Phylogenetic analysis of Silphium and subtribe Engelmanniinae (Asteraceae : Heliantheae) based on ITS and ETS sequence data

The phylogenetic relationships of Silphium and subtribe Engelmanniinae were examined using DNA sequence data. The internal transcribed spacer (ITS) re gion and the external transcribed spacer (ETS) region were sequenced for 39 specimens representing the six genera of subtribe Engelmanniinae (Berlandi era, Chrysogonum, Dugesia, Engelmannia, Lindheimera, and Silphium), plus fi ve additional genera identified as closely related to the Engelmanniinae by chloroplast DNA restriction site analysis, and three outgroups. Phylogenet ic analysis supported the monophyly of Silphium with Lindheimeru as sister. Silphium can be divided into two sections based upon two well-supported cl ades that correspond to root type and growth form. These results also suppo rted the expansion of subtribe Engelmanniinae to include Balsamorhiza, Borr ichia, Rojasianthe. Vigethia. and Wyethia. We hypothesize that subtribe Eng elmanniinae originated in Mesoamerica and later radiated to the United Star es. We suggest that the cypsela complex, which is present in Berlandiera, C hrysogonum, Engelmannia, and Lindheimera, arose only once and was subsequen tly lost in Silphium.