Genetic diversity of bluebunch wheatgrass cultivars and a multiple-origin polycross

Bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love = Agropyron spicatum Pursh: Poaceae) is a cross-pollinating perennial grass native to western North America. Two bluebunch wheatgrass cultivars, Goldar and Whitmar, are currently available for large-scale rangeland seeding. However, cultivars may lack the genetic diversity and adaptation necessary for dynamic non-local environments. The objective of this study was to quantify and compare genomic DNA variation within and between Goldar, Whitmar, and Generation 2 of P-7, a multiple-origin polycross (MOPX2) of 25 naturally diverse bluebunch wheatgrass collections. We assayed 1043 polymorphic amplified fragment length polymorphism (AFLP) products and 88 monomorphic AFLP products from three sample populations of 22 plants. The number of polymorphic loci (and unique alleles) within sample populations of P-7, Goldar, and Whitmar was 898 (99), 813 (49), and 746 (59), respectively. Conversely, the number of fixed AFLP loci within sample populations of P-7, Goldar, and Whitmar was 233, 318, and 385, respectively. The overall nucleotide-sequence diversity (pi +/- SE (x1000)] estimated for P-7, Goldar, and Whitmar was 100.2 +/- 7.1, 80.1 +/- 6.6, and 79.4 +/- 6.7, respectively. By all measures, genetic variation within P-7 is significantly higher than genetic variation within cultivars. However, the estimated number of inter-population nucleotide differences per site (d(x) +/- SE (x1000)] between Goldar and Whitmar, e.g., 36.6 +/- 1.6, is only slightly higher than pi within these cultivars, therefore the net nucleotide-sequence divergence (d(A) +/- SE (x1000)] between these cultivars is relatively small, e.g. 2.5 +/- 0.3. These results indicate that selectively neutral genetic diversity has not been dramatically reduced or inadvertently lost via genetic drift that may have occurred since the divergence of Goldar and Whitmar. No AFLP markers completely distinguish Goldar and Whitmar, therefore discrete morphological differences between these cultivars (e.g., the presence and absence of awns) most likely result from natural or artificial selection.