O÷nopsis is a monophyletic genus consisting of five species distributed mainly in central and eastern Wyoming and Colorado. The plants are herbaceous perennials that tend to grow on alkaline soils. All members of the genus are primary selenium indicators, and appear to be obligatory seleniophiles. The purpose of this study was to determine phylogenetic relationships among species of the genus O÷nopsis using DNA sequence data from two chloroplast intergenic spacers (trnL-trnF and psbA-trnH) and the nuclear ribosomal internal transcribed spacers (ITS). Sequence variation was lowest in the trnL-trnF spacer and greatest in the psbA-trnH region. The total data phylogeny placed O. multicaulis sister to the rest of the genus. O. foliosa var. foliosa and O. foliosa var. monocephala were placed in separate clades, in disagreement with morphology-based phylogenies. Low bootstrap and decay values indicate a low amount of divergence among sequences, suggesting a relatively recent origin for the genus.

Key words: Asteraceae, ITS, O÷nopsis, psbA-trnH, trnL-trnF