Generic Limits of Xanthocephalum, Gutierrezia, Amphiachyris, Gymnosperma, Greenella, and Thurovia (Compositae: Astereae)

Abstract

Xanthocephalum, Gutierrezia, Greenella, Gymnosperma, Amphiachyris, and Thurovia have been variously treated with regard to their generic limits by several authors in the last 3 decades. The generic delimitation presented here is based on a thorough re-evaluation of all 26 known North American members of these genera. The data obtained in the study strongly support the following disposition of the taxa, which differs from all previous treatments. Xanthocephalum (x = 6), a genus of montane areas in western Mexico and the southwestern United States, contains five species (one with three varieties) and is more closely related to Grindelia and Olivaea than to Gutierrezia and its allies. Gutierrezia (x = 4) encompasses seventeen specific and infraspecific taxa in North America, including not only the eight taxa recognized for it by Solbrig, but also five taxa formerly of Xanthocephalum, and the genera Greenella and Thurovia (the last resulting in Gutierrezia triflora, comb. nov.). The genus thus constituted ranges from central Mexico north and west through the central and western United States to Saskatchewan. Amphiachyris (n = 4 or 5) with two grassland species of Texas and central United States, and the monotypic Gymnosperma (x = 8), which ranges from northern Guatemala to Texas and westward to Arizona, are closely allied to Gutierrezia.