Résumé
Water warming and salinity increases due to climate change are having drastic effects on fish reproduction, sex differentiation, growth, survival and consequently in general biodiversity, particularly in Africa. In several fish species, such as the Nile tilapia, high temperatures may induce female to male sex-reversal. The proportion of phenotypic males and females, and the mismatch with their genotypic sex can now be analysed in wild populations thanks to the identification of the sex-determining gene. The master male gene is a Y chromosome gene of the anti-müllerian hormone (amh). Surprisingly, our genome analysis evidenced variations in the sex determiners within populations of Nile tilapia. When amhY was present, Y and X chromosome markers showed that the majority of the populations studied were sensitive to high temperatures, inducing sex reversals of F1 under control conditions, and with the finding of wild XX males. We have also found several XY females in wild populations, which are potentially due to feminising effects of the combination of high temperatures and xenobiotics. In Senegal, evaporation and reduced rainfall together with Sahelian droughts have led to the inverse Saloum estuary with hypersalinity (up to 130 g.kg-1) in the upper reaches, whereas salinity in downstream reaches is that of seawater. The black-chinned tilapia is the only fish species capable to adapt to these hypersalinities, but populations are also found in brackish and freshwaters. Experimental short-term salinity challenges have evidenced the incredible capacity of this species to osmoregulate, with increased GH levels with salinity, as well as an endocrine (liver) and auto/paracrine (gills, intestine, kidney) involvement of IGF-I and IGF-II. Salinity differences also affect modifications in immune organs. A genome-wide study of six wild populations in Senegal living along a freshwater to hypersaline gradient, showed significant genetic differentiation between populations. Population structure and gene diversity reflect historical processes that correlate with geography, but also suggest adaptive variation. Our analyses gave enriched functional terms related to osmosensing and in cellular stress response, but mostly showed that immune-related pathways appear to play an important role in the adaptive process of the black-chinned tilapia to salinities.