Phylogeny of the Asian Hedyotis-Oldenlandia complex (Spermacoceae, Rubiaceae): evidence for high levels of polyphyly and the parallel evolution of diplophragmous capsules

Abstract

Generic delimitation in the Hedyotis-Oldenlandia complex has a long taxonomically confused history because of the controversy of lumping or splitting these two taxa. Previous morphological and phylogenetic studies with a paucity of Asian taxa suggested that Hedyotis should include only Asian species characterized by diplophragmous capsules. In order to test the reliability of this conclusion, assess the phylogenetic value of capsular characters, and evaluate generic circumscriptions in this complex, a phylogenetic study based on expanded inclusion of 63 Asian species was performed using two nuclear regions and eight plastid regions with parsimony and likelihood analyses. The results show that the Hedyotis-Oldenlandia complex is a highly polyphyletic group. Hedyotis should only include most Asian species with erect, robust herbs or shrubs and diplophragmous capsules. Oldenlandia s. str. consists primarily of African species, including the type O. corymbosa, that are characterized by small herbs, paniculate or corymbose inflorescences, inserted styles and stamens, and loculicidally dehiscent capsules. Dimetia, Scleromitrion and Thecagonum are proposed to be resurrected to accommodate three newly resolved clades. Morphological character optimizations indicate that the diplophragmous capsule evolved independently twice within this complex. Plant habit, stipule shape, and capsular dehiscent pattern are of great value in generic circumscriptions.