Toward a Global Phylogeny of the Brassicaceae

TLDR

The Brassicaceae is a large plant family of 3,700 species, but its taxonomy is confounded by convergent evolution across morphological traits. The study aimed to test the monophyly of many Brassicaceae tribes and genera using novel ITS analysis and extensive sampling. Phylogenetic analysis of 746 nrDNA ITS sequences from 24 tribes, 146 genera, and 461 species, employing parsimony ratchet and Bayesian inference, produced the most comprehensive single‑locus phylogeny to date. The analysis confirmed monophyly for 12 tribes and several genera, identified polyphyly in others, uncovered problematic sequences, revealed weak backbone support due to rapid radiations, and suggested that a preliminary 10‑locus supermatrix could improve intertribal resolution.

Abstract

The Brassicaceae is a large plant family (338 genera and 3,700 species) of major scientific and economic importance. The taxonomy of this group has been plagued by convergent evolution in nearly every morphological feature used to define tribes and genera. Phylogenetic analysis of 746 nrDNA internal transcribed spacer (ITS) sequences, representing 24 of the 25 currently recognized tribes, 146 genera, and 461 species of Brassicaceae, produced the most comprehensive, single-locus–based phylogenetic analysis of the family published to date. Novel approaches to nrDNA ITS analysis and extensive taxonomic sampling offered a test of monophyly for a large complement of the currently recognized tribes and genera of Brassicaceae. In the most comprehensive analysis, tribes Alysseae, Anchonieae plus Hesperideae, Boechereae, Cardamineae, Eutremeae, Halimolobeae, Iberideae, Noccaeeae, Physarieae, Schizopetaleae, Smelowskieae, and Thlaspideae were all monophyletic. Several broadly defined genera (e.g., Draba and Smelowskia) were supported as monophyletic, whereas others (e.g., Sisymbrium and Alyssum) were clearly polyphyletic. Analyses of ITS data identified several problematic sequences attributable to errors in sample identification or database submission. Results from parsimony ratchet and Bayesian analyses recovered little support for the backbone of the phylogeny, suggesting that many lineages of Brassicaceae have undergone rapid radiations that may ultimately be difficult to resolve with any single locus. However, the development of a preliminary supermatrix including the combination of 10 loci for 65 species provides an initial estimate of intertribal relations and suggests that broad application of such a method will provide greater understanding of relationships in the family.