Molecular identification of weedy glyphosate-resistant Lolium (Poaceae) in California

The origins of invasive weed populations can involve multiple species, introductions and genotypes. In California, USA, self-incompatible (SI) Lolium species are highly successful agricultural weeds. During the last decade, resistance to glyphosate has evolved in California populations of Lolium . However, the species identity of glyphosate-resistant populations is unclear. Occurrence of three SI species, L. multiflorum , L. rigidum and L. perenne , and the lack of reproductive barriers between them, suggests that any or all species may be contributing to resistant populations. To elucidate the identity and evolutionary history of resistant Lolium , we assessed genetic variation of known species and unknown California individuals using five simple sequence repeat loci, a nuclear DNA sequence and a chloroplast DNA sequence. Employing two analytical approaches, we identified three major genetic groups within the accessions of SI species that were similar, but not identical, to the taxonomic species identities. California glyphosate-resistant and susceptible plants were most closely related to the L. multiflorum group. However, a few glyphosate-resistant plants and a third of the glyphosate-susceptible plants also identified with L. rigidum and L. perenne . While L. rigidum and L. perenne contribute to invasive populations within California, the majority of glyphosate-resistant individuals tested were L. multiflorum .