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

Unraveling hybridization consequences 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.

The Western Mediterranean population of the European seabass (Dicentrarchus labrax) results from hybridization between the Atlantic and the Mediterranean lineage in the Alboran Sea. By genotyping 1850 admixed fish, we assessed the relationship between individual admixture levels and fitness traits in different thermal regimes (19°C, 21°C and 23°C). Admixed individuals with more Atlantic ancestry displayed a biased sex ratio toward males, and were more sensitive to temperature variation. Additionally, they also had a lower body weight (which is correlated to fecundity in fish), compared to fish with less Atlantic ancestry. Atlantic ancestry is disadvantageous in the Mediterranean basin which is consistent with the previously observed purging of Atlantic ancestry tracts following hybridization.

Methods
Fish are originating from the Western Mediterranean (WEM) European Sea Bass (Dicentrarchus labrax) population.  Fish were reared in common garden at 16°C until 85 days post hatching (dph) in six replicate tanks before being divided into four different thermal treatments: 19°C, 21°C and 23°C with 4 replicates each, until the fish reached about 8 cm in length. Fish were individually tagged, fin-clipped for genotyping, weighed, and sexed through gonad observation.

Fish were genotyped using the Axiom Sea Bass 57k SNP DlabChip array.