The role of sex steroid hormones against global warming in species with temperature-dependent sex determination

全球变暖威胁着全球400多种依赖温度的性别决定物种，包括许多爬行动物，如海龟。在发育过程中，不断升高的温度会影响性类固醇激素的转化，只产生一种性别，进而危险地影响种群的性别比例。例如，在拥有世界第三大龟类聚集地的佛得角，预计到2100年，海龟数量增加3摄氏度将导致99%的雌性新生儿出生。如果这些预测是正确的，这个物种将灭绝，因此我们需要对性别决定有更好的生理理解，以便应用于管理策略。自从海龟首次出现在120 Mya的化石记录中以来，它们就暴露在大规模的气候变化中。因此，它们很可能进化出了尚不为人所知的生理机制，以缓冲温度变化导致的性别比偏差。重要的是，这些生理变化可能跟不上当前全球变暖前所未有的速度。但如果他们这样做了，我们预测未来的模型是错误的，因此我们需要重新分配有限的资源，例如重新考虑鸟巢搬迁的主要缓解战略。在海龟中，性别主要是通过催化酶芳香化酶将雄激素(睾酮)转化为雌激素(雌二醇)来决定的。起初，这些激素的浓度来自母体转移到卵子中。虽然这种进化机制可能在启动芳香酶活性方面发挥了关键作用，但它受到温度的调节以及随后对性别比的影响仍然完全未知。胚胎发育到一半时，胚胎本身就开始产生睾酮和芳香酶。这一催化过程是依赖于温度的，温度是否影响睾酮、芳香酶的浓度或其催化效率仍有待确定。然而，现在可以在大肠杆菌中表达芳香酶基因(CYP19A)，并进行体外测试，以揭示在不同范围的受控温度和母体启动条件下性别决定的机制。到目前为止，还不能非致死地确定新生儿的性别，因此性别比也不能确定，这限制了对濒危海龟性别决定的体内生理机制的研究。在最近的一份预印本中，我们介绍了一种新的方法，该方法基于对血液中睾酮和雌二醇的定量，可以在不杀死海龟新生儿的情况下确定它们的性别。在这里，我们将利用这种方法来测试依赖温度的母体激素转移到卵子中是如何提供底物的，以便在发育过程中被芳香酶催化。我们假设，这种生理机制与孵化温度相结合，形成了调整性别比例的过程，以应对全球气温上升导致的种群灭绝风险。我们将使用这些新的见解来设计预测模型，并测试缓解策略的效果，例如在不同温度下将巢重新安置到孵化室。

Global warming threatens over 400 species with temperature-dependent sex determination (TSD) worldwide including many reptiles such as sea turtles. During development, increasing temperatures affect the conversion of sex steroid hormones to produce only one sex and, in turn, dangerously bias populations' sex ratios. For example, in Cabo Verde, which holds the third largest aggregation of loggerhead turtles (Caretta caretta) in the world, an increase of 3C is predicted to result in >99% female neonates by 2100. If these predictions are correct this species will go extinct, and therefore we need a better physiological understanding of sex determination that can be applied to management strategies. Since their first appearance in the fossil records 120 Mya, sea turtles have been exposed to large scale climatic changes. It is therefore likely they have evolved still undescribed physiological mechanisms that buffer against sex ratio bias driven by temperature variation. Importantly, these physiological changes may not keep up with the unprecedented pace of current global warming. But if they do, our models that predict the future are wrong, and hence we need to redirect limited resources, for instance in rethinking the main mitigation strategy of nest relocation. In turtles, sex is mainly determined by the conversion of androgens (testosterone) to estrogens (estradiol) by the catalytic enzyme aromatase. At first, concentrations of these hormones originate from maternal transfer into the eggs. While this evolutionary mechanism may play a key role in priming the aromatase activity, its regulation by temperature and subsequent influence on sex ratios remains entirely unknown. Half way through embryogenesis, the embryo itself starts producing testosterone as well as the aromatase enzyme. This catalytic process is temperature-dependent, and whether temperature affects the concentration of testosterone, of aromatase or its catalytic efficiency remains to be established. It is, however, now possible to express the aromatase gene (CYP19A) into E. coli and implement in vitro tests to uncover the mechanisms of sex determination under a diverse range of controlled temperatures and maternal priming conditions. Until now, sex of neonates, and therefore sex ratios, could not be determined non-lethally, limiting the study of the in vivo physiological mechanisms underlying sex determination in endangered sea turtles. In a recent pre-print, we introduced a new method based on the quantification of testosterone and estradiol from a blood drop to determine the sex of turtle neonates without killing them. Here, we will exploit this method to test how temperature-dependent maternal hormone transfer into the eggs provides the substrates to be catalysed by the aromatase during development. We hypothesize this physiological mechanism, combined with incubation temperatures, forms the process that adjusts sex ratio against the risks of population extinction driven by globally increasing temperatures. We will use these new insights to design predictive models and test the effect of mitigation strategies such as nest relocation at different temperatures into hatcheries.