The natural delimitation of Centaurea (Asteraceae: Cardueae): ITS sequence analysis of the Centaurea jacea group.
Taxonomic complexity has hindered partitioning the genus Centaurea into natural subdivisions, even though it has long been recognized as an unwiedly, artificial assemblage. Most of the remaining difficulties are located in the Centaurea jacea group whose taxa share a common advanced type of pollen. Because it comprises half the species of the genus Centaurea, as well as five other disputed genera previously segregated from Centaurea (Chartolepis, Cheirolepis, Cnicus, Grossheimia and Tomanthea), the Centaurea jacea group is a significant taxonomic challenge. Newer molecular approaches are useful for resolving complex relationships because they provide more precise inferences of evolutionary relationships than traditional morphological characters. Sequences of the Internal Transcribed Spacers (ITS) of nuclear ribosomal DNA were analysed for a comprehensive sample of this group. Results indicated that the Centaurea jacea group is monophyletic and includes the segregated genera, but not two other genera (Oligochaeta and Zoegea), whose inclusion in the Centaurea jacea group was doubtful. In addition to pollen morphology, the ITS phylogeny is also supported by karyological evidence and by good correlation with biogeographic distribution of the species. The monophyly of the Centaurea jacea group suggests that a natural delimitation of Centaurea that minimizes nomenclatural changes is possible, but only if a new type of the genus is designated.