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

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.

The specially designed website will be our principle means of disseminating the outputs of our project to i) scientists, ii) NGOs, iii) the general public.
We will present our work at specialist meetings that include scientists and other interested parties from outside the University sector. In particular, we will target the Sea Turtle Symposium (https://internationalseaturtlesociety.org/) which brings together scientists and NGOs. This yearly meeting forms the core of the international sea turtle network with >3000 attendees. Because it brings together scientists, NGOs and stakeholders, it offers the opportunity to expose the new guidelines of management which will emerge from our research. We will also disseminate our results through more formal conference papers (by the PDRAs), scientific papers in top tier generalist journals and more specialist journals. By 2021, we will engage with the British Ecological Society to assemble a virtual issue on TSD species and the effect of climate variation. We are familiar with such special issues (e.g. Molecular Ecology [Impact factor: 6.1] http://onlinelibrary.wiley.com/doi/10.1111/mec.2013.22.issue-3/issuetoc or Evolutionary Applications [Impact factor: 5.0] http://onlinelibrary.wiley.com/doi/10.1111/eva.2014.7.issue-9/issuetoc) and British Ecological Society (https://besjournals.onlinelibrary.wiley.com/hub/eco-evo-dynamics2019) and therefore are confident to assemble high quality contributions.

We will also build a free application - HatchIt - which will integrate the detected maternal effects into a population dynamics model guided by temperature. Compared to other such applications which rely solely on temperature, here we will add the fact both local adaptation and maternal effect can modify the pivotal temperature of a clutch and hence affect population dynamics. This will provide a tool for conservation groups to guide the decision of translocating nests into hatchery or not. Ultimately those groups will better determine the sex ratio and the fate of their local population with regards to global warming.

From a global perspective, we will discuss with the IUCN sea turtle specialist group to recommend the new guidelines emerging from our research. Eizaguirre will contribute to the next re-evaluation of the sea turtles' status planned for June 2020, under the organization of the Exploration of Monaco. We will lobby to increase research to determine the role of sex steroid hormones on sex ratio and the initiation of a survey to verify the accuracy of model prediction.
How turtles and TSD-species can cope with global change is of significant general interest to the public. The fact that one Australian population was discovered to be composed by >99% adult females raised important public concern: the manuscript media metrics performed better than 99% than other articles published in Current Biology (https://bit.ly/2QGBy8l). We will exploit the public interest in sea turtles to produce press-releases on the proposed work as well as the results. We will use our contact with journalists like Matt McGrath (Environment correspondent, BBC News, http://www.bbc.com/news/science-environment-25505612) to produce such documents. We will also build on Eizaguirre being a National Geographic Explorer since 2019 (Grant NGS-59158R-19) having access to specific media training and network.

The milestones and measures of success are: 1) We anticipate publications to arise by the end of the second year of the project and extend well beyond the project end. 2) More immediate assessment of the success of the transmission of our scientific findings can be made by monitoring the degree to which specific pages of our web site are accessed (i.e. via "hit rates" monitored by access loggers) and 3) the usage of our free application. 4) Crucially, the success of the research we propose will be measured in terms of its impact on strategic policy development.