Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2009
Publication Date: 9/21/2009
Citation: Leger, E.A., Espeland, E.K., Merrill, K.R., Meyer, S.E. 2009. Genetic variation and local adaptation at a cheatgrass (Bromus tectorum) invasion edge in western Nevada. Molecular Ecology. 18(21): 4366-4379.
Interpretive Summary: Cheatgrass (Bromus tectorum) is an invasive weed in western North America with a distribution primarily at less than 2200 m in the west. We asked if cheatgrass is capable of becoming adapted to high elevations in the near future. In order for rapid adaptation to high elevation to occur, cheatgrass 1) must not be able to proliferate at high elevation in its current, unadapted state, 2) heritable variation must exist, and 3) natural selection must act on this variation. We planted 100 families (two seeds each) collected from low and high elevations into low and high elevation gardens. Survivorship was low at the high elevation, with only 46 plants surviving from 400 seeds planted, compared with 211 survivors out of 400 seeds planted at the low elevation. This indicates that cheatgrass must adapt in order to persist at high elevations. There was heritable variation for seed size and for both germination and emergence timing. Because survival at high elevation was low, we do not know if these traits confer fitness to plants and if natural selection can act upon them. Early emergence conferred fitness to plants at low elevation. Genotypic analysis showed evidence of outcrossing in this highly selfing species in the high elevation population. The presence of genetic variation, heritable phenotypes, and low survivorship at high elevation indicate that the potential for adaptation to high elevation exists within cheatgrass.
Technical Abstract: 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 high elevations at a population near a high elevation invasion edge, using a combination of methods, including field reciprocal transplants, controlled environment studies, and molecular analysis. 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. Survivorship was low at the high elevation (13%), compared to the low elevation (55%). Seed size positively affected emergence, survival, and plant performance in the field, and this trait is heritable. Timing of emergence affected survival at the low elevation site, and germination timing is 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.