Genetic structures of the Japanese stone loach Barbatula oreas (Cypriniformes: Nemacheilidae) in Sakhalin and Hokkaido: back dispersal from Hokkaido to Sakhalin

Understanding how dispersal and vicariance shape species distributions is a central goal in biogeography, yet the role of islands as sources of continental diversity remains poorly resolved. While multiple dispersal routes from the Eurasian continent to the Japanese Archipelago have been proposed, back dispersal from islands to the mainland is rarely documented, particularly in primary freshwater taxa constrained by marine barriers. Here, we investigated the population genetic structure and phylogeographic history of the Japanese stone loach Barbatula oreas, distributed in Hokkaido and Sakhalin, using mtDNA, nDNA, and genome-wide SNP data. We identified two differentiated Northern and Southern lineages within Hokkaido that diverged during the Pleistocene, indicating that geological events such as paleo-catchment reorganization, mountain uplift, and volcanic activity have shaped the present population structure. Ancestral area reconstruction based on mtDNA phylogeny identified Hokkaido as the origin of B. oreas and revealed dispersal from Hokkaido to Sakhalin, indicating back dispersal from islands toward the mainland. This pattern contrasts with the prevailing hypothesis of southward colonization from the continent via Sakhalin to Hokkaido. Additionally, low genetic differentiation between Hokkaido and Sakhalin suggested genetic exchange across the strait, consistent with paleo-catchment reconstruction indicating past catchment connectivity between the regions. These results highlight the combined roles of geological dynamics and sea-level fluctuations in shaping genetic structure, challenge the conventional continent-to-island dispersal paradigm. Moreover, our study demonstrates that island systems can act as biodiversity sources--not merely sinks--and provides a rare empirical example of back dispersal in primary freshwater species.