Phylogenetics of the slipper orchids (Cypripedioideae, Orchidaceae): nuclear rDNA ITS sequences

Cypripedioideae (Orchidaceae) have been the subject of numerous taxonomic treatments with conflicting interpretations of relationships among the five genera and the 150-170 species. We have produced nuclear ribosomal ITS nucleotide sequences for nearly 100 slipper orchid species and used parsimony analysis to investigate their relationships. Our results demonstrate that each genus, as currently circumscribed, is monophyletic (Mexipedium and Selenipedium being represented by a single taxon). Like rbcL data, ITS sequences place Mexipedium sister to Phragmipedium. Relationships at the sectional level in Paphiopedilum are largely as described by CRIBB. However, the division of Paphiopedilum into subgg. Brachypetalum and Paphiopedilum is not supported; subg. Brachypetalum is paraphyletic to subg. Paphiopedilum. Phragmipedium species are divided into the same three major clades as in the taxonomic scheme of McCooK. The plicate-leaved genera, Cypripedium and Sebenipedium, are successive sister groups to the rest of the subfamily, confirming generally held opinions that they display plesiomorphic characters compared to the conduplicate-leaved genera. A survey of karyotypes in the context of the ITS tree reveals a general trend toward increased chromosome number, probably brought about by centric fission. These data also accord with a previously suggested biogeographic hypothesis of a widespread Northern Hemisphere distribution, followed by range fragmentation due to Miocene cooling.