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 (TH), 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 versus the hormone hypothesis. Exogenous TH 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 oestradiol, 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 THs 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 THs, and thereby affect offspring sex in TSD reptiles.温度依赖型性别决定（temperature-dependent sex determination, TSD）的适应性意义已吸引了大量研究关注，但温度如何决定发育中胚胎的性别，其潜在机制仍未被充分阐明。本研究在胚胎发育阶段对甲状腺激素（thyroid hormone, TH）——三碘甲状腺原氨酸（triiodothyronine, T3）——的水平进行了调控（通过向具备TSD机制的爬行动物红耳滑龟（Trachemys scripta）的卵中添加过量T3），以检验关于TSD近因机制的两种竞争性假说：发育速率假说与激素假说。外源性甲状腺激素可加快胚胎心率（进而提升代谢速率）、发育速率以及孵化后早期生长速度。更关键的是，甲状腺功能亢进状态会抑制Cyp19a1（编码芳香化酶的基因）的表达以及雌二醇水平，并诱导产生更多雄性后代。该结果与发育速率假说所预测的性别比例偏移方向相悖，但与激素假说的预测结果相符。我们的研究结果表明，甲状腺激素在调控性类固醇激素方面发挥着重要作用，进而影响TSD爬行动物的性腺性别分化。本研究对于全球变化背景下的TSD爬行动物保护具有启示意义，因为环境污染物可能会破坏甲状腺激素的活性，从而影响TSD爬行动物的后代性别比例。