Allozyme Diversity in Populations of Bromus tectorum (cheatgrass) from Central Asia: Evidence for Founder Effects in Native Populations at the Edge of the Species’ Geographic Distribution
Bromus tectorum (cheatgrass) is an annual, self-fertilizing grass with an enormous native range across much of Eurasia. The grass has been introduced into many regions around the globe, and it invasive in the western United States (US) causing degradation and loss of sagebrush steppe habitats. Here we determined the genetic diversity of 13 populations of cheatgrass from Central Asia: 12 populations from Xinjiang Province, China, and one population from Kazakhstan. These populations are located at the northwestern edge of the plants’ native range, and allow us to explore the pattern of genetic diversity of this broadly distributed species. Genetic diversity of these populations was assessed using enzyme electrophoresis (allozymes), and the 15 enzymes employed in this analysis were coded for by 25 putative loci. Across all 13 populations, 30 alleles were detected at these 25 loci (1.2 alleles/locus) and five loci (20%) were polymorphic. On average, these populations display 1.02 alleles per locus (A), 1.85 percent polymorphic loci per population (%P), and an expected mean heterozygosity (Hexp) value of 0.005. Heterozygotes, which have been rarely detected for cheatgrass, were not detected in this analysis. The genetic diversity across and within the 13 Central Asian populations in our study is very low compared with that previously reported for native Eurasian populations, especially populations from the Mediterranean region. The populations analyzed here exhibit a reduction in genetic diversity due to “founder effects” during range expansion of cheatgrass from its center of diversity in the Mediterranean to the edge of its distribution in Central Asia. The genetic diversity of Central Asian populations is similar to that of invasive populations from North America previously reported.