Dual northward overseas radiation from Japanese islands shaped the phylogeography of North-East Asian tree frog

The tree frog Dryophytes japonicus has a broad geographic distribution in northeastern Asia, comprising two major genetically divergent clades whose geographic boundary is located within the Japanese mainland. However, the evolutionary and adaptive mechanisms underlying this wide overseas distribution remain unclear. To uncover the phylogeographic history of the species, we conducted comprehensive genetic and reproductive analyses. Mitochondrial gene ( cytochrome b and 16 S rRNA ) and nuclear genomic (single nucleotide polymorphism, SNP) analyses confirmed the two major clades: the West Japan clade, comprising western to southern Japan and the continent, and the East Japan clade, comprising eastern to northern Japan and Sakhalin (Russia). SNP analyses also revealed nuclear genome admixture around the boundary between the two clades in central Japan. Although the reciprocal hybrids were fully viable and fertile in captivity, the geographic cline of hybrid index was extremely steep, suggesting a tension zone formed by persistent selection against hybrids through reproductive isolation. Genomic analyses further demonstrated that a dual northward overseas radiation originating from the Japanese archipelago shaped the current phylogeography and extensive distribution in northeastern Asia. One radiation involved the West Japan subclade spreading from Tsushima Island through the Korean Peninsula to the continent, while the other involved the East Japan subclades reaching Hokkaido and Sakhalin. These results suggested that D. japonicus possesses a high adaptive capacity and an effective strategy for overseas dispersal.