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.