Results of phylogenetic analyses of nuclear 18S-26S rDNA ITS-region sequences for representatives of all but four recognized genera of helenioid Heliantheae (i.e., Helenieae s. lat.) and various members of Heliantheae s. str. and Eupatorieae help to clarify major lineages in the clade corresponding to Heliantheae s. lat. plus Eupatorieae. Most subtribes of helenioid Heliantheae circumscribed by Robinson (1981) correspond closely with ITS clades. Polygeneric subtribes of helenioid Heliantheae that appear to be monophyletic based on ITS data include Flaveriinae s. Turner and Powell (1977), Madiinae s. Carlquist (1959), and Peritylinae s. Robinson (1981). Chaenactidinae s. Robinson (1981) is polyphyletic but most members of the group are encompassed within only four ITS clades. Based on the ITS data, Heliantheae s. str. and Eupatorieae represent clades nested among clades of helenioid Heliantheae, as previously suggested. Loss of receptacular bracts appears to have occurred rarely during radiation of Heliantheae s. str.; only one of the epaleate taxa that we sampled (Trichocoryne) was placed within the ITS clade corresponding to Heliantheae s. str., a principally paleate group. We conclude that receptacular bracts were acquired independently in the ancestors of Heliantheae s. str. and Madiinae. We also conclude that pappi of bristles have evolved in various lineages of helenioid Heliantheae and often have received too much weight in circumscriptions of suprageneric taxa. Multiple examples of extreme dysploidy from high ancestral chromosome numbers in helenioid Heliantheae are evident from the phylogenetic data. Bidirectional ecological shifts between annual and perennial habits and repeated origins of woodiness from herbaceous ancestors also must be concluded for helenioid Heliantheae. Based on modern distributions of taxa and evident phylogenetic patterns, the most recent common ancestor of Heliantheae s. lat. and Eupatorieae probably occurred in southwestern North America (including northern Mexico). Baeriinae, Madiinae, and the x = 19 "arnicoid" taxa may share a common Californian ancestry.

Key words: Asteraceae, Compositae, Helenieae, Heliantheae, ITS, Phylogeny