Associate Professor University of San Diego San Diego, California
The legume genus Astragalus encompasses over 400 species in the western U.S.A. It has a remarkable diversity of fruiting morphology and defensive chemistry, likely linked to seed predation by beetles in the genus Acanthoscelides (Chrysomelidae: Bruchinae). However, there are only seven described species of seed beetle associated with this plant genus. Given that most species of seed beetle are specialists and that these legumes are highly toxic, this disconnect between host species diversity and beetle species diversity demands explanation. Are these seed beetles generalist across Astragalus species? Are they cryptic species? Is there a time lag between host plant diversification and beetle diversification? In this study, we present the results based on field collections of and associated seed beetle rearings from more than 100 species of Astragalus in the western U.S. Interspecific phylogeographic analysis reveals complicated evolutionary patterns of biogeographic distributions and host plant associations, with no consistent or predictable trends that would suggest a common theme generating or limiting the diversification of these beetles. These results suggest that diversification of seed beetles in this group has been highly idiosyncratic.
