Phylogenetic Relationships in Tribe Cardueae (Asteraceae) Based on ITS Sequences

Abstract

The Cardueae pose vexing systematic problems at several taxonomic levels. Tribal delimitation is controversial, with views ranging from a broadly defined Cardueae comprising subtribes Echinopsidinae, Carlininae, Carduinae, and Centaureinae to a more restrictive Cardueae (comprising only Carduinae and Centaureinae) with tribal rank also given to Echinopsideae (or Echinopeae) and Carlineae. Furthermore, the circumscription of groups within a broad Cardueae is also controversial, with divergent views emphasizing differences in floral or achene morphology; the distinction between Carduinae and Centaureinae is particularly weak. The generic limits of Centaurea, comprising 500–600 species and great diversity in both chromosome number and pollen morphology, are also unclear. Patterns of morphological, chromosomal, and pollen diversity in Centaurea do not completely support either the current taxonomy or any of the proposed modifications. Clearly, the Cardueae in general and Centaurea in particular require a phylogenetic approach to resolve problems of taxonomic circumscription and rank and to understand patterns of pollen and chromosomal evolution. We therefore analyzed DNA sequences of the internal transcribed spacers (ITS) of the nuclear ribosomal RNA genes (rDNA) of 35 taxa to reconstruct the phylogeny of Cardueae and assess 1) the positions of Echinopsidinae and Carlininae, 2) the circumscription of the Carduinae-Centaureinae complex, and 3) the limits of Centaurea and related genera. Phylogenetic analysis of ITS sequence variation strongly supports the monophyly of Cardueae relative to outgroup taxa from Mutisieae and Cichorieae. Both Carlininae and Echinopsidinae occupy basal branches in Cardueae. Carduinae are paraphyletic, forming a basal grade in the tribe. The Centaureinae, however, form a well-supported clade. As typically circumscribed, Centaurea is paraphyletic, comprising species of several other genera in addition to those currently recognized as Centaurea. Proposed evolutionary trends in pollen morphology and, to a lesser extent, base chromosome numbers are largely supported by the ITS phylogeny for Cardueae.