Unraveling the potential structure of a Parnassius butterfly in Japan: Insights into the expansion history

The Japanese Archipelago consists of a series of isolated yet interconnected islands off the Eurasian continent. The linear topography of the archipelago presents a unique biogeographic context for the dispersal of organisms from the continent. In this study, mitochondrial DNA (mtDNA) and single-nucleotide polymorphism (SNP) variation were employed to elucidate the dispersal history of the Japanese clouded butterfly (Parnassius glacialis) across the Japanese Archipelago, including the northern island (Hokkaido), the main island (Honshu), and Shikoku Island. Network analysis of 1192 bp of mtDNA (cytochrome oxidase I and II) regions revealed 49 haplotypes and three distinct haplotype groups, which correspond geographically to Eastern Japan, Western Japan, and Chugoku-Shikoku. The Chugoku-Shikoku group is the most ancient lineage. Divergence time estimates using whole-genome sequencing of mtDNA suggest that the Japanese lineage diverged from the continental P. glacialis approximately 3.08 million years ago (Ma). Subsequently, the Eastern Japan and Western Japan lineages diverged from the Chugoku-Shikoku lineage around 1.06 Ma, with subsequent divergence of the Eastern and Western Japan lineages at approximately 0.62 Ma. P. glacialis is estimated to have expanded its distribution via a land bridge that once connected China and the Japanese Archipelago. Population structure analysis based on 3067 SNP genotypes revealed five distinct genetic structures within the Japanese Archipelago, indicating geographical differentiation. Through mtDNA and SNP variation analyses, four primary genetic barriers were identified: between Hokkaido and Honshu, between Eastern and Central Japan, within the Kansai region, and in the Chugoku region. The first three barriers correspond to notable geographical features, the Blakiston Line, a line parallel to the Itoigawa-Shizuoka Tectonic Line, and a boundary crossing Lake Biwa. These findings suggest that Japanese P. glacialis diverged from the continental P. glacialis and expanded its range across the Japanese Archipelago via western routes, leading to its current distribution.