Population genomic study of the threespine stickleback hybrid zone in the Misty Lake watershed (Vancouver Island, Canada)

In the Misty Lake watershed (Vancouver Island, Canada), lake- and inlet stream-resident threespine stickleback fish ecotypes exhibit a steep phenotypic and genomic shift across the lowest section of the inlet stream. Based on individual-level whole-genome sequence data, this study focuses on stickleback from this contact zone and demonstrates extensive dispersal of lake individuals into the stream, as well as hybridization between the ecotypes. The hybrids are generally deeply admixed, indicating long-standing population mixing. With increasing distance from the habitat transition, hybrids rapidly converge toward the genomic composition typical of the stream ecotype, consistent with strong selection against genomic material from the lake within the stream habitat. Overall, this study demonstrates that divergent selection can maintain strong reproductive isolation on a small spatial scale even in the absence of physical dispersal barriers.