Seascape genetics of a flatfish reveals local selection under high levels of gene flow

Summary

Local adapt ation is often found to be in a delicate balance with gene flow in marine species with high dispersal potential. Genotyping with mapped transcriptome-derived markers and advanced seascape statistical analyses are proven tools to uncover t he genomic basis of bio logically relevant traits under environmental selection. Using a panel of 426 gene-linked single nucleotide polymorphisms, we scanned 17 samples and 539 individuals of sole Solea solea L. from the Northeast Atlantic Ocean and applied a node-based seascape analysis. Neutral loci confi rmed a clear distinction between t he North Sea - Baltic Sea transition zone and t he other Eastern Atlantic samples. At a more subt le leve l, the latter unit split in an English Channel and North Sea group, and a Bay of Biscay and Atlantic Iberian coast group. A fourth group, the Irish and Celtic Sea, was identified with 19 outlier loci. A pattern of isolation by distance charact erized t he latitudinal distribution. Seascape analyses identified winter seawat er temperature, food availability and coasta l currents to explain a significant component of geographi ca lly distributed genetic variation, suggesting that these factors act as drivers of local adaptation. The evidence for local adaptation is in line with the cu rrent understanding on the impact of two key ecological factors, the life-history trait winter mortality and the behavior of inshore and offshore spawning. We conclude that the subtle differentiation observed between two metapopulations, North Sea and Bay of Biscay, mirrors local adaptation. At least t hree genomic regions with strong population differentiation point to locally divergent selection. Further functional characterization of t hese genomic regions should help with formulating adaptive management policies.