One species or two? Multilocus analysis of nucleotide variation of Melastoma penicillatum and Melastoma sanguineum (Melastomataceae) in Hainan, China

Melastoma comprises more than 20 shrub species and is distributed in tropical Asia and Oceania. Melastoma penicillatum, the most narrowly distributed taxon of this genus in China, was recently incorporated into another morphologically similar species, M. sanguineum. Based on its distinct morphological traits, unique habitat and flowering time, we propose that M. penicillatum should be a distinct species. In this study, we sequenced three nuclear genes and four chloroplast intergenic spacers of M. sanguineum and M. penicillatum at two locations in Hainan to test this hypothesis. There was no sequence variation at all four chloroplast intergenic spacers within and between M. sanguineum and M. penicillatum, suggesting that they should have diverged recently. However, they showed strong divergence at all three nuclear genes and no shared haplotypes were observed between them. Bayesian clustering-based STRUCTURE analysis revealed that individuals of M. sanguineum and M. penicillatum were clustered into their own clades. Fst calculations showed that both taxa had marked population genetic differentiation, but population differentiation in M. penicillatum was stronger than that in M. sanguineum. Stronger population differentiation in M. penicillatum may be caused by its discontinuous high-elevation and understory habitats, which is disadvantageous for bird-mediated dispersal. Our analysis based on the isolation-with-migration (IM) model revealed bidirectional, asymmetrical gene flow in M. sanguineum and little gene flow in M. penicillatum. Taken together, our results indicate that M. penicillatum is a distinct species and should not be incorporated into M. sanguineum. The two species might have diverged recently, and lineage sorting has been incomplete. Differential adaptation to different elevations might have led to speciation in this case.