ARS | Publication request: Phylogenetic, morphological, and chemotaxonomic incongruence in the North American endemic genus Echinacea (Moench)

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 4, 2008
Publication Date: May 20, 2008
Citation: Flagel, L.E., Rapp, R.A., Grover, C.E., Widrlechner, M.P., Hawkins, J., Grafenberg, J.L., Álvarez Fernández, I., Young Chung, G., Wendel, J.F. 2008. Phylogenetic, morphological, and chemotaxonomic incongruence in the North American endemic genus Echinacea (Moench). American Journal of Botany. 95:756-765.

Interpretive Summary: Purple coneflowers, members of the genus Echinacea, are widely used as ornamental plants and as botanical dietary supplements. The evolution of this group of plants is poorly understood. To learn more about the evolutionary history of these plants and their nearest relatives, we sequenced DNA from several nuclear and chloroplast genes sampled from across the native ranges of all nine Echinacea species. Chloroplast data indicates that Echinacea is sister to the genus Sanvitalia, a common bedding plant. However, levels of chloroplast DNA variation are too low to resolve evolutionary relationships within Echinacea. We can only say that there are three dominant types, one on each side of the Mississippi River and one that spans the entire geographic range. DNA sequence data from nuclear genes did not resolve hypotheses about species-level relationships. This is likely due to the combined effects of incomplete lineage sorting, hybridization and backcrossing among different types following secondary contact after the glacial retreat. Our DNA results stand in strong contrast to well-resolved and taxonomically supported biochemical and morphological data, indicative of an evolutionary process called phenotypic canalization, which can produce clearly distinguishable morphological species. We attempt to explain differences between our DNA-sequence data and the biochemical/morphological approach by examining the impacts of natural history and biogeography on the genetic organization and evolutionary history of Echinacea. These findings should be useful to geneticists, taxonomists, and medicinal-plant researchers who work to link patterns of genetic and morphological variation.

Technical Abstract: We sequenced several nuclear and chloroplast loci from range-representative populations of all species and subspecies of the North American endemic genus of purple coneflower, Echinacea Moench. Chloroplast data support Echinacea as sister group within the Heliantheae to the American endemic genus Sanvitalia Lam. Plastid data from Echinacea generate unresolved phylogenies due to low levels of polymorphism, but identifies three dominant cytotypes, one on each side of the Mississippi River and one that spans the entire range of the genus. Mantel tests against Euclidean distances indicate that cytotype structure is governed by physical proximity and not taxonomic boundaries. Nuclear-locus data provide neither resolved topologies nor congruent hypotheses about species-level relationships, likely due to the combined effects of incomplete lineage sorting, hybridization and backcrossing following secondary contact. Additionally, these neutral molecular markers are in strong contrast to the well-resolved and taxonomically supported metabolic and morphological data, indicating potential phenotypic canalization resulting in identifiable morphological species. Here we explore the impact of natural history on the genetic organization and phylogenetic relationships of Echinacea.