Genetic characteristics of the amphidromous fish Ayu Plecoglossus altivelis altivelis (Osmeriformes: Plecoglossidae) on Yaku-shima Island in Japan, the southernmost population of the subspecies.

The Ayu (Plecoglossus altivelis altivelis) population on Yaku-shima Island, Kagoshima Prefecture, Japan represents the southernmost population of the subspecies and is considered to be facing extinction. We investigated the genetic characteristics of Ayu in the Miyanoura River (MYU) in the northeast of the island and the Kurio River (KRO) in the southwest of the island, using partial sequences of mitochondrial (mt) control region and polymorphic microsatellite (simple sequence repeat: SSR) loci. The mtDNA analysis revealed that the Yaku-shima Island population was significantly differentiated from other Ayu populations; additionally, the MYU and KRO populations were significantly different from each other in terms of mtDNA (pairwise Φ_ST = 0.5826, P < 0.01) and SSR (pairwise Φ_ST = 0.1598, P < 0.01) analyses, indicating very little or no gene flow between them. The mtDNA haplotype diversity values were minimal for KRO and somewhat lower for MYU (h = 0.8176) than for each population from the mainland of Japan (Honshu-Kyushu) and the Korean Peninsula (h = 0.9905-1.0000). The mean values of expected heterozygosity (He) of SSRs were also lower in KRO (mean He = 0.555) than in MYU (mean He = 0.649). A considerably small effective population size (N_e = 100.1 for MYU, 151.2 for KRO) and a bottleneck effect for Yaku-shima Island population were suggested by SSR analysis. These findings underscore the importance of monitoring the genetic diversity of Ayu on Yaku-shima Island and the necessity of designing conservation policies for each river's population.