Molecular systematics and biogeography of Descurainia Webb & Berthel, (Brassicaceae)

Descurainia is a genus in the Brassicaceae distributed throughout temperate areas of the Old and New World. The genus is well-known for its taxonomic complexity, especially within New World species, on account of its numerous intergrading forms coupled with circumscriptions dependent upon inconsistent and overlapping characters. Descurainia is most diverse in western North America and western South America, with a smaller center of distribution in the Canary Islands and three additional Old World species. This distribution makes the genus well-suited for addressing biogeographical issues related to New World intercontinental dispersal and evolution in island systems. A molecular-based analysis of Descurainia was conducted using DNA sequences from nuclear ribosomal internal transcribed spacer (ITS), single-copy nuclear Target of Rapamycin (TOR), and non-coding chloroplast regions. The genus, with the inclusion of the monotypic genera Hugueninia and possibly Robeschia, is strongly supported as monophyletic, and appears to be of Old World origin with recent diversification within the Canary Islands and the New World. A phylogeny recovered from combined ITS and chloroplast data is not well-resolved with respect to relationships between some major New World lineages, but suggests that multiple independent dispersals of Descurainia have taken place between North and South America. Substantial incongruence between ITS, chloroplast, and TOR phylogenies, as well as the presence of mixed ITS and TOR sequences, point to a complex evolutionary history involving extensive gene flow and hybridization for North American Descurainia. The molecular data highlight possible problems with current species circumscriptions, especially within North American taxa such as D. incisa, D. obtusa, and D. pinnata. ITS and chloroplast data indicate that species of Descurainia in the Canary Islands are derived via a single colonization event, most likely from southwestern Europe onto the lowland scrub zone on Tenerife. Both intra-island adaptive radiation and inter-island colonization have played a prominent role in the evolution of this genus in the islands. The results presented in this dissertation represent the first comprehensive molecular study of Descurainia, and may serve as a phylogenetic framework for future research on the genus as well as phenomena such as speciation and hybridization in recently-evolved groups.