Genetic variation and local adaptation at an invasion edge of cheatgrass (Bromus tectorum)

Background/Question/Methods Cheatgrass (Bromus tectorum) is an invasive weed in western North America primarily found growing at elevations less than 2200 m. We asked whether cheatgrass is capable of becoming adapted to a peripheral habitat, investigating a population near a high elevation invasion edge. We used a combination of methods, including field reciprocal transplants, controlled environment studies, and molecular analysis. Results/Conclusions High levels of SSR gene diversity (0.50 - 0.43) and comparable variation in phenotypic traits were observed between high and low elevation populations. Three heterozygotes were observed in the high elevation population, which is unusual in this predominantly self-pollinating organism. Survivorship was low at the high elevation (13%), compared to the low elevation site (55%). Seed size positively affected emergence, survival, and plant performance in the field, and this trait was heritable. Timing of emergence affected survival at the low elevation site, and germination timing was also heritable. Plants from high elevations germinated more slowly than plants from high elevations and had lower seedling growth rates. The strongest local adaptation was seen in the low, rather than in the high, elevation site. There were no differences in the size or reproductive output of surviving plants or in the size or reproduction of high and low elevation seed sources in either environment. While there was only weak evidence for local adaptation to the high elevation observed in the field, family-level and genotype-level differences in traits that affect fitness, high genetic diversity, and evidence of outcrossing in this highly selfing species indicate that the potential for adaptation to high elevation exists within this population.