Genetic structure in chloroplast and nuclear microsatellites in Rosa rugosa around sea straits in northern Japan

Sea‐strait geological history and seawater dispersal patterns can affect the genetic structure of coastal plants. We investigated microsatellite (simple sequence repeat, SSR) variations in chloroplast (cp) and nuclear (n) DNA of the coastal shrub R osa rugosa, native to northeastern Asia, and three species closely related to it. In northern Japan, we sampled R. rugosa along coasts around the Soya and Tsugaru straits with different histories of the Quaternary land formation as a result of different water depths. Both cpSSR and nSSR variations suggest rare hybridization between R . rugosa and the closely related species. Variations in one of two cpSSRs showed genetic differentiation between eastern and western Hokkaido. This genetic structure may result from introgression from R. davurica in eastern Hokkaido and/or isolation between the eastern and western coastlines of Hokkaido as a result of the Quaternary land formation in the Soya Strait. On the other hand, variations in 10 nSSRs were geographically homogeneous with weak isolation by distance along coastlines. These results suggest that extensive gene flow has been homogenizing the genetic structure of R . rugosa in northern Japan. Sea‐strait geological history and seawater dispersal patterns can affect the genetic structure of coastal plants. The genetic structure of Rosa rugosa in northern Japan may result from introgression from R . davurica in eastern Hokkaido and/or isolation between the eastern and western coastlines of Hokkaido by the Quaternary land formation in the Soya Strait, which has been homogenized by extensive gene flow.