Selection Against Hybrids Maintains Genetic Divergence Between Populations of a Coastal Cleaner Fish Translocated Across a Genetic Break

ABSTRACT Genetic divergence between populations and/or species is mainly driven by reproductive isolation, that is, reduced gene flow, due to either pre‐ and/or postzygotic barriers. In this study, we investigate the role of (i) reproductive ecological interactions and (ii) mechanisms of evolutionary isolation between two genetically differentiated populations of a coastal fish species, corkwing wrasse Symphodus melops , in a large semi‐natural mesocosm basin over a 2‐year period. Corkwing wrasse is one of the main cleaner fish species used by the salmon aquaculture industry in Europe, where translocation is a common practice. The parentage analysis performed on the group of offspring survivors at the end of the translocation experiment evidenced strong selective winter mortalities in hybrid offspring (with mixed parental origin). Additionally, it suggested unprecedented assortative mating patterns in corkwing wrasse. Fish of west Norwegian origin experienced stronger intensity of sexual selection and displayed higher relative reproductive success (RRS) than individuals of southern origin under the experimental conditions. Size of the breeder and alternative sneaking behavior in males are two phenotypic traits often selected to attract potential mates and maximize reproductive fitness. However, neither of them had a significant effect on offspring production. Hence, our results suggest that other traits related to mate choice and competition, not examined here, could play a major role. Our results revealed strong selection against hybrids and potential assortative mating between southern and western corkwing wrasses population as two major contemporary drivers reshaping the historical genetic divergence associated to founder events during post‐glacial recolonization in Norway. We discuss the relevance of our findings for aquaculture‐based fisheries enhancement systems, resource management and conservation.