Shaping of genetic structure along Pleistocene and modern river systems in the hydrochorous riparian azalea, Rhododendron ripense (Ericaceae).

To determine the effects of hydrochory on the formation of the present range of a species and the spatial distribution of genetic variation, we assessed the rangewide genetic structure of a hydrochorous riparian Japanese species (Rhododendron ripense) using four nuclear microsatellite loci. The patterns of isolation by distance and Bayesian clustering analyses of 33 populations suggested that the present range, characterized by both localized and disjunct distributions across the sea, arose from two contrasting colonization events: (1) primary colonization along two Pleistocene rivers that have been submerged and become partly isolated by marine transgression by 6000 years ago, and (2) additional range expansions from these rivers into unconnected neighboring rivers as a result of river captures. Along the Pleistocene rivers, frequent gene flow by hydrochory resulted in the retention of considerable genetic diversity within each population and genetic homogenization among populations. Within unconnected neighboring rivers, genetic diversity was also retained by the simultaneous redistribution of many individuals as a result of river captures, whereas restricted gene flow within a river resulted in genetic divergence among the river populations. Thus, the evolutionary history of hydrochorous R. ripense appears to have been strongly shaped by both ancient and modern rivers.