Genetic structure of refugial populations of the temperate plant Shortia rotundifolia (Diapensiaceae) on a subtropical island

Continental island systems harbour relict biota and populations that might have migrated during glacial periods due to the formation of landbridges. Here we analysed the genetic structure of relict populations of the temperate plant Shortia rotundifolia on the subtropical island of Iriomotejima, Japan. This plant, which inhabits riparian environments, is designated "near threatened". Only five extant populations have been found on the island. Our analyses of 10 nuclear microsatellite loci detected genetic diversity of H _E = 0.488 and H _O = 0.358 for all populations of S. rotundifolia on the island. A high inbreeding coefficient for all populations together (F _IS = 0.316) and each population separately (F _IS = 0.258-0.497) might be attributable to crossing among closely related descendants within a population, an idea that is supported by the relatedness coefficient. These results and an examination of the populations' demographic histories suggest that the extant populations on Iriomotejima have not experienced a recent population bottleneck. The five extant populations were genetically differentiated (F _ST = 0.283; P < 0.001), suggesting low seed dispersal by gravity and/or low pollen flow via pollinators in the riparian environment. In addition, population differentiation was not related to genetic distance, implying that at one time, ancestral populations might have been distributed over a wider area of the island. However, population fragmentation and range contraction might have occurred at random during the postglacial period.