A Molecular Phylogenetic Framework for the Ryukyu Endemic Rodents Tokudaia osimensis and Diplothrix legata

Diplothrix legata and Tokudaia osimensis are rodent species endemic to the Ryukyu Islands (Okinawa area) of the southern part of Japan. To place them within a phylogenetic framework of Asian rodent species, we examined the molecular relationships among these species and Apodemus, Micromys, Mus, and Rattus, using an analysis of mitochondrial cytochrome b gene (1140 bp) and nuclear IRBP gene (782 bp) sequences. The results indicate that the four main murine genera and Tokudaia diverged at similar evolutionary times, namely at the time of the radiation of Murinae, 14 to 40 million years ago, depending on different concepts for the divergence times of Rattus and Mus. In contrast, Diplothrix separated from the Rattus group more recently. Our results indicate that D. legata and the Rattus group diverged at 20–30% of the divergence time of Rattus and Mus under the assumption of the molecular clock, suggesting that D. legata established its lineage at least by the beginning of the Pleistocene era. It is thus evident that the Okinawa area preserves rare indigenous species with various levels of genetic endemicity.