A fluorescence-based AFLP fingerprinting was applied to investigate genetic diversity in 22 natural populations of two wheatgrasses from Qinghai-Tibetan Plateau at different altitudes: the hexaploid Elymus nutans Griseb and the tetraploid E. burchan-buddae (Nevski) Tzvelev (Poaceae). Five selective primer combinations used in this study generated a total of 637 AFLP fragments across all the samples, with 612 fragments in E. nutans and 570 in E. burchan-buddae. About 45% of the scored fragments were \200 bp and about 13% of the fragments were [400 bp. Results showed that genetic diversity within populations of the two Elymus species increased gradually with the increase in altitudes from the lowest sampling sites (2800 m) and reached a plateau at the medium altitudes, and then started to decrease with the increase in altitudes. Regression analysis demonstrated a clear pattern between the expected heterozygosity (H e ) or Shannon index (I) and altitude variation, where the highest H e values (0.3449 for E. nutans and 0.3167 E. burchan-buddae) and I values (0.5123 and 0.4759) were expected at the altitudes 3399 m and 3418 m across all sampling sites, respectively for E. nutans and E. burchan-buddae. In other words, higher genetic diversity was observed in populations occurring at the medium altitudes (3200-3600 m) than those at the low and high altitudes for the two Elymus species. Principal coordinate analysis (PCA) did not show clear association between genetic relationships of populations and their occurrences at a particular altitude. The above results suggest that efforts for conservation and utilization of two wheatgrasses species should focus more on populations occurring at the medium altitudes.

Seed-mediated gene flow can considerably affect population genetic structure of strictly self-pollinating species, but little is known on the extent and nature of such gene flow among pastoral plant populations. Molecular fingerprints provide a powerful tool to address the relevant issues. Genetic structure of 22 populations of two self-pollinating pasture species, Elymus nutans and E. burchan-buddae, collected from various altitudes of the Qinghai-Tibetan Plateau was studied using fluorescencebased amplified fragment length polymorphism technique. Analysis of molecular variance revealed 42.97% and 37.63% among-population variation for the two Elymus species, respectively, indicating that the majority of the total variation presented within populations. This result contradicts the common genetic variation pattern for a selfing plant species: lower genetic variation within populations. Further analysis suggested higher level of gene flow among populations within the same region than among different regions across the sampled area for the two Elymus species. STRUCTURE analyses of the Elymus populations indicated an evident admixture genetic structure, particularly among neighboring populations from the same region, supporting the hypothesis of considerable seed dispersal among populations. The excess withinregion gene flow of E. nutans and E. burchan-buddae might be caused by grazing animals that promote seed dispersal when moved around the pastoral lands during foraging. The among-population gene flow promulgated by grazing animals may promote the maintenance of genetic diversity in the pasture species, particularly in small and fragmented populations within a given region.