Hybridisation between the indigenous Japanese pond turtle, Mauremys japonica, and the exotic Reeves’ pond turtle, Mauremys reevesii, is widespread in Japan. In this study, we examined this hybridisation using an analysis of mtDNA and 11 microsatellite markers (MS) combined with morphometry. In a Bayesian clustering analysis of MS, the admixture of two clusters equivalent to the two species was detected in the Seto Inland Sea and Ise Bay regions. While mtDNA showed reciprocal hybridisation between the two species, their hybrids tended to possess the mtDNA of the rarer species at each locality. Contrary to preceding studies, F2/later generations outnumbered F1 generations. In addition, the admixture of two mtDNA lineages (Chinese and Korean) of M. reevesii was widely observed in all regions, together with the range expansion of the recently introduced Chinese lineage. A significant correlation was observed between plastron morph and assignment probability of MS, showing that plastron morphology reflects the degree of introgression by M. reevesii in M. japonica. This finding means that the reproductive interference of the dominant species on a rarer species is a major factor driving the hybridisation of the two Mauremys species. In addition, the range expansion of the Chinese lineage of M. reevesii appears to enhance the introgression by M. reevesii in M. japonica.

See Matsuda et al. (2024), Density-dependent unidirectional hybridization between the Japanese pond turtle, Mauremys japonica and the exotic Reeves’ pond turtle, Mauremys reevesii in Japan, inferred from molecular and morphological analyses. Zoological Journal of the Linnean Society.