Natural hybridization between two Japanese medaka species (Oryzias latipes and Oryzias sakaizumii) observed in the Yura River basin, Kyoto, Japan

To determine the actual state of hybridization between two Japanese medaka species ( Oryzias latipes and Oryzias sakaizumii ) in their natural environment, we used nuclear DNA markers at 10 loci to analyze 215 individuals from eight wild populations in the middle reaches of the Yura River basin in Japan, where the two species are sympatric. Despite large genetic differentiation between the two species, reproductive isolation between them could not be confirmed. We also discussed the formation of the current distribution patterns of the two species and their hybridization zone in the Yura River basin.