Phylogeography of Opsariichthys platypus in Japan based on mitochondrial DNA sequences

To investigate the phylogeography of the Japanese pale chub Opsariichthys platypus, we examined the genetic differentiation, phylogenetic relationships, and historical demography using mitochondrial cytochrome b gene sequences of 788 O. platypus from 124 localities throughout the Japanese Archipelago. Molecular phylogenetic analyses revealed that Japanese O. platypus is divided into three major clades (western Japan: WJ; eastern Japan: EJ; and Kyushu: KY), and that among these clades, KY is remarkably differentiated from the others. The distribution of the EJ and KY clades clearly reflects their respective geographical ranges: the EJ and KY clades are restricted to the eastern region of the Ibuki–Suzuka Mountains in central Honshu and northern Kyushu, respectively. In contrast to the EJ and KY clades, the WJ clade is widely distributed throughout the Japanese Archipelago, including areas where O. platypus is not naturally distributed (e.g., northern part of Honshu, southern Kyushu, and Tokunoshima). In addition, nearly all the WJ haplotypes in the non-indigenous regions were the same as or similar to the haplotypes in Lake Biwa, and the WJ haplotypes in the distribution ranges of the EJ and KY clades were also same as or similar to those of Lake Biwa, indicating that the distribution of WJ clade had been the result of inadvertent releases in conjunction with releases of Plecoglossus altivelis from Lake Biwa. The estimated divergence time indicated that each clade was formed during or before the Pleistocene, and mismatch distribution test suggested the occurrence of the population expansion in three clades and the time since expansion was 120,000–226,000 years. This study demonstrates that O. platypus exhibits clear genetic differentiation among regional populations, and that range expansion following divergence caused by uplifting of mountains is important for distribution and genetic structuring of O. platypus. In addition, artificial introductions of non-indigenous populations have proceeded throughout the Japanese Archipelago, indicating ongoing loss of the genetic features due to introductions or introgression. Further efforts should be made toward a comprehensive understanding of the current state of introductions of non-indigenous O. platypus populations and the potential influence of invasive populations, including loss of genetic diversity.