Breeding Biology of the Threadstalk Milkvetch, Astragalus Filipes (Fabaceae), with a Review of the Genus
Watrous, Kristal M., Cane, James H., The American Midland Naturalist
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.
Plants in the legume family (Fabaceae sensu lato Lindi.) are of global economic and ecological significance. The Fabaceae is second only to the grasses in economic importance and is only smaller than the Orchidaceae and Asteraceae in numbers of species. Papilionoid legumes are particularly valuable as ground cover, forage and food crops (Allen and Allen, 1981).
The genus Astragalus L. is the largest of the flowering plant genera (Frodin, 2004). Comprising some 3270 species, Astragalus is most diverse in the Sino-Himalayan region, Russia, the Andes mountains of South America and across western North America (Allen and Allen, 1981; Isely, 1998). Nearly 400 species of Astragalus occur in North America, with 156 species occurring in the Intermountain West alone (Barneby, 1964). Plants in this genus are economically significant as a source of gum tragacanth, as indicators of selenium and uranium and as toxic locoweeds in rangelands (Allen and Allen, 1981). The more widespread and common species can support diverse elements of the region's pollinating bee communities as well (e.g., Green and Bohart, 1975; Clement et al, 2006)
Few Astragalus breeding biologies are known despite the size, geographic extent and prevalence of the genus. A literature search revealed known breeding biologies for only 29 Astragalus species worldwide, accounting for <1% of all Astragalus species (Table 1). These few species have breeding biologies that span the range of self fertility, from self compatible to self incompatible; a few are obligately xenogamous. One annual species (A. cymbicarpos Brot.) is even cleistogamous (Gallardo et al, 1993). This range of breeding biologies within the genus, coupled with a general paucity of knowledge about most species, makes predicting the breeding biology of a given Astragalus species dubious at best.
Our primary objective was to experimentally characterize the breeding biology of Astragalus filipes Torrey ex. A. Gray (threadstalk milkvetch or basalt milkvetch), which is widespread in western North American (Isely, 1998), and compare it with a compilation of other such studies with Astragalus. This species has been evaluated and now propagated for seed to use in future rangeland rehabilitation projects throughoLtt its range (Shaw et al, 2005). Wildland seed production is erratic and susceptible to beetle prédation, making it more costly and unpredictable than cultivated seed production (Youtie and Miller, 1986; Cane, 2008a). The first tested germplasm for A. …