Phylogenetic relationships in Helianthus (Asteraceae) based on nuclear ribosomal DNA internal transcribed spacer region sequence data

The internal transcribed spacer (ITS) region of the nuclear ribosomal DNA w as sequenced from 73 samples representing 46 of the 49 species of Helianthu s and both species of Phoebanthus. Overall there was only a small amount of sequence divergence, less than 4% within Helianthus and less than 5.5% bet ween Phoebanthus and Helianthus. Phylogenetic analysis supported a close re lationship between Phoebanthus and Helianthus as sister groups within subtr ibe Helianthinae. Within Helianthus, four species of the southeastern U.S. were separated successively as basally diverging clades: H. heterophyllus, H. carnosus, H. porteri, and H. agrestis. There was little differentiation among the remaining species. The annual species of sect. Helianthus were pl aced in a weakly supported clade, within which three further clades were al so weakly supported: H. annuus/H. argophyllus/H. bolanderi; H. debilis/H. p raecox; H. petiolaris/H. neglectus/H. deserticola/H niveus subsp. niveus an d subsp. canescens. The perennial species did not form a monophyletic clade . Within the perennials, there were a few clades of weak to moderate suppor t: H. augustifolius/H. floridanus/H. simulnns; H. giganteus/H grosseserratu s/H, maximiliani/H. nuttallii/H. divaricatus; H. arizonensis/H, ciliaris/H. laciniatus; and H, atrorubens/H. mollis/H. occidentalis/H silphioides. The phylogenetic placement of species with narrow geographic distributions in the extreme southeastern part of North America as the sister group (Phoeban thus) and as basally diverging branches of Helianthus in the ITS trees sugg ests that the ancestor to the genus may have been confined to this area pri or to divergence and dispersal leading to the extant array of species.