The genus Dahlia Cav. presently consists of 30 species, primarily from Mexico. Species are usually placed in four subgeneric sections: Pseudodendron, Epiphytum, Entemophyllon, and Dahlia, based largely on morphological characters, supplemented with cytological, geographical, and biochemical data. Combined molecular sequence data from both the internal and external transcribed spacer regions (ITS and ETS), located within the nuclear ribosomal gene repeat unit, were used to infer a phylogeny of the genus. Basal nodes were also supported by RAPD data. Sect. Entemophyllum appears to be very distinct from the remainder of the genus in almost every parameter measured. Dahlia merckii and D. tubulata are also quite distinct, both from each other and the remaining taxa. Sections Pseudodendron and Epiphytum are closely allied with each other as well as with a few species from sect. Dahlia, and this clade, the variable root clade (VRC), incorporates all species with unusual root forms, along with some species exhibiting the more typical tuberous type. The remaining species of sect. Dahlia form a well-defined clade, the core Dahlia clade (CDC). Comparisons of this phylogeny with morphological characters suggest some traits of an ancestral Dahlia.

Key words: Coreopsideae, Dahlia, ETS, external transcribed spacer, Heliantheae s.l., internal transcribed spacer