Molecular Data Indicate Complex Intra- and Intercontinental Differentiation of American Draba (Brassicaceae)

Abstract

The genus Draba includes about 350 species distributed primarily in the Northern Hemisphere, with some 80 species in South and Central America. Although species of Draba are well described morphologically, the existing sectional classification is highly controversial. American taxa exhibit enormous morphological differences even among species of the same section. We tested the hypothesis that variation accumulated during migration and differentiation of American Draba. The present phylogenetic study is based on analyses of the ITS (internal transcribed spacer) regions of the nuclear ribosomal DNA and the chloroplast trnL-intron and spacer sequences from 72 American Draba taxa and 6 Draba species. Results suggest that some intrageneric groupings correspond primarily to phytogeography, and that only to a small degree do these findings agree with previous sectional classification. Differences between ITS-and trnL-derived phylogenies suggest extensive genetic contact may have existed between some of the groups or sections and that this disjunction between and American Draba is demonstrated by ITS sequence data. Plastid DNA sequences suggest that the European plastome types may be more widely distributed among the American Draba species, perhaps through multiple transmissions of Eurasian chloroplast types into American Draba. Additional systematic analysis demonstrates that the genus Erophila has to be integrated into Draba. Analysis on the tribal level reveals the entire Draba complex to be close to Arabis and Aubrieta. The data provide additional support for previous assumptions that the existing tribal classifications of the Brassicaceae are mostly artificial and that the segregation of Draba and Arabis into separate tribes or subtribes does not accurately reflect their phylogenetic relationship.