Hybridization and introgression in Daphniphyllum macropodum (Daphniphyllaceae) on Ulleungdo Island

Biodiversity on an island presents complicated and unexpected patterns of evolution. Ulleungdo Island is a well-known example of such an oceanic island. Most systematic and evolutionary studies of Ulleungdo Island have focused on endemic species, for which the ancestral population derived from the mainland isolated and ultimately evolved into a distinct species. To gain a better understanding of the origin of the biodiversity of Ulleungdo, non-endemic taxa on the island should also be examined. Daphniphyllum macropodum, distributed in warm-temperate regions in China, Taiwan, Korea, and Japan, is an evergreen tree commonly found in the maritime broad-leaf vegetation on Ulleungdo. Thirty-two populations of D. macropodum from Ulleungdo and other areas in Korea and Japan and its close relative D. teijsmannii were included in a phylogeographic analysis using nuclear ITS and chloroplast DNA regions. Each species was found to form a distantly related monophyletic group in the MP trees based on the ITS data. In contrast, the chloroplast DNA data showed that the clade of D. macropodum was mixed with certain samples of D. teijsmannii. Some individuals of D. macropodum on Ulleungdo and those on southeastern Honshu consist of haplotypes typically found in D. teijsmannii. Our results suggest that some of the D. macropodum on Ulleungdo Island likely derived via introgression between D. teijsmannii and D. macropodum and that D. teijsmannii, currently not distributed on Ulleungdo, likely served as the maternal source. Our results also imply that plants of D. macropodum in the Kanto region derived independently from introgression between D. teijsmannii and D. macropodum with different maternal parents.