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

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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.