Corallorhiza Gagnebin (Orchidaceae) comprises eleven species of leafless, rootless, mycotrophic orchids. Closely related genera (Aplectrum, Cremastra, and Oreorchis) all possess leaves and roots. Because morphological synapomorphies for Corallorhiza are all loss characters, additional evidence for its monophyly was sought with molecular characters; these data were also used to study relationships among the species of Corallorhiza and among genera of the Corallorhizinae. The ITS region of nuclear ribosomal DNA and the chloroplast gene matK were sequenced for multiple accessions of Corallorhiza (representing 8 of 11 spp.), Aplectrum (1 of 1 sp.), Cremastra (2 of 2 spp.), and Oreorchis (1 of 16 spp.). Govenia and Coelia were used as outgroups in the parsimony analysis. ITS data suggest that the relationships among the genera are Aplectrum (Cremastra (Oreorchis, Corallorhiza)), while matK differs in the positions of Aplectrum and Cremastra. Molecular data also support the monophyly of Corallorhiza. Relationships among the species are congruent with previous phylogenetic hypotheses based on plastid RFLP patterns. Corallorhiza bentleyi, a newly described West Virginian species, is sister to a small-flowered accession of C. striata from southern Mexico. Since previous analyses provided evidence of deletions in the plastid genome in Corallorhiza, we investigated this more finely by amplifying a segment of the chloroplast genome that includes the gene psbA in Corallorhiza and outgroups. The size of this segment is ca. 2150 bp in Govenia, 2050 bp in Aplectrum, 2250-2350 bp in Cremastra, and 2000 bp in Oreorchis. Within Corallorhiza it appears that both insertions and deletions have occurred, with sizes ranging from 2300 bp among larger flowered C. striata to 1750 bp in C. bentleyi and its sister C. striata accession. Other species exhibit less pronounced variation, but multiple accessions of a species often exhibit the same segment length.

Key words: Aplectrum, Corallorhiza, Cremastra, molecular phylogeny, Orchidaceae, Oreorchis