BackgroundSingle nucleotide polymorphisms (SNPs) are ideally suited for the construction of high-resolution genetic maps, studying population evolutionary history and performing genome-wide association mapping experiments. Here, we used a genome-wide set of 1536 SNPs to study linkage disequilibrium (LD) and population structure in a panel of 478 spring and winter wheat cultivars (Triticum aestivum) from 17 populations across the United States and Mexico.ResultsMost of the wheat oligo pool assay (OPA) SNPs that were polymorphic within the complete set of 478 cultivars were also polymorphic in all subpopulations. Higher levels of genetic differentiation were observed among wheat lines within populations than among populations. A total of nine genetically distinct clusters were identified, suggesting that some of the pre-defined populations shared significant proportion of genetic ancestry. Estimates of population structure (FST) at individual loci showed a high level of heterogeneity across the genome. In addition, seven genomic regions with elevated FST were detected between the spring and winter wheat populations. Some of these regions overlapped with previously mapped flowering time QTL. Across all populations, the highest extent of significant LD was observed in the wheat D-genome, followed by lower LD in the A- and B-genomes. The differences in the extent of LD among populations and genomes were mostly driven by differences in long-range LD ( > 10 cM).ConclusionsGenome- and population-specific patterns of genetic differentiation and LD were discovered in the populations of wheat cultivars from different geographic regions. Our study demonstrated that the estimates of population structure between spring and winter wheat lines can identify genomic regions harboring candidate genes involved in the regulation of growth habit. Variation in LD suggests that breeding and selection had a different impact on each wheat genome both within and among populations. The higher extent of LD in the wheat D-genome versus the A- and B-genomes likely reflects the episodes of recent introgression and population bottleneck accompanying the origin of hexaploid wheat. The assessment of LD and population structure in this assembled panel of diverse lines provides critical information for the development of genetic resources for genome-wide association mapping of agronomically important traits in wheat.

Wheat stem sawfly (WSS), Cephus cinctus N., is a major insect pest of winter and spring wheat, Triticum aestivum L., in areas of the northern Great Plains. The primary control measure is use of resistant cultivars containing solid stems. Environmental effects on expression of the trait can be problematic, thus genetic markers would be useful. In this study, a doubled haploid (DH) winter wheat population derived from a ‘Rampart’ (solid stems) × ‘Jerry’ (hollow stems) cross was analyzed to identify molecular markers linked to genes controlling stem solidness. The DH population was genotyped with GWM and BARC microsatellite primers that spanned the wheat genome. To genotype the population efficiently, bulked segregant analysis (BSA) was used to identify polymorphism between groups of solid stem and hollow stem individuals. Four microsatellite markers (GWM247, GWM340, GWM547, and BARC77) were found linked to a single solid stem QTL (designate Qss.msub‐3BL) on chromosome 3BL. However GWM247, GWM340, and GWM547 were found to be more closely linked to the QTL than BARC77. Single marker analysis showed Qss.msub‐3BL contributes at least 76% of the total variation for stem solidness. Additionally, no significant relationship existed between Qss.msub‐3BL and other agronomic traits, including yield. These microsatellite markers (GWM247, GWM340, and GWM547) will be useful for selecting solid‐stemmed wheat cultivars to help control the wheat stem sawfly.

The grain fill (GF) period of spring wheat (Triticum aestivum L.) grown in the Northern Great Plains coincides with periods of drought and high temperature stress, whicho ften reduce crop yield and quality. Rate and duration of GF were determined for 20 spring wheat genotypes in four rainfed North Dakota environments to evaluate genotypic variation for rate and duration of GF, and to examine relationships between estimated GF parameters, environmental limitations, wheat productivity, and stress tolerance in environments prone to postanthesis drought and high temperature stresses. A quadratic polynomial was used to describe the relationship between kernel weight and accumulated growing‐degree days from anthesis to maturity. Rate and duration of GF were estimated from fitted curves. Genotypes varied for both GF rate and duration, but increasing temperatures during GF tended to stop grain growth prematurely and to hasten physiological maturity. Rate, but not duration of GF, was correlated with kernel weight. Increasing kernel weight by extension of the GF period does not appear to be a promising strategy for increasing grain yield in these environments. Rate and duration of GF were not associated in these genotypes. Results suggest that simultaneous selection for high GF rate and high kernel weight is possible without lengthening GF duration, and that selection for high GF rate through selection for high kernel weight is possible. High rate and short duration of GF appeared to contribute to increased stress tolerance in these genotypes, but only one of the 20 genotypes investigated had both characteristics. High GF rates with short to medium GF durations appear to be desirable objectives in environments in which the growing season frequently is shortened because of severe stress.

There is growing interest in the use of hard white wheat (Triticum aestivum L.) for both the production of oriental noodles and making bread. It would be desirable if new hard white wheat cultivars produced flour suitable for both purposes. Our objective was to address the feasibility of developing such dual purpose white wheats. We tested several hard white spring wheat lines grown in three locations in Montana for quality characteristics associated with both end‐use products. The hard white spring wheat lines had bread‐making quality similar to that observed for high quality hard red spring wheat check cultivars. The hard white spring wheat flours differed from Australian Standard White by producing significantly harder noodles that were not as bright, particularly after 24 h of noodle storage. Significant variation existed among the hard white lines for noodle color, suggesting selection during breeding might improve color characteristics. However, superior noodle color was correlated with low protein in two environments and with poor loaf volume in one environment. The challenge facing breeders in the development of dual purpose hard white spring wheats will be to improve noodle characteristics while maintaining good bread‐making qualities.

Development of stress tolerant cultivars is an objective of many breeding programs, but success has been limited by inadequate screening techniques, and the lack of genotypes that show clear differences in response to well defined environmental stresses. Twenty spring wheat (Triticum aestivum L.) genotypes were evaluated over a range of water and high temperature‐stressed environments (15 location‐years) to establish standards for evaluation of stress tolerance screening techniques and to characterize genotypic stress tolerance and adaptation to stress environments. Overall and postanthesis stress tolerances were determined using grain yield and kernel weight responses, respectively. Stress tolerances were estimated using stress susceptibility indices (S), grain yields and kernel weights predicted for a hypothetical severe‐stress environment from regression parameters, and yields and kernel weights per se in five stress environments. Yield‐based stress tolerance estimates were correlated among all estimation methods. The stress susceptibility index identified stress tolerant genotypes that did not have outstanding yield performance per se in stress environments due to low yield potential, but which minimized yield loss under stress conditions. Kernel weightbased estimates of postanthesis stress tolerance using S were not correlated with predicted kernel weights in a hypothetical severestress environment or with kernel weights per se in five stress environments. Genotypes evaluated in this study exhibited a wide range in stress tolerance and adaptation to stress‐prone environments. Stress tolerant and susceptible genotypes were identified to use in evaluation of stress tolerance screening techniques. Used in tandem, regression analyses of genotype ✕ environment interactions, stress susceptibility indices, and stress performance per se, provided a more complete description of genotypic stress tolerances than did any analysis used alone.