Evolutionary process of iwame, a markless form of the red-spotted masu salmon Oncorhynchus masou ishikawae, in the Ono River, Kyushu

A markless form of the red-spotted masu salmon Oncorhynchus masou ishikawae (iwame) co-exists with the normal color form (amago) in several mountain streams in south-west Japan. In a headwater stream of the Ono River, Kyushu, amago has recently disappeared from the upper reach above a natural waterfall barrier, and only iwame has survived in this area. In addition, a unique gradual distribution of the two forms has been developed below the waterfall, where the frequency of iwame increases with elevation. To explore the evolutionary process of iwame in this river, the population structure and phylogenetic relationships among subspecies of O. masou were investigated through AFLP and mtDNA analyses. Bayesian admixture analysis revealed two distinct subpopulations within the Ono River, although both shared a single mtDNA haplotype. All iwame above the waterfall, as well as 87.5 % of iwame below the waterfall, were estimated to have pure ancestry from one subpopulation with extremely low genetic diversity, suggesting that genetic drift could account for the fixation of the markless phenotypes (i.e., disappearance of amago) in this subpopulation. The remaining iwame and most amago below the waterfall showed admixture with another subpopulation. Gene flow from an adjacent branch stream occupied only by amago into the lower part of the lower reach below the waterfall may contribute to the maintenance of the gradual distribution of the two forms. Phylogenetic analysis based on AFLP data revealed that iwame were independently derived from amago in different river systems (i.e., polyphyletic). The present results may provide a framework for genetic management to prevent further loss of genetic diversity in the unique Ono River iwame.