Genetic structures of Calophyllum inophyllum L., a tree employing sea-drift seed dispersal in the northern extreme of its distribution

• Context: Little is known about the potential of gene flow and resulting genetic structures of trees employing sea-drifting seed dispersal in island populations. • Aims: Current genetic structure and the magnitude of historical gene flow were estimated in island populations of Calophyllum inophyllum L., a typical plant employing sea-drifting seed dispersal. • Methods: Samples were collected from the northern extreme of the species' distribution (Taiwan and the Sakishima, Daito, and Ogasawara Islands, Japan) and genotyped using 15 EST-SSR markers. Genetic differentiation (F _ST and AMOVA), genetic structure (STRUCTRE analysis), and historical gene flow (assignment testing) were determined. • Results: Frequent gene flow within and rare gene flow among island groups was determined using assignment testing. Clear genetic structures were also detected using the STRUCTURE analysis, which demonstrated differentiation between dominant clusters among geographically constructed island groups. • Conclusions: The potential for gene flow via sea-drifting seed dispersal was high, and this was possible even among small islands. However, the extent and frequency of gene flow were not great enough to prevent genetic differentiation in a range of over a few hundred kilometers.