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 andSelenipedium being represented by a single taxon). LikerbcL data, ITS sequences placeMexipedium sister toPhragmipedium. Relationships at the sectional level inPaphiopedilum are largely as described byCribb. However, the division ofPaphiopedilum into subgg.Brachypetalum andPaphiopedilum 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 ofMcCook. The plicate-leaved genera,Cypripedium andSelenipedium, 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.