Abstract
The Japanese rosy bitterling (Rhodeus ocellatus kurumeus) is a cyprinid subspecies endemic to Japan. It is now on the verge of extinction due to hybridization with its subspecies R. o. ocellatus, introduced from mainland China. In this study, we examined 13 populations of R. ocellatus from Nara Prefecture, Japan, using morphological analysis and molecular analyses of mtDNA and microsatellites. One of the populations in Nara Park (Stn. 9) closely matched the morphological characteristics of R. o. kurumeus, while other populations more closely resembled R. o. ocellatus. The former population possessed mitotypes of R. o. kurumeus only, while the latter possessed those of both subspecies or those of R. o. ocellatus only. Notably, mitotypes of R. o. kurumeus were widely observed in the Nara Basin. In addition to the presence of mitotypes reported in an Osaka population of R. o. kurumeus, the low genetic diversity of the population at Stn. 9 was comparable to that of the Osaka R. o. kurumeus population. In a principal component analysis of microsatellites data, three clusters were recognized; 1. R. o. kurumeus (including the population at Stn. 9), 2. R. o. ocellatus and 3. their probable hybrids. These results suggest that R. o. kurumeus was once distributed widely in the middle reaches of the Yamato River, but the remaining populations have hybridized with R. o. ocellatus, except for the population at Stn. 9. Low genetic diversity in the Nara Park population is considered a result of a population bottleneck in a small isolated habitat.
