Lack of mitochondrial gene flow between populations of the endangered amphidromous fish Plecoglossus altivelis ryukyuensis inhabiting Amami-oshima Island

The Ryukyu-ayu, Plecoglossus altivelis ryukyuensis, is an amphidromous fish that is endemic to Amami-oshima Island in southernmost Japan. Its abundance, however, has been appreciably reduced during the last two decades such that the subspecies is now considered to be endangered. The variation of the mitochondrial DNA control region was investigated among specimens of the extant populations in the eastern and western parts of Amami-oshima Island (Sumiyo Bay and Yakeuchi Bay areas, respectively), using restriction fragment length polymorphism analysis in order to estimate the extent of gene flow between the two areas. Of a total of 165 fish including temporally different samples, four haplotypes were detected and each area possessed the two haplotypes. However, the common haplotypes shared between these areas were not observed, which indicates that recent gene flow has not occurred between these populations. The nucleotide divergence between populations was much higher than the nucleotide diversity within each population, and the neighbor-joining phylogram among haplotypes showed that the haplotypes are associated with their geographic area. These results suggest that the two populations of Ryukyu-ayu on Amami-oshima Island have been historically formed.