Astragalus L. (Fabaceae) is an enormous and diverse plant genus with a cosmopolitan distribution, but relatively few breeding biologies are known for its member species. Threadstalk (or basalt) milkvetch, Astragalus filipes Torrey ex. A. Gray, is common and widespread throughout the U.S. Intermountain West, including the Great Basin. It is being studied and ultimately propagated for extensive rangeland restoration projects throughout the sagebrush steppe. Understanding the breeding biology of A. filipes will be necessary for reliable and consistent commercial seed production with this species. We examined reproductive output from four manual pollination treatments (autogamy, geitonogamy, xenogamy and distant xenogamy) in a common garden. As measures of fitness, we counted fruit and seed set, then germinated viable seeds, to assess reproductive output. This species is weakly self compatible; xenogamous pollen transfer results in nine times more seed per pollination. Pollen transfer between geographically distant seed accessions resulted in a decrease in seed germination, but no difference in fruit or seed set. Cross pollination by bees will be necessary for copious seed production by this species. In the wild, flowers of A. filipes are visited most commonly and ubiquitously by a diversity of Osmia bee species plus several bee species each of Eucera, Anthidium, Bombus and sometimes Hoplitis.
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Vol. 165 • No. 2