Population structure of two ecologically distinct forms of ninespine stickleback, Pungitius pungitius: gene flow regimes and genetic diversity based on mtDNA sequence variations

摘要

The population structure and its effect on genetic diversity were investigated in two ecologically distinct forms (fresh- and brackish-water types) of ninespine stickleback (Pungitius pungitius L.), which coexist on the coast of eastern Hokkaido, Japan. Obvious population subdivision (phi(ST) = 0.102 to 0.668) between freshwater systems in the freshwater type compared with that in the brackish-water type (phi(ST) = <0 to 0.078) was revealed on the basis of mitochondrial DNA (mtDNA) control region sequence variations. Results of the nested clade analysis suggested that the geographic distribution patterns of mtDNA lineages were mainly due to the contemporary population structure, rather than population histories. Although local populations were sampled from a similar geographic range for the two types, gene and nucleotide diversities of the freshwater type were significantly greater (0.94 and 0.016) than those of the brackish-water type (0.55 and 0.003). The increment of genetic diversity should be attributable to the obvious population structure and existence of divergent mtDNA lineages in the former, which clearly increased the effective population size of a subdivided population. Although these two forms maintain distinct gene pools, small sequence differences among mtDNA haplotypes obtained from each type suggested that mtDNA introgression has occurred between them.