Introduction and Spread of \u3cem\u3eBromus tectorum\u3c/em\u3e (Cheatgrass) into Midwestern United States: Population Genetic and Evolutionary Consequences

High propagule pressure is correlated with invasion success, and has important implications for the genetic diversity and evolutionary potential of a species in its introduced range. Here, I examine an invasive annual grass Bromus tectorum and document the population genetic consequences that resulted from multiple introductions of genotypes native to different Eurasian regions into the North American Midwest. Herbarium collections showed that B. tectorum was first recorded near-contemporaneously throughout the Midwest in the late 1800s. Allozyme diversity data from 60 populations were used to assess the origin and frequency of introductions into the Midwest. Genetic variation and structure was compared to similar measures for the Eurasian native range and other North American regions to infer the introduction pathways of B. tectorum across the continent. A minimum of four to five introduction events contributed to genetic diversity in the Midwest. A genetic bottleneck associated with introduction was detected across populations, yet more Midwestern populations were polymorphic (53%), and populations were more diverse (HT = 0.187) and less structured (GST = 0.582) than native range populations, indicating high propagule pressure was associated with the invasion of B. tectorum into the Midwest. Within the Midwest, genetic diversity and structure values were compared for individual polymorphic loci to infer propagule pressure for individual genotypes, and scenarios for their likely invasion pathways were drawn based on historical collections and the history of the region. The intermixing of previously allopatric genotypes within populations in the Midwest may have allowed the formation, via outcrossing, of a novel multilocus genotype found only in this region of North America

Propagule Pressure and Introduction Pathways of \u3cem\u3eBromus Tectorum\u3c/em\u3e (Cheatgrass; Poaceae) in the Central United States

An introduced species\u27 propagule pressure strongly influences the genetic diversity and evolutionary potential of its descendants and even the likelihood of biological invasion. We examined population genetic consequences arising from introduction of the invasive annual grass Bromus tectorum into the central United States (U.S.). The origin and frequency of introductions were investigated by assembling allozyme diversity data from 60 widely spaced populations. At least five introduction events contributed to the grass’s genetic diversity in the central U.S. Populations in this region have fewer alleles (30 vs. 43) and polymorphic loci (5 vs. 13) than native populations, evidence of a genetic bottleneck. However, the populations are, on average, more genetically diverse and less structured than native populations. Assembly within central U.S. populations of previously allopatric genotypes may have allowed the formation, via outcrossing, of a rare multilocus genotype. Genetic admixtures may have occurred through any combination of east-to-west spread coincident with nineteenth-century arrival of European settlers, dispersal from southern Ontario via Great Lakes shipping and commerce, and direct introduction from the native range. Our results illustrate the population genetic consequences of relatively high propagule pressure (i.e., repeated immigrations to a new range from multiple sources)