The genus Chrysothamnus Nutt. is comprised of a group of asteraceous shrubs distributed throughout the western United States but with a center of diversity on the Colorado Plateau. The most well-known, widely distributed and studied species is C. nauseosus (Pall.) Britton. This singular taxon tends to dominate most informal concepts of the genus as a whole. Recent phylogenetic studies of the tribe Astereae, using cpDNA restriction site data, have placed C. nauseosus as either sister to or within the genus Ericameria Nutt. This is a position long supported by morphological and chemical analyses and as a natural consequence has precipitated the transfer of C. nauseosus into Ericameria. None of the other species of Chrysothamnus have been included in these recent phylogenetic studies, yet there is morphological and chemical data to suggest that at least C. viscidiflorus (Hook.) Nutt. and possibly some of the other core Chrysothamnus species are not closely related to Ericameria but are phylogenetically allied to Petradoria Greene and its sister taxa within the subtribe Solidagininae. This apparent phylogenetic contradiction within Chrysothamnus would suggest that the genus is polyphyletic, as generally construed. The purpose of the current study is to test the monophyletic nature of Chrysothamnus and resolve its putative phylogenetic relationships using DNA sequence data, in addition to morphological data.

Key words: Chrysothamnus, DNA, Ericameria, Petradoria, phylogeny