Much argument has been occasioned by questions having to do with gener
ic concepts in the tribe Heliantheae, subtribe Ambrosiinae, especially
involving the genus Iva. Some workers recognize five genera (e.g., Le
uciva, Chorisiva, Oxytenia, Cyclachaena, and Iva s. str.), while other
s recognize only a single genus Iva s. 1. Although chemical, pollen ul
trastructure, and other morphological data have challenged the latter
concept, relationships among the segregate taxa are still unclear. The
current study used cladistic analyses of chloroplast DNA variation to
investigate evolutionary relationships within Iva s. 1., especially w
ithin the controversial section Cyclachaena. Nuclear rDNA was also uti
lized to test for hybrid origin of selected taxa. The results support
the dismemberment of Iva s. 1. and recognition of Iva s. str., largely
because Iva s. 1. is shown to be paraphyletic. Most members of the se
ction Cyclachaena were found to have relatively close interspecific re
lationships. Nevertheless, cpDNA data strongly support two lineages wi
thin Cyclachaena. One lineage, including I. xanthifolia, I, acerosa, I
, nevadensis, and I. dealbata, has a close relationship with the genus
Euphrosyne. The second lineage includes only one species, I. ambrosia
efolia, which is closely related to Dicoria. The origin of I. ambrosia
efolia was shown to be monophyletic, not involving hybridization.