Molecular phylogeny of the genus Dahlia Cav. (Asteraceae, Heliantheae–Coreopsidinae) using sequences derived from the internal transcribed spacers of nuclear ribosomal DNA

Sequences of the internal transcribed spacers (ITS) of the 18S–26S nuclear ribosomal DNA were used to resolve phylogenetic relationships among 13 species of the genus Dahlia. Phylogenetic reconstruction from both parsimony analysis and pairwise distance data produced a single tree in which four clades could be distinguished, these broadly separating the species into the different chromosome numbers found in Dahlia (x=16, x=17 and x=18). Although species based on x=16 were divided into two clades, there is the suggestion that this separation may be artificial and that these should be combined as a single clade. The phylogeny produced here has been discussed with reference to the existing taxonomic treatment of the genus. It currently appears that the genus Dahlia is monophyletic and it is evident that the different basic chromosome numbers within the dysploid series in Dahlia have evolved only once. This proposed monophyly of the genus coupled with the successful production of hybrids within the dysploid series (2 n=32, 2 n=34 and 2 n=36) suggests that the species of Dahlia have evolved from a common ancestor.