Reduced fitness associated with introgression within the Western Mediterranean admixed population of European seabass

Unraveling the consequences of hybridization on fitness is of main concern in ecology and evolution. Many studies report how evolutionary mechanisms modulate the mosaic of introgression within genomes, but few assessed the associated effect of admixture on fitness traits. Here we took advantage of the Western Mediterranean population of the European seabass (Dicentrarchus labrax), a population resulting from hybridization between the Atlantic and the Mediterranean lineage in the Western Mediterranean Sea. By genotyping 1,488 admixed fish with the Axiom Sea Bass 57k SNP DlabChip array, we assessed the relationship between individual admixture levels and fitness traits in different thermal regimes (19°C, 21°C and 23°C). We first reveal that for admixed individuals with higher levels of Atlantic ancestry, the sex ratio was biased toward males and was more sensitive to temperature treatments. Then we show that admixed individuals with more Atlantic ancestry also had a lower body weight (which is correlated to fecundity in fish), compared to fish with less Atlantic ancestry. These results highlight the fact that introgression of Atlantic ancestry is likely disadvantageous in the Mediterranean basin, which is consistent with the previously observed purging of Atlantic ancestry tracts following hybridization.