Contrasting diversification history between insular and continental species of three‐leaved azaleas ( Rhododendron sect. Brachycalyx ) in East Asia

Abstract

Aim To reconstruct the diversification history of Rhododendron sect. Brachycalyx (Ericaceae), and to elucidate the differences in evolutionary history between 18 insular and two continental species. Location Islands of Japan, South Korea and Taiwan and the mainland of China in East Asia. Methods Sequences of seven non‐coding chloroplast DNA regions (3971 bp) were obtained from 74 populations covering all 20 species in Rhododendron sect. Brachycalyx . The diversification history of the section was estimated by phylogenetic analysis and molecular dating. Genetic diversity estimates within species and species groups (continental and insular species) were calculated. Differences in patterns of genetic structure within each species group were estimated by analysis of the spatial genetic structure. Results The section was monophyletic and started to diversify in the late Miocene or Pliocene. We detected 61 haplotypes in the section, of which 44 were observed in the insular species and 17 in the continental species. The two species groups were almost monophyletic. Genetic diversity across species was higher for the group of insular species than for the group of continental species, corresponding to the difference in species richness. In contrast, the insular species showed a tendency towards low genetic diversity within species and populations, whereas the continental species showed the opposite trend. Main conclusions The rapid diversification in the insular species during the Quaternary may be a result of genetic drift due to population isolation and population decline on the islands. In contrast, widespread migration or colonization may have prevented allopatric speciation in the continental species.

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