Genetic Disturbance in Wild Minami-Medaka Populations in the Kyushu Region, Japan

In wild populations of the Japanese endemic freshwater fish Minami-medaka (Oryzias latipes), genetic disturbances caused by crossing with artificially introduced exotic populations, especially an orange-red commercial variety of medaka commonly called himedaka, have been reported throughout Japan. Minami-medaka populations in the Kyushu-Ryukyu Islands region include many genetically distinct groups, indicating that the region is important for the conservation of genetic diversity in Minami-medaka populations. In this study, genetic analyses using two DNA markers (one for a mitochondrial gene, the other for a nuclear gene) were conducted to reveal the current status of genetic disturbance in wild Minami-medaka populations in the Kyushu-Ryukyu Islands region. In 125 individuals from 18 populations analyzed, one individual, from the northern Kyushu Islands, was found to carry alleles originating from the himedaka strain. Based on the results of all available surveys, including those of the present study, a total of six introgressed populations are currently known in the region. All other populations are considered unaffected. The native genetic diversity of Minami-medaka populations in the Kyushu-Ryukyu Islands region is comparatively well preserved; results of surveys throughout the rest of the range of the species indicate that almost half of all Minami-medaka populations have been introgressed by non-native genes. Conservation measures and regular genetic monitoring are recommended to preserve the genetic diversity of this species.