Phylogeny and biogeography of the Viola iwagawae-tashiroi species complex (Violaceae, section Plagiostigma) endemic to the Ryukyu Archipelago, Japan

Abstract

In continental island biogeography, geologically age-old straits have been considered to be the most likely barriers in determining geographical patterns of speciation/genetic differentiation among islands. Straits with similar ages may have had different influences if geographical width across the straits had fluctuated differently in the past. However, this issue has seldom been discussed. We explored it by studying the Viola iwagawae-tashiroi species complex in the Ryukyu Archipelago, Japan. The archipelago is divided into three island groups, the northern, central, and southern Ryukyus, by two old straits called the Tokara and Kerama gaps, which are roughly of the same age. Bayesian and maximum parsimony phylogenetic analyses of section Plagiostigma and Bayesian molecular dating using multiple calibration points were conducted based on nuclear ribosomal DNA internal transcribed spacer (ITS) sequences. To reveal the detailed genealogy of the species complex, statistical parsimony networks were estimated separately for the ITS and chloroplast DNA (cpDNA) sequences and a calibrated multispecies coalescent tree based on both ITS and cpDNA sequences was constructed. Results suggest that the V. iwagawae-tashiroi species complex originated in the Ryukyu Archipelago when this region formed part of the East Asian continental margin or was formed by a few larger islands and that the complex was already distributed across the archipelago by the late Pliocene–early Pleistocene. Divergence time estimations suggest that the Kerama Gap has been a long-term effective barrier to dispersal preventing gene flow across it, whereas recent dispersal over the Tokara Gap was inferred. These contrasting results are likely explained by the fact that the minimum geographical width across the straits is geohistorically and currently greater over the Kerama Gap than at the Tokara Gap.