Molecular phylogeny and character evolution of Didymocarpus (Gesneriaceae) in Thailand

Until recently the genus Didymocarpus Wall. (Gesneriaceae) was used in an unwarrantably wide sense and included more than 180 species. It has now been remodelled and restricted to around 70 species. Of these, 18 species and one variety are known to occur in Thailand. To clarify the relationships among Thai species of Didymocarpus we sequenced the internal transcribed spacers (ITS) of nuclear ribosomal DNA (nrDNA) from a sample of 23 taxa, including 15 from Thailand, four from China, three from Malaysia and one from Bhutan. Seventeen morphological characters were coded for all 23 taxa and optimized onto a retention index (RI) reweighted maximum parsimony (MP) tree. The phylogenetic analyses suggested that Didymocarpus taxa formed a strongly supported monophyletic clade, with several supported subclades. The combination of molecular phylogeny and optimization of morphological characters suggests the presence of three distinct groups: the first, corresponding to Didymocarpus sect. Elati Ridl., includes plants with tall stems, yellow or white flowers and one-celled conoid or two-celled headed pigment glands; the other two groups, which represent Didymocarpus sect. Didymocarpus, both contain plants with dwarfed stems and violet or purple flowers, but are distinguished by the presence of both four-celled conoid or onecelled globose glands in one, and the absence in the other. Optimization of geographical locality onto the phylogeny led us to propose the hypothesis that, based on this sample, the geographical origin of Didymocarpus is the Malay Peninsula, and the ancestral corolla colour is white/yellow. Subsequent dispersal northward through southern and northern Thailand to China and Bhutan was accompanied by the evolution of a purple/violet corolla colour.