Mating System, Introduction and Genetic Diversity of Bromus tectorum in North America, the Most Notorious Product of Evolution Within Bromus Section Genea

Abstract

The genus Bromus (Poaceae) consists of approximately 150–160 species that occur in temperate regions worldwide. Members of section Genea in Bromus are native to Eurasia, and most are invasive in North America (NA). Most environmentally and economically important among these species is Bromus tectorum, which along with Bromus rubens and B. diandrus are some of the most destructive invasive grasses in the Western United States (USA). Members of section Genea form a polyploid series. Molecular data suggest a reticulate speciation pattern (a netlike pattern of diversification) for the polyploid members of Genea. Seed set, floral characteristics, sex allocation patterns, and genetic markers reveal that B. tectorum is predominately self-pollinating. Historical records and genetic data indicate that B. tectorum entered repeatedly into NA and that its current distribution can be attributed to multiple phenomena, including direct introduction from its native range and multidirectional range expansion. Across invasive populations of B. tectorum, overall genetic diversity (e.g., allelic richness and number of polymorphic loci) is lower compared to the diversity among native populations. Conversely, invasive populations of B. tectorum exhibit, on average, higher within-population genetic diversity than found in native populations, suggesting that multiple introductions have partially offset founder effects within invasive populations. Additionally, invasive populations appear to be genetic admixtures composed of two or more independently derived native genotypes. These admixtures have produced invasive populations with generally less genetic differentiation than native populations. The genetic consequences of multiple introductions have implications for the management of invasive populations of B. tectorum, especially in Western NA.