Temperate origin and diversification via southward colonization in Fatsia (Araliaceae), an insular endemic genus of the West Pacific Rim

Abstract

Islands isolated by oceans that act as a geographical barrier for plant migration often possess high species endemism and have been deemed as a natural laboratory for studying species divergence. Fatsia Decne. & Planch. (Araliaceae), with three species, is one of the few plant genera absent in continents while exclusively spanning continental and oceanic islands. The nuclear ribosomal internal transcribed spacer (nrITS) phylogeny uncovered a pattern with reciprocal monophyly of Fatsia oligocarpella Koidz. (Bonin) and Fatsia polycarpa Hayata (Taiwan) vs. paraphyly of Fatsia japonica (Thunb.) Decne. & Planch. (Japan and Ryukyus), suggesting ancestry of the species in Japan and a likely temperate origin; whereas, lack of monophyly of all three allopatrically distributed species at chloroplast DNA (cpDNA) trnL–trnF spacer likely resulted from lineage sorting. In spite of the limited habitats for F. oligocarpella, unexpectedly high genetic variations in this species of oceanic islands were likely attributable to multiple colonizations and recurrent gene introgression. Biogeographical analyses suggested that Fatsia likely diverged via southward colonization in Bonin Islands and Taiwan during the late Pliocene to Pleistocene. Besides, Fatsia species with an allopatric distribution provide a perfect model for testing speciation modes of insular endemics. Nonzero gene flow between species was detected based on MIGRATE and STRUCTURE analyses of DNA sequences and microsatellite fingerprints, suggesting that allopatric speciation is less likely.