QTL analysis of durable stripe rust resistance in the North American winter wheat cultivar Skiles.

KEY MESSAGE: This study determined the effects of growth stage and temperature on expression of high-temperature adult-plant resistance to stripe rust, mapped six QTL for durable resistance in winter wheat Skiles using a doubled haploid population, and selected breeding lines with different combinations of the QTL using marker-assisted selection. The winter wheat cultivar Skiles has a high level of high-temperature adult-plant (HTAP) resistance to stripe rust caused by Puccinia striiformis f. sp. tritici (Pst). The Skiles HTAP resistance was highly effective at the adult-plant stage even under low temperatures, but high temperatures induced earlier expression and increased levels of resistance. To map resistance genes, Skiles was crossed with the susceptible cultivar Avocet S and a doubled haploid (DH) population was developed. The DH population was tested in fields at Pullman, WA, in 2016, 2017 and 2018, Mount Vernon, WA, in 2017 and 2018 under natural infection, and an environmentally controlled greenhouse at the adult-plant stage with the currently predominant race PSTv-37. The population was genotyped using the 90 K Illumina iSelect wheat SNP chip and selected SSR markers on specific chromosomes. In total, 2526 polymorphic markers were used for QTL mapping and six QTL were detected. Two of the six QTL had major effects across all environments, with one mapped on chromosome 3BS, explaining up to 28.2% of the phenotypic variation and the other on chromosome 4BL, explaining up to 41.8%. Minor QTL were mapped on chromosomes 1BL, 5AL, 6B and 7DL. Genotyping 140 wheat cultivars from the US Pacific Northwest revealed high polymorphism of markers for five of the QTL, and five highly resistant lines with the five QTL were selected from Skiles-derived breeding lines using the markers. This study demonstrated that multiple QTL with mostly additive effects contributed to the high-level HTAP resistance in Skiles.

Combination of all-stage and high-temperature adult-plant resistance QTL confers high-level, durable resistance to stripe rust in winter wheat cultivar Madsen.

KEY MESSAGE: Wheat cultivar Madsen has a new gene on the short arm of chromosome 1A and two QTL for all-stage resistance and three QTL for high-temperature adult-plant resistance that in combination confer high-level, durable resistance to stripe rust. Wheat cultivar Madsen has maintained a high-level resistance to stripe rust over 30 years. To map quantitative trait loci (QTL) underlying the high-level, durable resistance, 156 recombinant inbred lines (RILs) developed from cross Avocet S × Madsen were phenotyped with selected races of Puccinia striiformis f. sp. tritici in the greenhouse seedling tests, and in naturally infected fields during 2015-2017. The RILs were genotyped by SSR and SNP markers from genotyping by sequencing and the 90 K wheat SNP chip. Three QTL for all-stage resistance were mapped on chromosomes 1AS, 1BS and 2AS, and two QTL for high-temperature adult-plant (HTAP) resistance were mapped on 3BS and 6BS. The most effective QTL on 2AS, explaining 8.97-23.10% of the phenotypic variation in seedling tests and 8.60-71.23% in field tests, contained Yr17 for all-stage resistance and an additional gene for HTAP resistance. The 6BS QTL, detected in all field tests, was identified as Yr78. The 1AS QTL, conferring all-stage resistance, was identified as a new gene, which explained 20.45 and 30.23% of variation in resistance to races PSTv-37 and PSTv-40, respectively, and contributed significantly to field resistance at Pullman in 2015-2017, but was not detected at Mount Vernon. The interactions among QTL were mostly additive, and RILs with all five QTL had the highest level of resistance in the field, similar to Madsen. Genotyping 148 US Pacific Northwest wheat cultivars with markers for the 1AS, 2AS and 6BS QTL validated the genes and markers, and indicated their usefulness for marker-assisted selection.

Molecular Mapping of Stripe Rust Resistance Gene Yr76 in Winter Club Wheat Cultivar Tyee.

Tyee, one of the wheat cultivars used to differentiate races of Puccinia striiformis f. sp. tritici in the United States, was identified to have a single gene for all-stage resistance, tentatively named YrTye. To map the gene, Tyee was crossed with 'Avocet Susceptible' (AvS). Genetic analysis of the F1, F2, F2:3, and BC1 progenies confirmed a single dominant gene for resistance to race PSTv-37 that is avirulent to YrTye. A mapping population of 135 F2 plants was phenotyped with PSTv-37 and the derived F2:3 lines were tested with races PSTv-37, PSTv-40, and PSTv-79. The F2 mapping population was genotyped with simple sequence repeat (SSR) markers. A genetic map comprising 13 SSR markers located YrTye in chromosome 3AS flanked distally by SSR marker wmc11 and proximally by wmc532 at 2.6 and 3.4 cM, respectively. Amplification of Chinese Spring 3A deletion lines placed the gene in the distal bin 3AS4-0.45 to 1.00. Because YrTye is different from all formally named Yr genes in chromosomal location, we permanently name the gene Yr76. A near-isogenic line of spring common wheat was developed and selected by testing F3 lines derived from a AvS*4/Tyee cross with Tyee-avirulent and virulent races and the flanking markers. The specific SSR alleles flanking Yr76 were validated using cultivars and breeding lines with and without the gene, and showed high polymorphisms. The specificity of Yr76 is useful in differentiating P. striiformis f. sp. tritici races, and its tightly linked markers will be useful in developing resistant cultivars when combining the gene with other genes for resistance to stripe rust.

Grass Hosts Harbor More Diverse Isolates of Puccinia striiformis Than Cereal Crops.

Puccinia striiformis causes stripe rust on cereal crops and many grass species. However, it is not clear whether the stripe rust populations on grasses are able to infect cereal crops and how closely they are related to each other. In this study, 103 isolates collected from wheat, barley, triticale, rye, and grasses in the United States were characterized by virulence tests and simple sequence repeat (SSR) markers. Of 69 pathotypes identified, 41 were virulent on some differentials of wheat only, 10 were virulent on some differentials of barley only, and 18 were virulent on some differentials of both wheat and barley. These pathotypes were clustered into three groups: group one containing isolates from wheat, triticale, rye, and grasses; group two isolates were from barley and grasses; and group three isolates were from grasses and wheat. SSR markers identified 44 multilocus genotypes (MLGs) and clustered them into three major molecular groups (MG) with MLGs in MG3 further classified into three subgroups. Isolates from cereal crops were present in one or more of the major or subgroups, but not all, whereas grass isolates were present in all of the major and subgroups. The results indicate that grasses harbor more diverse isolates of P. striiformis than the cereals.

Molecular Mapping of YrSP and Its Relationship with Other Genes for Stripe Rust Resistance in Wheat Chromosome 2BL.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is an important disease of wheat worldwide. Resistance is the best way to control the disease. YrSP, a gene originally from 'Spaldings Prolific' wheat and providing resistance to a broad spectrum of races, is used for differentiating P. striiformis f. sp. tritici races but its chromosomal location is not clear. To map YrSP, a near-isogenic line (AvSYrSPNIL) was backcrossed to the recurrent parent, Avocet S. Genetic analysis of the BC7F1, BC8, BC7F2, and BC7F3 progenies confirmed a single dominant gene for resistance. In total, 182 BC7F2 plants and their derived BC7F3 lines were phenotyped with an avirulent P. striiformis f. sp. tritici race and genotyped with simple-sequence repeat (SSR), single-nucleotide polymorphism (SNP), and sequence-tagged site (STS) markers. A linkage map was constructed with 3 SSR, 17 SNP, and 3 STS markers covering 23.3 centimorgans (cM). Markers IWA638 and dp269 were 0.6 cM proximal and 1.5 cM distal, respectively, to YrSP. The gene was mapped in chromosome bin 2BL-C-0.5, physically within the proximal 50% of the chromosome 2BL arm. Allelism tests based on F2 phenotypes indicated that YrSP is closely linked to but not allelic with genes Yr5, Yr7, Yr43, Yr44, and Yr53. Infection type data from tests with 10 historical and currently predominant P. striiformis f. sp. tritici races in the United States also demonstrated differences in specificity between YrSP and the other genes. The specificity of YrSP is useful in differentiating P. striiformis f. sp. tritici races and studying the plant-pathogen interactions, and the information of chromosomal location of the gene and its tightly linked markers should be useful in developing resistant cultivars when combined with other genes for resistance to stripe rust.

Molecular mapping of genes Yr64 and Yr65 for stripe rust resistance in hexaploid derivatives of durum wheat accessions PI 331260 and PI 480016.

KEY MESSAGE: This manuscript reports two new genes ( Yr64 and Yr65 ) for effective resistance to stripe rust and usefulness of their flanking SSR markers for marker-assisted selection. Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases of wheat worldwide and resistance is the best control strategy. Durum wheat accessions PI 331260 and PI 480016 were resistant to all tested Pst races. To transfer the resistance genes to common wheat and map them to wheat chromosomes, both accessions were crossed with the stripe rust-susceptible spring wheat 'Avocet S'. Resistant F3 plants with 42 chromosomes were selected cytologically and by rust phenotype. A single dominant gene for resistance was identified in segregating F4 lines from each cross. F6 populations for each cross were developed from single F5 plants and used for genetic mapping. Different genes from PI 331260 and PI 480016 were mapped to different loci in chromosome 1BS using simple sequence repeat markers. The gene from PI 331260 was flanked by Xgwm413 and Xgdm33 in bin 1BS9-0.84-1.06 at genetic distances of 3.5 and 2.0 cM; and the gene from PI 480016 was flanked by Xgwm18 and Xgwm11 in chromosome bin C-1BS10-0.50 at 1.2 and 2.1 cM, respectively. Chromosomal locations and race and allelism tests indicated that the two genes are different from previously reported stripe rust resistance genes, and therefore are named as Yr64 from PI 331260 and Yr65 from PI 480016. These genes and their flanking markers, and selected common wheat lines with the genes should be valuable for diversifying resistance genes used in breeding wheat cultivars with stripe rust resistance.

Permanent genetic resources added to Molecular Ecology Resources Database 1 February 2012 - 31 March 2012.

This article documents the addition of 171 microsatellite marker loci and 27 pairs of single nucleotide polymorphism (SNP) sequencing primers to the Molecular Ecology Resources Database. Loci were developed for the following species: Bombus pauloensis, Cephalorhynchus heavisidii, Cercospora sojina, Harpyhaliaetus coronatus, Hordeum vulgare, Lachnolaimus maximus, Oceanodroma monteiroi, Puccinia striiformis f. sp. tritici, Rhea americana, Salmo salar, Salmo trutta, Schistocephalus solidus, Sousa plumbea and Tursiops aduncus. These loci were cross-tested on the following species: Aquila heliaca, Bulweria bulwerii, Buteo buteo, Buteo swainsoni, Falco rusticolus, Haliaeetus albicilla, Halobaena caerulea, Hieraaetus fasciatus, Oceanodroma castro, Puccinia graminis f. sp. Tritici, Puccinia triticina, Rhea pennata and Schistocephalus pungitii. This article also documents the addition of 27 sequencing primer pairs for Puffinus baroli and Bulweria bulwerii and cross-testing of these loci in Oceanodroma castro, Pelagodroma marina, Pelecanoides georgicus, Pelecanoides urinatrix, Thalassarche chrysostoma and Thalassarche melanophrys.

Identification of quantitative trait loci (QTL) for resistance to Fusarium crown rot (Fusarium pseudograminearum) in multiple assay environments in the Pacific Northwestern US.

Fusarium crown rot (FCR), caused by Fusarium pseudograminearum and F. culmorum, reduces wheat (Triticum aestivum L.) yields in the Pacific Northwest (PNW) of the US by as much as 35%. Resistance to FCR has not yet been discovered in currently grown PNW wheat cultivars. Several significant quantitative trait loci (QTL) for FCR resistance have been documented on chromosomes 1A, 1D, 2B, 3B, and 4B in resistant Australian cultivars. Our objective was to identify QTL and tightly linked SSR markers for FCR resistance in the partially resistant Australian spring wheat cultivar Sunco using PNW isolates of F. pseudograminerarum in greenhouse and field based screening nurseries. A second objective was to compare heritabilities of FCR resistance in multiple types of disease assaying environments (seedling, terrace, and field) using multiple disease rating methods. Two recombinant inbred line (RIL) mapping populations were derived from crosses between Sunco and PNW spring wheat cultivars Macon and Otis. The Sunco/Macon population comprised 219 F(6):F(7) lines and the Sunco/Otis population comprised 151 F(5):F(6) lines. Plants were inoculated with a single PNW F. pseudograminearum isolate (006-13) in growth room (seedling), outdoor terrace (adult) and field (adult) assays conducted from 2008 through 2010. Crown and lower stem tissues of seedling and adult plants were rated for disease severity on several different scales, but mainly on a numeric scale from 0 to 10 where 0 = no discoloration and 10 = severe disease. Significant QTL were identified on chromosomes 2B, 3B, 4B, 4D, and 7A with LOD scores ranging from 3 to 22. The most significant and consistent QTL across screening environments was located on chromosome 3BL, inherited from the PNW cultivars Macon and Otis, with maximum LOD scores of 22 and 9 explaining 36 and 23% of the variation, respectively for the Sunco/Macon and Sunco/Otis populations. The SSR markers Xgwm247 and Xgwm299 flank these QTL and are being validated for use in marker-assisted selection for FCR resistance. This is the first report of QTL associated with FCR resistance in the US.