Phylogenetic analysis of the Internal and External Transcribed Spacer regions of nuclear ribosomal DNA was used to elucidate the taxonomic limits of the genus Gochnatia and generic relationships in Gochnatioideae. These analyses place Richterago and Cnicothamnus sister to Gochnatia and collectively sister to the discoid genera Anastraphia and Nahuatlea. Gochnatia is monophyletic and has three main clades each corresponding to formal sections of the genus, namely Gochnatia, Moquiniastrum, and Pentaphorus. The recently described genus Nahuatlea was not monophyletic with its species found in a grade of three clades and only one species sister to the Caribbean genus Anastraphia. Nahuatlea magna is sister to Anastraphia and a new genus, Tehuasca, is erected with this species as its type. Tehuasca shares with Anastraphia corollas with glandular trichomes but differs from Anastraphia in having white to yellow corollas, recurved lobes of the corollas and leaf margins without spiny mucros. Tehuasca magna is a native species of northeastern Mexico.

The name Adesmia arborea Bertero has been considered a nomen nudum since Colla's assertation that all new names in Bertero's publication detailing his Chilean collections of 1828 were nomina nuda. Subsequent authors seeking to validate the name A. arborea have provided conflicting descriptions adding confusion about the identity of the species. These problems arose from equating this name with other new names published by Bertero in the same work and because material collected and annotated by Bertero as A. arborea represents two different taxa. Here I show that A. arborea was, in fact, validly published by Bertero, provide evidence as to the collection number from which type must be chosen, and designate a Lectotype and Isolectotypes.

Molecular phylogenetic studies were conducted to clarify the phylogenetic placement of Conoclinium within Eupatorieae and to analyze interspecific relationships in the genus. Analysis of a six gene data set placed Conoclinium sister to Ageratum and closely related to Fleischmannia and Paneroa, a result that is consistent with the overall appearance and the phytochemistry of these genera but not with their current subtribal placement. Results of an analysis of noncoding DNA, including the chloroplast trnH-psbA spacer and the nuclear ITS and ETS regions, was consistent with the close relationship of Conoclinium to Ageratum, Fleischmannia, and Paneroa, and showed Conoclinium to be monophyletic. Within Conoclinium, the results of the phylogenetic analyses were slightly at variance with current classification, and it is suggested that C. dichotomum (populations from peninsular Florida previously included within C. coelestinum) and C. oligolepis (formerly C. betonicifolium var. integrifolium) be recognized as distinct species. The low overall sequence variability suggests that Conoclinium is a relatively recent arrival in eastern North America. Conoclinium mayfieldii is not monophyletic and the Sierra Madre Occidental populations of the species are herein recognized as a new species, C. gonzaleziae. Neotypes for C. dichotomum and C. oligolepis are designated.