Ragweeds and relatives: Molecular phylogenetics of Ambrosiinae (Asteraceae)

Ambrosiinae are one of the most distinct subtribes in the Heliantheae alliance (Asteraceae), mainly due to specialization toward wind pollination. Taxa of the subtribe are principally native to the Americas, although some species have attained a cosmopolitan distribution. Members of subtribe Engelmanniinae are considered close to Ambrosiinae, due to shared morphological traits. However, the placement of Ambrosiinae within the Heliantheae alliance has not yet been corroborated by phylogenetic analyses. In the present study, we test the circumscription of subtribe Ambrosiinae and examine relationships among its genera. We used sequence information from three plastid (psbA-trnH, trnQ-rps16 and trnL-F) and two nuclear (ITS and D35) marker regions. Phylogenetic inference analyses were conducted, applying Bayesian Inference (BI) and Maximum Likelihood (ML). Subtribe Ambrosiinae is found monophyletic or nearly so in all analyses. The genera Dugesia and Rojasianthe (previously considered part of subtribe Engelmanniinae) in some cases cluster together with Ambrosiinae; these genera are clearly not part of Engelmanniinae. Within Ambrosiinae, the genera Parthenium and Parthenice occupy basal positions, whereas members of the genus Ambrosia are the most derived representatives of the subtribe. Previous subdivision of Ambrosiinae into "Iveae" (members having androgynous capitula and free achenes) and "Ambrosieae" (genera with unisexual heads and achenes enclosed in burs) is not corroborated. Results also allow consideration of relationships among species and subgeneric groups within Parthenium, Iva, and Ambrosia.