Genome‐wide single‐generation signatures of local selection in the panmictic E uropean eel

Abstract

Next‐generation sequencing and the collection of genome‐wide data allow identifying adaptive variation and footprints of directional selection. Using a large SNP data set from 259 RAD ‐sequenced E uropean eel individuals (glass eels) from eight locations between 34 and 64 o N, we examined the patterns of genome‐wide genetic diversity across locations. We tested for local selection by searching for increased population differentiation using F ST ‐based outlier tests and by testing for significant associations between allele frequencies and environmental variables. The overall low genetic differentiation found ( F ST = 0.0007) indicates that most of the genome is homogenized by gene flow, providing further evidence for genomic panmixia in the E uropean eel. The lack of genetic substructuring was consistent at both nuclear and mitochondrial SNP s. Using an extensive number of diagnostic SNP s, results showed a low occurrence of hybrids between E uropean and A merican eel, mainly limited to Iceland (5.9%), although individuals with signatures of introgression several generations back in time were found in mainland Europe. Despite panmixia, a small set of SNP s showed high genetic differentiation consistent with single‐generation signatures of spatially varying selection acting on glass eels. After screening 50 354 SNP s, a total of 754 potentially locally selected SNP s were identified. Candidate genes for local selection constituted a wide array of functions, including calcium signalling, neuroactive ligand–receptor interaction and circadian rhythm. Remarkably, one of the candidate genes identified is PERIOD , possibly related to differences in local photoperiod associated with the >30° difference in latitude between locations. Genes under selection were spread across the genome, and there were no large regions of increased differentiation as expected when selection occurs within just a single generation due to panmixia. This supports the conclusion that most of the genome is homogenized by gene flow that removes any effects of diversifying selection from each new generation.

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