ARS | Publication request: Microsatellite evidence of heterozygosity in cheatgrass (Bromus tectorum): implications for the evolution of invasiveness

Submitted to: Society for Range Management Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: August 31, 2006
Publication Date: N/A

Interpretive Summary: Cheatgrass is a highly invasive exotic annual grass that has caused severe ecological impacts in the western United States. Disagreement exists in the literature as to whether cheatgrass always self-pollinates or, if it does cross-pollinate, to what extent may this occur. Cross pollination is an important factor of plant evolution. Evidence of cross-pollination in cheat grass may help to explain the advance of this species into new and ever more extreme habitat types since its introduction to North America in the late nineteenth century. Genetic analysis using microsatellite markers is a powerful tool for answering such questions. Heterozygosity, having two different alleles (fragment sizes) for the same marker, is highly indicative of cross-pollination, especially in a species that usually self-pollinates. We analyzed individuals from four populations in western Nevada with seven microsatellite markers and obtained evidence of heterozygosity in some of the individuals from two of those populations. While heterozygosity rates were low in the two populations that exhibited this trait, there was still enough evidence to infer that cross pollination had likely occurred between plants within these two populations in the previous year.

Technical Abstract: We assayed genetic variation and the presence or absence of heterozygous individuals for four cheatgrass (Bromus tectorum) populations in north-western Nevada using seven polymorphic species-specific di-nucleotide microsatellite markers. Allelic polymorphisms were found in all populations but not at all loci within each population. Mean genetic diversity across loci and populations was reasonably high (_ = 0.355 ± 0.085) for a predominantly inbreeding species. Within-population mean genetic diversity ranged from 0.023 ± 0.014 to 0.550 ± 0.095. Fifteen individuals (8.15%) from the total sample (n = 184) were heterozygotic at one or more loci. Two populations exhibited no heterozygosity while the remaining two populations had relative proportions of heterozygotic individuals of 21.88 % (7 of 32) and 11.94% (8 of 67). Mean heterozygosity across loci and populations was 0.018 ± 0.007 with values for the two heterozygote bearing populations of 0.059 ± 0.025 and 0.022 ± 0.009. Genetic distance between population pairs ranged from 0.0006 to 0.3462. Facultative out-crossing is discussed as a mechanism promoting the evolution of invasive genotypes and a graphical conceptual model of reproductive outcome for such a system is presented.