The phylogenetic relationships within two major locoweed genera, Astragalus and Oxytropis, and among varieties of woolly loco found in New Mexico were analyzed by comparing their chloroplast rpoC1 and rpoC2 gene sequences. Nucleic acids from locoweed species and varieties collected from different geographical locations in New Mexico were amplified using specific primer sets and subjected to restriction fragment analyses. Identity of the amplicons was confirmed by determining the 5′-end sequences from pea and woolly loco var. matthewsii. The amplified sequences from all samples were digested with 16 different restriction enzymes. Presence or absence of individual restriction fragments was scored as binary characters and used to develop a similarity coefficient matrix for cladistic analyses to determine the phylogenetic relationships. The target sequence was conserved, yielding 7% polymorphic data. Oxytropis species were monophyletic and, as expected, formed a clade distinct from Astragalus. The average similarity coefficient among woolly loco varieties was very high (0.9733), but the varieties still separated into three different clades. The phylogenetic relationship among woolly loco varieties coincided with their geographic distribution but was unrelated to insect feeding preference.

Nomenclature: Woolly loco, Astragalus mollissimus Torr. ASAML; pea, Pisum sativum L. ‘Alaska’.