Quantitative genetics and GWAS reveal population-specific sex-ratio responses in wild European seabass (Dicentrarchus labrax) under various temperature scenarios.

In populations of fish species, polygenic sex determination (PSD) can evolve in different directions, depending on environmental fluctuations and migration rates. The European seabass provides an interesting model to explore such evolutionary phenomena, because it exhibits a PSD, mainly driven by temperature and has three genetically differentiated populations evolving from a common PSD ancestor. We produced experimental offspring of the three populations (AT = Atlantic, WM = Western Mediterranean, and EM = Eastern Mediterranean) and reared them in four thermal regimes mimicking temperatures encountered in the Atlantic (rAT), Western Mediterranean (rWM), Eastern Mediterranean (rEM), plus a husbandry regime maximizing the proportion of females (rAQUA). For AT, we found a relatively balanced sex-ratio and a significantly higher proportion of females than WM and EM. The WM population had male-biased sex-ratios, especially in the colder regimes (rAT, rWM), with no differences from the EM population. The WM had sex-ratios intermediate between AT and EM in the warmer regimes (rEM, rAQUA). Genetic correlations for sex tendencies were high, and the heritability, around 0.62 ± 0.07, was consistent among populations and thermal regimes. Significant interactions of population x thermal regime were found for sexual size dimorphism (SSD), favoring females, where the SSD for AT fish increased with rising temperatures. GWAS showed seven significant SNPs located in LG19 for the AT population, with one QTL region containing one gene involved in fish sex determination. No QTLs were found for WM and EM. Overall, these findings reveal different adaptive evolution of the sex determination system across the populations.