Putative monophyly of New World Cirsium based on phylogenetic analysis of the ITS and ETS regions of ribosomal DNA (rDNA) suggests that Cirsium invaded North America only once before the advent of European peoples. Therefore, indigenous North American members of the genus are each equally closely related to weedy thistles introduced into North America from the Old World. Use of rDNA transcribed spacers for resolving phylogeny within New World Cirsium is limited by low sequence variation among species but evidently not by the rate of concerted evolution. Ribosomal DNA transcribed spacers in Cirsium appear to have undergone sufficiently rapid concerted evolution for use in phylogeographic study of at least one widespread species, Canada thistle (C. arvense). This dioecious, extensively clonal species is native throughout Eurasia, but it is also widely adventive throughout the world. Analysis of the ITS and the 3' end of the ETS regions in 13 accessions of Canada thistle revealed that the Japanese C. a. var. setosum may not belong in the C. arvense clade. Among 12 accessions within the C. arvense clade, there are 40 variable sites in the ITS and 25 in the ETS. This represents about 42% and 50% of the number of variable sites found among the ITS and ETS, respectively, of 35 samples representing the New World radiation (ca. 80 spp.). The C. arvense clade has 14 ITS and 18 ETS potentially phylogenetically informative sites (P.P.I.S.) versus 39 ITS and 19 ETS P.P.I.S. for New World Cirsium. The low genetic diversity of New World Cirsium compared to a single, widespread Old World species supports a relatively recent origin for the New World Clade. This represents a continental parallel to insular composite lineages displaying low rDNA divergence and extensive ecological diversity.

Key words: Cirsium arvense, Cirsium, ETS, ITS, radiation, systematics