Abstract
Understanding how genetic differentiation is maintained between closely related species with overlapping habitats is a central question in evolutionary biology. The sticklebacks Gasterosteus aculeatus and Gasterosteus nipponicus occur in sympatry with reproductive isolation on the Pacific coast of Hokkaido Island. However, their distributions and hybridization patterns in more northern regions remain largely elusive. Here, we analyzed species distributions and hybrid occurrences in Sakhalin, the Northern Territories, the Kuril Islands, the Pacific and Okhotsk coasts of Kamchatka, Chukotka, and the open Bering Sea using genome-wide nucleotide variant data. We found that G. nipponicus is largely restricted to Sakhalin, while G. aculeatus predominates in other regions, with only rare hybrids detected. These findings indicate largely allopatric distributions around the Sea of Okhotsk and the Bering Sea. These allopatric populations provide valuable opportunities for comparing patterns of genetic differentiation and phenotypic divergence between allopatric and sympatric species pairs.