Data from: Reproductive isolation in a three-way contact zone

Contact zones between divergent forms within a species provide insight into the role of gene flow in adaptation and speciation. Previous work has focused on contact zones involving only two divergent forms, but in nature many more than two populations may overlap simultaneously and experience gene flow. Patterns of introgression in wild populations are, therefore, likely much more complicated than is often assumed. We begin to address this gap in current knowledge by investigating patterns of divergence and introgression across a complex natural contact zone. We use phenotypic and genomic data to confirm the existence of a three-way contact zone between divergent freshwater resident, saltwater resident and saltwater migratory three-spined stickleback (Gasterosteus aculeatus) on the island of North Uist, Scottish Western Isles. We find evidence for hybridisation, mostly between saltwater resident and saltwater migratory forms. Despite hybridisation, genomic analyses reveal pair-wise islands of divergence between all forms that are maintained across the contact zone. Genomic cline analyses also provide evidence for selection and / or hybrid incompatibilities in divergent regions. Divergent genomic regions occur across multiple chromosomes and involve many known adaptive loci and several chromosomal inversions. Our results suggest that reproductive isolation is maintained in this three-way contact zone, despite some hybridisation, and that reduced recombination in chromosomal inversions may play an important role in maintaining this isolation.