Mitochondrial DNA analysis in ayu (Plecoglossus altivelis) raises a urgent conservation need for its southernmost population

The central-marginal hypothesis predicts that genetic diversity decreases and genetic differentiation increases toward the species distribution margin. We analyzed the mitochondrial DNA control region of ayu ( Plecoglossus altivelis ) collected from different localities, including central and marginal populations, to test the hypothesis. The results show that the ayu populations at southern marginal habitats (Tien Yen River in Vietnam and Yakugachi River in Amami-oshima Island) displayed low genetic diversity (Haplotype diversity, H = 0.52 and 0.67; Nucleotide diversity, Pi = 0.01 and 0.006, respectively) and significantly differed from the central populations ( p < 0.001), consistent with the central-marginal hypothesis. However, ayu populations at the center of their distribution range (Shinano River in Honsyu and Hamoji River in Sado Island) and the northern marginal habitats (Yoichi River in Hokkaido and Aonae River in Okushiri Island) exhibited high genetic diversity ( H = 1 for all rivers; Pi = 0.024–0.029). The genetic differences between the populations at the center and the northern marginal habitats were insignificant ( p > 0.05). This study showed that in ayu, the central-marginal hypothesis is valid in the southern direction but not in the northern direction. Urgent conservation is needed for the genetically distinct population in the degraded southern habitat with severely low genetic diversity. ### Competing Interest Statement The authors have declared no competing interest.