Ecological significance of microsatellite variation in western North American populations of Bromus tectorum

Bromus tectorum (cheatgrass or downy brome) is an exotic annual weed that is abundant in western USA. We examined variation in six microsatellite loci for 17 populations representing a range of habitats in Utah, Idaho, Nevada and Colorado (USA) and then intensively sampled four representative populations, for a total sample size of approximately 1000 individuals. All loci were homozygous, indicating that the species is strongly selfing. Populations consisted of a few common genotypes and variable numbers of rare genotypes. Small sample sizes (n = 10 individuals) were adequate for distinguishing among populations, but larger sample sizes were needed to characterize more diverse populations, particularly in terms of genotype. Large populations contained more genetic diversity than small populations in terms of both number of alleles per locus and number of genotypes. Genetic distance among survey populations was much more strongly correlated with ecological distance (habitat) than with geographical distance, and was also strongly correlated with a suite of adaptively significant seed germination traits. This suggests that similar habitats across the range of B. tectorum in western USA select for specific self-pollinating lines from an array of widely distributed genotypes. Because all traits are effectively linked in this selfing organism, the distribution of adaptively significant genetic variation can be successfully inferred from an examination of microsatellite marker variation.