Comparison of genetic population structure between two cyprinids, Hemigrammocypris rasborella and Pseudorasbora pumila subsp., in the Ise Bay basin, central Honshu, Japan

Two small cyprinid fishes, Hemigrammocypris rasborella and Pseudorasbora pumila subsp. (sensu Nakamura 1963), inhabit similar habitats and often occur sympatrically in the Ise Bay basin, central Honshu Island, Japan. Their genetic population structures were revealed, using sequence data from the mitochondrial cytochrome b gene, and then compared. Hemigrammocypris rasborella populations in the Ise Bay area formed a monophyletic group that has been isolated from eastern (Tenryu River) and western (Lake Biwa–Yodo River) populations at least for several hundred thousand years. Pseudorasbora pumila subsp., endemic to the Ise Bay area, was estimated to have become isolated from its sister subspecies, P. p. pumila , about 5 million years ago. Both H. rasborella and P. pumila subsp. had centers of genetic diversity around the Okazaki Plain in the eastern part of the basin and showed trans-bay distribution of haplotypes or haplotype groups. Their common population structure was explained by geological features in the Ise Bay area, in which a large paleo-river system developed in regression periods, suggesting gene flow among populations of each species in the mid to lower reaches of the paleo-river. Based on the estimated expansion or divergence time, however, not all populations experienced gene flow during the Last Glacial. In contrast to the maintenance of high genetic diversity in H. rasborella , almost all populations of P. pumila subsp. have lost mitochondrial DNA genetic diversity. This implies that effective population size of P. pumila subsp. tended to be smaller, probably because of differences in reproductive ecology, even though the two species have been exposed to similar environmental changes. For conservation of the two species, genetic and adaptive differentiation among local populations should be considered, and attention should be paid to inbreeding depression, especially in P. pumila subsp.