Piper section Muldera (Piperaceae) of the Malay Peninsula: variation and species limits.

Species limits in Piper spp. have been unstable, with specimens often assigned to different species by different botanists. Using a strongly supported clade of Asian Piper section Muldera, I investigated species boundaries in the eight Malay Peninsula species using morphological, anatomical and molecular approaches. I assessed morphological variation using principal component analysis (PCA) and a model-based clustering method. Herbarium specimens and samples collected in the field were examined, and the latter were also used to examine variation in vegetative anatomy. Potential evidence of reciprocal monophyly of species came from the molecular phylogenetic analyses of nuclear ITS and g3pdh and plastid trnL-F. Morphological analyses did not lend support for any of the hypothesized species from the Malay Peninsula. Nuclear phylogenetic trees supported a combination of Piper schizonephros, Piper polygynum and Piper maingayi as a single species that might also have a distinct distribution of calcium oxalate crystals in the mesophyll of the leaf and a prominently raised leaf midrib raise. Nevertheless, shared characters between this species and some, but not all, of the other morphogroups, and paraphyly of other morphogroups in the phylogenetic analyses, might suggest hybridization or lateral gene transfer among species of the section Muldera. In-depth population studies coupled with examination of morphological variation for all species of the section Muldera, including those from outside the Malay Peninsula, are necessary to disentangle the species limits and ultimately the evolutionary history in this group. [ABSTRACT FROM AUTHOR]