A 21 base-pair sequence, invariant among Carduus nutans and Cirsium andrewsii, was used to design primer ETS-Car-1, beginning at base pair 561 upstream from the 3 end of ETS (referring to the sequence of Cirsium andrewsii). A subsequent PCR of 10 diverse Cardueae taxa using primers ETS-Car-1 and 18S-E produced products of identical length. Maximum corrected (HKY85) pairwise ETS-region sequence divergence was 14.8% between Centaurea calcitrapa and Atractylodes japonica. Within Cirsium, the maximum corrected (HKY85) pairwise ETS-region sequence divergence was found between C. henryi and C. arvense (5.2%). For phylogenetic analysis, 52 taxa were sequenced for ETS and ITS, including 34 North American native Cirsium species, 13 Old World Cirsium species, and two species each of Carduus and Onopordum. Divergence between the ITS and ETS sequences were similar. Phylogenetic analysis supported the monophyly of Cirsium and C. subgenus Eucirsium (New World thistles). Two clades of West Coast species appeared; one including Pacific Northwest and montane species with n=34 and one comprising California endemics with n=32. ETS and ITS divergence among Cirsium are significantly lower than in other studied groups, as determined by comparison of divergence in six California plant groups. The relatively low levels of rDNA spacer sequence divergence contrast with the great ecological diversity displayed by new World Cirsium.

Key words: Cardueae, Cirsium, Compositae, ETS, phylogeny