ARS | Publication request: Microsatellite evidence of facultative outcrossing in cheatgrass (Bromus tectorum): Implications for the evolution of invasiveness

Submitted to: Plant Species Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2008
Publication Date: December 16, 2007
Citation: Ashley, M.C., Longland, W.S. 2007. Microsatellite evidence of facultative outcrossing in cheatgrass (Bromus tectorum): Implications for the evolution of invasiveness. Plant Species Biology. 22:197-204.

Interpretive Summary: It has been a long held assumption that cheatgrass (Bromus tectorum), a highly invasive exotic annual grass, is an exclusive self-pollinator or at best only very rarely cross-pollinates. This phenomenon has been reported only once, two individuals sampled from twenty-five Eurasian populations, from a study of this species with protein markers. Two studies to date using microsatellite markers showed differences in genetic variation among cheatgrass populations but no evidence cross-pollination. This microsatellite study of four cheatgrass populations in northern Nevada shows similar degrees of genetic variation and the presence of fifteen heterozygous individuals, the product of cross-pollination, in two of the populations sampled. These results are important in that they demonstrate the value of microsatellites as a tool for further studies of cheatgrass genetics. It also shows the need for incorporation of cross-pollination as a factor of how cheatgrass has been able to evolve and adapt to a wide variety of habitats since its introduction to North America.

Technical Abstract: Genetic variation and the presence or absence of heterozygotes were determined for the invasive exotic annual grass Bromus tectorum from four northern Nevada populations using seven polymorphic species-specific microsatellite primers. Allelic polymorphisms were present in all populations but not for all loci within each population. Mean genetic variation across populations was 0.0.234 ± 0.052, ranging from 0.023 ± 0.014 to 0.550 ± 0.095 within populations. Fifteen individuals from the total sample (8.24%) were heterozygotic at one to four loci with relative abundances of 21.88% and 17.91% in the two populations containing heterozygotes. The marked presence of heterozygotes in cheatgrass populations is a significant aspect for an improved understanding of the evolution of invasive genotypes for this species.