) and levels of estradiol, and induced more male offspring. This result is contrary to the direction of sex-ratio shift predicted by the developmental rate hypothesis, but consistent with that predicted by the hormone hypothesis. Our results suggest an important role for thyroid hormones in regulating sex steroid hormones, and therefore in affecting gonadal sex differentiation in TSD reptiles. Our study has implications for the conservation of TSD reptiles in the context of global change because environmental contaminants may disrupt the activity of thyroid hormones, and thereby affect offspring sex in TSD reptiles.