The internal transcribed spacer (ITS) region of 18S–26S nuclear ribosomal DNA (rDNA) was sequenced in 65 taxa representing most coneflowers (i.e., species in Dracopis, Echinacea, Ratibida, and Rudbeckia) and other taxa representing 21 outgroup genera of tribe Heliantheae. Results of parsimony analysis of the rDNA dataset by itself and in combination with the cpDNA dataset uphold the hypothesis from an earlier cpDNA restriction site study that Echinacea is not closely related to the other three genera of coneflowers. The data support placement of Echinacea in subtribe Zinniinae. The remaining three coneflower genera represent a monophyletic lineage corresponding to subtribe Rudbeckiinae sensu H. Robinson. The rDNA data support two sublineages in Rudbeckia congruent with the two traditionally recognized subgenera, subg. Macrocline and subg. Rudbeckia. In subg. Macrocline, two geographic areas of diversification are indicated: southeastern and western United States. The widespread species R. laciniata is placed strongly with the western lineage of subg. Macrocline. The rDNA data support transfer of Dracopis to Rudbeckia subg. Macrocline, a relationship also supported by multiple morphological characters. The rDNA data do not confidently resolve the sister group of Rudbeckiinae from among the members of Heliantheae investigated. Multiple hypotheses are suggested for the outgroup taxa, such as expanding subtribe Zinniinae to include Echinacea and Trichocoryne, a genus previously regarded as belonging to subtribe Hymenopappinae (Heleneae or Heliantheae sensu lat.). Our findings further support expansion of subtribe Engelmanniinae to include Balsamorhiza, Borrichia, and Wyethia even though these taxa lack ray floret complexes and have fertile disc ovaries. We suggest that bioprospectors might usefully search among taxa of Zinniinae for bioactive substances similar to the immune stimulants of Echinacea.
Communicating Editor: Alan Whittemore