Reconstruction of the native distribution range of a Japanese cryptic dojo loach species (Misgurnus sp. Type I sensu Okada et al. 2017): has the Type I loach dispersed beyond the Blakiston's Line?

Recent genetic studies have revealed that the Japanese dojo loach populations include a cryptic biological species (Misgurnus sp. Type I sensu Okada et al. 2017, hereafter called Type I loach), which is distributed in discontinuous restricted areas in certain parts of northeastern Japan, including the Hokkaido Island. The current exceptional distribution of the Type I loach beyond the major faunal boundary line between the Honshu and Hokkaido Islands (Blakiston's Line) may provide novel and vital perspectives on the formation of the Japanese archipelago and distribution patterns of primary freshwater fishes. In this study, we reconstructed the native distribution range of the Type I loach based on the nucleotide sequence of the cytochrome oxidase b (cytb) gene in mitochondrial DNA (mtDNA) from samples covering its entire current distribution range. Three major clades consisting of eight subclades were identified among the Type I loach haplotypes. The Hokkaido population comprised haplotypes belonging to only two subclades and had some common haplotypes with the Japanese Sea side of Honshu Island. Considerably low genetic divergence and unclear geographic genetic structures in Hokkaido haplotypes indicate their origin of artificial introduction. This speculation was further supported by the cultural background and recorded history, accompanied by human migration from the Honshu to Hokkaido Islands. Our reconstruction of the native distribution of the Type I loach by removing the non-indigenous populations revealed geographically cohesive haplotype distribution patterns. It provided some divergent scenarios for each clade and subclade.

Automated summary

Recent genetic studies have identified a cryptic species of loach, called Type I loach, in populations from northeastern Japan. This species is distributed in certain areas, including Hokkaido Island. The distribution of Type I loach beyond the major faunal boundary line between Honshu and Hokkaido Islands may provide insights into the formation of the Japanese archipelago and distribution patterns of freshwater fishes. The study reconstructed the native distribution range of Type I loach based on mitochondrial DNA analysis and identified multiple clades and subclades. The Hokkaido population showed low genetic divergence and unclear geographic genetic structures, suggesting artificial introduction.