Previous research on the systematics of Machaeranthera and its relatives using chloroplast DNA and ITS sequence data have left several questions unresolved. New sequence data from the external transcribed spacer of nuclear rDNA (ETS) were generated in an attempt to resolve these questions. ETS sequence data were obtained from the same taxa that were used for previous analyses with chloroplast DNA and ITS sequences. ETS evidence provides greater resolution than does ITS evidence, and greater support for monophyletic groups, permitting further resolution of the relationships of Machaeranthera. Phylogenetic analysis of these data supports relationships that differ substantially from those supported by chloroplast DNA for several species and groups of species. These taxa include Machaeranthera gypsitherma, M. viscida, M. heterophylla, the Machaeranthera section Arida, and the genus Pyrrocoma. The relationships that the molecular data support for these taxa suggest that their chloroplast DNA and their nuclear rDNA have come from separate lineages, and that they may have experienced reticulate evolution in their phylogenetic history.

Key words: Asteraceae, external transcribed spacer, Machaeranthera, nrDNA