Changes over a 10-year Period in the Distribution Ranges and Genetic Hybridization of Three Pelophylax Pond Frogs in Central Japan

In this study, we focused on interspecial interactions in Central Japan where three closely related pond frog species, Pelophylax nigromaculatus , Pelophylax porosus porosus and Pelophylax porosus brevipodus (two species and two subspecies) are distributed and are in contact and/or overlap. The presence or absence of hybridization between these species was evaluated by comparing the sequences of the mitochondrial DNA Cyt-b (586-bp) and the nuclear gene Tyrosinase (747-bp) regions of each specimen collected. By comparing interspecific hybridization in the same area in both 2010 (119 specimens) and 2020 (152 specimens), we clarified population distributions and corresponding genetic dynamics over a 10-year period; there was no major change in the degree of interspecific hybridization between P. nigromaculatus and P. p. brevipodus in the Ina Basin, but it was revealed that the proportion of “pure”-bred Pelophylax p. brevipodus remained low. On the other hand, in the Matsumoto Basin, interspecific hybridization between P. nigromaculatus and P. p. porosus was evaluated to have progressed slightly. The proportion of “pure”-bred Pelophylax p. porosus declined to less than half, while the proportion of hybrid individuals showed a tendency to increase. Moreover, the dispersal of P. nigromaculatus was confirmed in areas along the Sai River, where only the pure-bred P. p. porosus was previously known to inhabit. Since the three frogs targeted in this study are all “Red List” species that are endangered, continuous monitoring of population structures and genetic dynamics is required.