Molecular Differentiation of Astragalus Species and Varieties from the Western United States: The Chloroplast DNA Bridge Between Evolution and Molecular Systematics

Locoweeds are the most widespread poisonous plant problem in the world and have been reported in the Western United States since the 1800s, causing tremendous losses in livestock. Consumption of locoweeds by grazing animals stimulates the neurological disease, locoism, characterized by weight loss, ataxia, and lack of muscular coordination. The name locoweed is used for Astragalus and Oxytropis species known to contain swainsonine, the toxic principle produced by the plant endophytic fungus Undifilum. Astragalus includes 2,500-3,000 species and many varieties that have almost identical morphological characteristics that overlap among species, leading to improper identification. Therefore, the aim of this study was to develop effective markers for differentiation of Astragalus at low taxonomic level. Genomic DNA extraction, followed by PCR amplification were carried out with species and varieties of Astragalus. Primers targeted regions from the chloroplast genome (DE, trnH-psbA, rpl32-trnL) and the nuclear genome from the plants, (ITS1-ITS4) and phylogenetic trees were generated. Sequences from the chloroplast genome of Astragalus species and populations within a species had sufficient variation to support taxonomic differentiation at intraspecific and interspecific levels. The noncoding rpl32-trnL region of the chloroplast genome represent more specific differentiation of populations within a species than other chloroplast noncoding regions tested in this study, due to high numbers of potentially informative characteristics. All primer sets used in this study successfully amplified across species and varieties of Astragalus, and their combined sequences constructed well supported concatenated phylogenetic tree. This molecular systematics study aimed to provide a reliable plant identification system using high-resolution molecular markers at low taxonomic levels, to investigate the evolutionary diversification of locoweed, to improve understanding of the plant-fungal relationship, and ultimately, benefit management of locoweed