We shall meet again - Genomics of historical admixture in the sea

We studied the impact of genetic introgression in evolution and on evolutionary studies with whole-genome data from two divergent lineages of sticklebacks. Our results reveal that the hybrid zone between the lineages ranges across the entire Baltic Sea and parts of the North Sea with the foreign ancestry decreasing with increasing distance to the source population. Introgression has also penetrated currently isolated freshwater populations. We identified footprints of selection on regions enriched for introgressed variants, suggesting that some of the introgression has been adaptive. However, the levels of introgression were in general negatively correlated with the recombination rate, suggesting that the introgression has been largely neutral and adaptive ancestral standing variation likely had a more important role in shaping the genomic landscape. Our results further suggest that overlooked introgression can mislead analyses of local adaptation and phylogenetic affinities, highlighting the importance of accounting for introgression in studies of local adaptation.