The introgressed segment carrying rust resistance genes Yr17, Lr37 and Sr38 in wheat can be assayed by a cloned disease resistance gene-like sequence

A cloned gene sequence (Vrga1D), with features of the nucleotide-binding-site leucine-rich repeat class of disease resistance (R) gene sequence super family, was previously shown to belong to a family of five gene members derived from a Triticum ventricosum Ces. (syn. Aegilops ventricosa Tausch) segment in wheat (Triticum aestivum L.). This gene family was introgressed, together with the linked rust resistance genes Yr17, Lr37 and Sr38 from T. ventricosum, to wheat chromosome 2AS. An independently derived T. ventricosum segment carrying a leaf rust resistance gene in a French wheat cultivar, was shown to exhibit a rust resistance response equivalent to Lr37 as well as Yr17 and Sr38. DNA probes from different regions of the Vrga1D clone consistently detected the presence of RFLPs associated with the introgressed segment carrying the resistance genes Yr17, Lr37 and Sr38 present in diverse wheat genotypes from Australia, Canada, France and the UK. Our results showed that the transfer of the T. ventricosum- derived Vrga1 gene members and the rust resistance genes were always accompanied by the loss of a corresponding set of Vrga1-related gene members in recipient wheat cultivars presumed to be of homoeoallelic origin. A PCR assay, based on sequences from the 3"-untranslated region of a Vrga1 gene member isolated from the T. ventricosum donor line of the introgressed segment, was developed. The PCR assay detected the presence of the introgressed rust resistance genes across the diverse wheat backgrounds and should be useful in marker- assisted selection in wheat breeding. Received: 24 December 1999 / Accepted: 13 June 2000     

A mutagenesis-derived broad-spectrum disease resistance locus in wheat

Wheat leaf rust, stem rust, stripe rust, and powdery mildew caused by the fungal pathogens Puccinia triticina, P. graminis f. sp. tritici, P. striiformis f. sp. tritici, and Blumeria graminis f. sp. tritici, respectively, are destructive diseases of wheat worldwide. Breeding durable disease resistance cultivars rely largely on continually introgressing new resistance genes, especially the genes with different defense mechanisms, into adapted varieties. Here, we describe a new resistance gene obtained by mutagenesis. The mutant, MNR220 (mutagenesis-derived new resistance), enhances resistance to three rusts and powdery mildew, with the characteristics of delayed disease development at the seedling stage and completed resistance at the adult plant stage. Genetic analysis demonstrated that the resistance in MNR220 is conferred by a single semidominant gene mapped on the short arm of chromosome 2B. Gene expression profiling of several pathogenesis-related genes indicated that MNR220 has an elevated and rapid pathogen-induced response. In addition to its potential use in breeding for resistance to multiple diseases, high-resolution mapping and cloning of the disease resistance locus in MNR220 may lead to a better understanding of the regulation of defense responses in wheat.     

Physical mapping of an adult plant stripe rust resistance gene from Triticum monococcum

Stripe rust (Puccinia striiformis f. sp. tritici) is one of the major devastating disease which causes large reduction in wheat yield. T. monococcum is an attractive diploid species for gene discovery in wheat with smaller genome size of 5700 Mb compared to 17,300 Mb of bread wheat. An adult plant stripe rust resistance QTL QYrtm.pau-2A was mapped on chromosome 2A flanked by two SSR markers Xwmc170 and Xwmc407. In the present study, two gene based markers Pau_Ta2AL_Gene45 and Pau_Ta2AL_Gene54 developed from 2A specific ESTs were found to map close to QYrtmpau-2A to narrow down the region for candidate gene identification. Utilizing sequence information of these two markers, four BAC clones were identified from the Minimum Tiling Path of 2AL assembly and were sequenced. SSR markers were designed from these BAC sequences and mapped to chromosome 2A. A 50 Mb region of wheat chromomse 2A was identified to harbor stripe rust resistance gene of T. monococcum. Gene based markers identified in the present investigation can be used for marker assisted introgression of QYrtm.pau-2A from T. monococcum to cultivated wheat.     

Molecular mapping of a non-host resistance gene YrpstY1 in barley (Hordeum vulgare L.) for resistance to wheat stripe rust

Cultivated barley (Hordeum vulgare L.) is considered as a non-host or inappropriate host species for wheat stripe rust caused by Puccinia striiformis f. sp. tritici. Most barley cultivars show a broad-spectrum resistance to wheat stripe rust. To determine the genes for resistance to wheat stripe rust in barley, a cross was made between a resistant barley line Y12 and a susceptible line Y16. The two parents, F(1) and 147 BC(1) plants were tested at seedling stage with Chinese prevalent race CYR32 of Puccinia striiformis f. sp. tritici by artificial inoculation in greenhouse. The results indicated that Y12 possessed one dominant resistance gene to wheat stripe rust, designated YrpstY1 provisionally. A total of 388 simple sequence repeat (SSR) markers were used to map the resistance gene in Y12 using bulked segregant analysis. A linkage map, including nine SSR loci on chromosome 7H and YrpstY1, was constructed using the BC(1) population, indicating that the resistance gene YrpstY1 is located on chromosome 7H. It is potential to transfer the resistance gene into common wheat for stripe rust resistance.     

Genome analysis at different ploidy levels allows cloning of the powdery mildew resistance gene Pm3b from hexaploid wheat

In wheat, race-specific resistance to the fungal pathogen powdery mildew (Blumeria graminis f. sp. tritici) is controlled by the Pm genes. There are 10 alleles conferring resistance at the Pm3 locus (Pm3a to Pm3j) on chromosome 1AS of hexaploid bread wheat (Triticum aestivum L.). The genome of hexaploid wheat has a size of 1.6 x 1010 bp and contains more than 80% of repetitive sequences, making positional cloning difficult. Here, we demonstrate that the combined analysis of genomes from wheat species with different ploidy levels can be exploited for positional cloning in bread wheat. We have mapped the Pm3b gene in hexaploid wheat to a genetic interval of 0.97 centimorgan (cM). The diploid T. monococcum and the tetraploid T. turgidum ssp. durum provided models for the A genome of hexaploid wheat and allowed to establish a physical contig spanning the Pm3 locus. Although the haplotypes at the Pm3 locus differed markedly between the three species, a large resistance gene-like family specific to wheat group 1 chromosomes was consistently found at the Pm3 locus. A candidate gene for Pm3b was identified using partial sequence conservation between resistant line Chul and T. monococcum cv. DV92. A susceptible Pm3b mutant, carrying a single-base pair deletion in the coding region of the candidate gene was isolated. When tested in a single cell transformation assay, the Pm3b candidate gene conferred race-specific resistance to powdery mildew. These results demonstrate that the candidate gene, a member of the coiled-coil nucleotide binding site leucine-rich repeat (NBS-LRR) type of disease resistance genes, is the Pm3b gene.     

Resistance to wheat leaf rust and stem rust in Triticum tauschii and inheritance in hexaploid wheat of resistance transferred from T. tauschii.

Twelve accessions of Triticum tauschii (Coss.) Schmal. were genetically analyzed for resistance to leaf rust (Puccinia recondita Rob. ex Desm.) and stem rust (Puccinia graminis Pers. f.sp. tritici Eriks. and E. Henn.) of common wheat (Triticum aestivum L.). Four genes conferring seedling resistance to leaf rust, one gene conferring seedling resistance to stem rust, and one gene conferring adult-plant resistance to stem rust were identified. These genes were genetically distinct from genes previously transferred to common wheat from T. tauschii and conferred resistance to a broad spectrum of pathogen races. Two of the four seedling leaf rust resistance genes were not expressed in synthetic hexaploids, produced by combining tetraploid wheat with the resistant T. tauschii accessions, probably owing to the action of one or more intergenomic suppressor loci on the A or B genome. The other two seedling leaf rust resistance genes were expressed at the hexaploid level as effectively as in the source diploids. One gene was mapped to the short arm of chromosome 2D more than 50 cM from the centromere and the other was mapped to chromosome 5D. Suppression of seedling resistance to leaf rust in synthetic hexaploids derived from three accessions of T. tauschii allowed the detection of three different genes conferring adult-plant resistance to a broad spectrum of leaf rust races. The gene for seedling resistance to stem rust was mapped to chromosome ID. The degree of expression of this gene at the hexaploid level was dependent on the genetic background in which it occurred and on environmental conditions. The expression of the adult-plant gene for resistance to stem rust was slightly diminished in hexaploids. The production of synthetic hexaploids was determined to be the most efficient and flexible method for transferring genes from T. tauschii to T. aestivum, but crossing success was determined by the genotypes of both parents. Although more laborious, the direct introgression method of crossing hexaploid wheat with T. tauschii has the advantages of enabling selection for maximum expression of resistance in the background hexaploid genotype and gene transfer into an agronomically superior cultivar.     

Rust resistance in Triticum cylindricum Ces. (4x, CCDD) and its transfer into durum and hexaploid wheats.

In order to counteract the effects of the mutant genes in races of leaf rust (Puccinia recondita f.sp. tritici Rob. ex Desm.) and stem rust (P. graminis f.sp. tritici Eriks. &Henn.) in wheat, exploration of new resistance genes in wheat relatives is necessary. Three accessions of Triticum cylindricum Ces. (4x, CCDD), Acy1, Acy9, and Acy11, were tested with 10 races each of leaf rust and stem rust. They were resistant to all races tested. Viable F1 plants were produced from the crosses of the T. cylindricum accessions as males with susceptible MP and Chinese Spring ph1b hexaploid wheats (T. aestivum, 6x, AABBDD), but not with susceptible Kubanka durum wheat (T. turgidum var. durum, 4x, AABB), even with embryo rescue. In these crosses the D genome of hexaploid wheat may play a critical role in eliminating the barriers for species isolation during hybrid seed development. The T. cylindricum rust resistance was expressed in the F1 hybrids with hexaploid wheat. However, only the cross MP/Acy1 was successfully backcrossed to another susceptible hexaploid wheat, LMPG-6. In the BC2F2 of the cross MP/Acy1//LMPG-6/3/MP, monosomic or disomic addition lines with resistance to either leaf rust race 15 (infection types (IT) 1=, 1, or 1+; addition line 1) or stem rust race 15B-1 (IT 1 or 1+; addition line 2) were selected. Rust tests and examination of chromosome pairing of the F1 hybrids and the progeny of the disomic addition lines confirmed that the genes for rust resistance were located on the added T. cylindricum C-genome chromosomes rather than on the D-genome chromosomes. The T. cylindricum chromosome in addition line 2 was determined to be chromosome 4C through the detection of RFLPs among the genomes using a set of homoeologous group-specific wheat cDNA probes. Addition line 1 was resistant to the 10 races of leaf rust and addition line 2 was resistant to the 10 races of stem rust, as was the T. cylindricum parent. The added C-genome chromosomes occasionally paired with hexaploid wheat chromosomes. Translocation lines with rust resistance (2n = 21 II) may be obtained in the self-pollinated progeny of the addition lines through spontaneous recombination of the C-genome chromosomes and wheat chromosomes. Such translocation lines with resistance against a wide spectrum of rust races should be potentially valuable in breeding wheat for rust resistance.     

小麦遗传背景对黑麦抗叶锈基因Lr26的抗性表达的影响

利用1套从小麦纯系和黑麦自交系培育出的1R附加系、代换系和易位系,研究了1RS上的抗叶锈基因Lr26在小麦中的表达。结果发现,1R二体附加系和纯合1RS/1BL易位系高抗小麦叶锈病;而其小麦亲本、1R(1B)代换系和1BS/1RL易位系重感叶锈病。这一结果指出了黑麦染色体臂1RS上的抗小麦叶锈病基因Lr26在小麦中的表达受小麦染色体臂1BL上的基因的强烈影响,指出了外源基因在小麦中的表达可受染色体臂或基因水平上的相互作用的制约。文中讨论了外源基因与小麦遗传背景相互作用在小麦育种中的意义。     

Molecular Mapping of Wheat: Major Genes and Rearrangements in Homoeologous Groups 4, 5, and 7

A molecular-marker linkage map of hexaploid wheat (Triticum aestivum L. em. Thell) provides a framework for integration with the classical genetic map and a record of the chromosomal rearrangements involved in the evolution of this crop species. We have constructed restriction fragment length polymorphism (RFLP) maps of the A-, B-, and D-genome chromosomes of homoeologous groups 4, 5, and 7 of wheat using 114 F(7) lines from a synthetic X cultivated wheat cross and clones from 10 DNA libraries. Chromosomal breakpoints for known ancestral reciprocal translocations involving these chromosomes and for a known pericentric inversion on chromosome 4A were localized by linkage and aneuploid analysis. Known genes mapped include the major vernalization genes Vrn1 and Vrn3 on chromosome arms 5AL and 5DL, the red-coleoptile gene Rc1 on 7AS, and presumptively the leaf-rust (Puccinia recondita f.sp. tritici) resistance gene Lr34 on 7DS and the kernel-hardness gene Ha on 5DS. RFLP markers previously obtained for powdery-mildew (Blumeria graminis f.sp. tritici) resistance genes Pm2 and Pm1 were localized on chromosome arms 5DS and 7AL.     

Molecular mapping of stripe rust resistance gene YrSN104 in Chinese wheat line Shaannong 104

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a serious yield-limiting factor for wheat production worldwide. The objective of this study was to identify and map a stripe rust resistance gene in wheat line Shaannong 104 using SSR markers. F(1) , F(2) and F(3) populations from Shaannong 104/Mingxian 169 were inoculated with Chinese Pst race CYR32 in a greenhouse. Shaannong 104 carried a single dominant gene, YrSN104. Six potential polymorphic SSR markers identified in bulk segregant analysis were used to genotype F(2) and F(3) families. YrSN104 was closely linked with all six SSR markers on chromosome 1BS with genetic distances of 2.0 cM (Xgwm18, Xgwm273, Xbarc187), 2.6 cM (Xgwm11, Xbarc137) and 5.9 cM (Xbarc240). Pedigree analysis, pathogenicity tests using 26 Pst races, haplotyping of associated markers on isogenic lines carrying known stripe rust resistance genes, and associations with markers suggested that YrSN104 was a new resistance gene or an allele at the Yr24/Yr26 locus on chromosome 1BS. Deployment of YrSN104 singly or in combination to elite genotypes could play an effective role to lessen yield losses caused by stripe rust.     

Molecular markers for leaf rust resistance genes in wheat

Over 100 genes of resistance to rust fungi: Puccinia recondita f. sp. tritici, (47 Lr - leaf rust genes), P. striiformis (18 Yr - yellow rust genes) and P. graminis f. sp. tritici (41 Sr - stripe rust genes) have been identified in wheat (Triticum aestivum L.) and its wild relatives according to recent papers. Sixteen Lr resistance genes have been mapped using restriction fragments length polymorphism (RFLP) markers on wheat chromosomes. More than ten Lr genes can be identified in breeding materials by sequence tagged site (STS) specific markers. Gene Lrk 10, closely linked to gene Lr 10, has been cloned and its function recognized. Available markers are presented in this review. The STS, cleaved amplified polymorphic sequence (CAPS) and sequence characterized amplified regions (SCAR) markers found in the literature should be verified using Triticum spp. with different genetic background. Simple sequence repeats (SSR) markers for Lr resistance genes are now also available.     

Development of resistance gene analog polymorphism markers for the Yr9 gene resistance to wheat stripe rust.

The Yr9 gene, which confers resistance to stripe rust caused by Puccinia striiformis f.sp. tritici (P. s. tritici) and originated from rye, is present in many wheat cultivars. To develop molecular markers for Yr9, a Yr9 near-isogenic line, near-isogenic lines with nine other Yr genes, and the recurrent wheat parent 'Avocet Susceptible' were evaluated for resistance in the seedling stage to North American P s. tritici races under controlled temperature in the greenhouse. The resistance gene analog polymorphism (RGAP) technique was used to identify molecular markers for Yr9. The BC7:F, and BC7:F3 progeny, which were developed by backcrossing the Yr9 donor wheat cultivar Clement with 'Avocet Susceptible', were evaluated for resistance to stripe rust races. Genomic DNA was extracted from 203 BC7:F2 plants and used for cosegregation analysis. Of 16 RGAP markers confirmed by cosegregation analysis, 4 were coincident with Yr9 and 12 were closely linked to Yr9 with a genetic distance ranging from 1 to 18 cM. Analyses of nullitetrasomic 'Chinese Spring' lines with the codominant RGAP marker Xwgp13 confirmed that the markers and Yr9 were located on chromosome 1B. Six wheat cultivars reported to have 1B/1R wheat-rye translocations and, presumably, Yr9, and two rye cultivars were inoculated with four races of P. s. tritici and tested with 9 of the 16 RGAP markers. Results of these tests indicate that 'Clement', 'Aurora', 'Lovrin 10', 'Lovrin 13', and 'Riebesel 47/51' have Yr9 and that 'Weique' does not have Yr9. The genetic information and molecular markers obtained from this study should be useful in cloning Yr9, in identifying germplasm that may have Yr9, and in using marker-assisted selection for combining Yr9 with other stripe rust resistance genes.     

Genetic studies of leaf and stem rust resistance in six accessions of Triticum turgidum var. dicoccoides.

Six accessions of Triticum turgidum var. dicoccoides L. (4x, AABB) of diverse origin were tested with 10 races of leaf rust (Puccinia recondita f.sp. tritici Rob. ex Desm.) and 10 races of stem rust (P. graminis f.sp. tritici Eriks. &Henn.). Their infection type patterns were all different from those of lines carrying the Lr or Sr genes on the A or B genome chromosomes with the same races. The unique reaction patterns are probably controlled by genes for leaf rust or stem rust resistance that have not been previously identified. The six dicoccoides accessions were crossed with leaf rust susceptible RL6089 durum wheat and stem rust susceptible 'Kubanka' durum wheat to determine the inheritance of resistance. They were also crossed in diallel to see whether they carried common genes. Seedlings of F1, F2, and BC1F2 generations from the crosses of the dicoccoides accessions with RL6089 were tested with leaf rust race 15 and those from the crosses with 'Kubanka' were tested with stem rust race 15B-1. The F2 populations from the diallel crosses were tested with both races. The data from the crosses with the susceptible durum wheats showed that resistance to leaf rust race 15 and stem rust race 15B-1 in each of the six dicoccoides accessions is conferred by a single dominant or partially dominant gene. In the diallel crosses, the dominance of resistance appeared to be affected by different genetic backgrounds. With one exception, the accessions carry different resistance genes: CI7181 and PI 197483 carry a common gene for resistance to leaf rust race 15. Thus, wild emmer wheat has considerable genetic diversity for rust resistance and is a promising source of new rust resistance genes for cultivated wheats.     

Chromosomal location of genes for resistance to powdery mildew in Chinese wheat lines Jieyan 94-1-1 and Siyan 94-1-2

Two Chinese wheat lines Jieyan 94-1-1 and Siyan 94-1-2 are resistant to all 120 isolates of Blumeria graminis f. sp. tritici maintained in Weihenstephan, Germany. Monosomic analyses employing the susceptible set of 21 Chinese Spring monosomic lines revealed that the line Jieyan 94-1-1 carries one dominant gene on translocated wheat/rye chromosome 1B/1R and one recessive gene on chromosome 7B, whereas line Siyan 94-1-2 possesses one recessive gene on chromosome 7B and one dominant gene on chromosome 5D. Allelism tests in combination with the use of specific isolates comfirmed that the dominant genes in Jieyan 94-1-1 and Siyan 94-1-2 are Pm8 and Pm2, respectively. The recessive genes present in each of the two lines are shown to be new alleles located on chromosome 7B at the pm 5 locus. The two genes are tentatively designated mljy in Jieyan 94-1-1 and mlsy in Siyan 94-1-2, respectively.     

陇南地区不同海拔区域内小麦条锈菌群体遗传多样性的分析

小麦条锈病是世界范围内小麦上最重要的流行性病害之一,可造成严重的产量损失。陇南地区是我国小麦条锈菌主要越夏易变区和新小种发源地,了解该地区不同海拔高度区域内条锈菌遗传多样性有重要意义。本研究采用TP-M13-SSR荧光标记技术对11个种群330个小麦条锈菌分离株基因组DNA进行了SSR标记分析。不同海拔区域的条锈菌遗传多样性有明显的差异,高山区的遗传多样性比较丰富,半山区次之,川道区相对比较低。不同生态区域内,小麦条锈菌群体遗传分化程度不同,高山区和半山区遗传分化程度大,基因流小,川道区群体遗传分化程度比较小,基因流大。来自不同海拔区域的菌系具有相同的基因型,这一结果从分子水平证明了在陇南地区小麦条锈菌在山区与川地之间存在广泛的菌源交流,可就地完成周年循环。     

Homoeologous set of NBS-LRR genes located at leaf and stripe rust resistance loci on short arms of chromosome 1 of wheat

Homoeologous group 1 chromosomes of wheat contain important genes that confer resistance to leaf, stem and stripe rusts, powdery mildew and Russian wheat aphid. A disease resistance gene analog encoding nucleotide binding site-leucine rich repeat (NBS-LRR), designated RgaYr10, was previously identified at the stripe rust resistant locus, Yr10, located on chromosome 1BS distal to the storage protein, Gli-B1 locus. RgaYr10 identified gene members in the homoeologous region of chromosome 1DS cosegregating with the leaf rust resistance gene, Lr21, which originally was transferred from a diploid D genome progenitor. Four RgaYr10 gene members were isolated from chromosome 1DS and compared to two gene members previously isolated from the chromosome 1BS homeologue. NBS-LRR genes tightly linked to stripe rust resistance gene Yr10 on chromosome 1BS were closely related in sequence and structure to NBS-LRR genes tightly linked to leaf rust resistance gene Lr21 located within the homoeologous region on chromosome 1DS. The level of sequence homology was similar between NBS-LRR genes that were isolated from different genomes as compared to genes from the same genome. Electronic Publication     

NBS-LRR sequence family is associated with leaf and stripe rust resistance on the end of homoeologous chromosome group 1S of wheat

A detailed RFLP map was constructed of the distal end of the short arm of chromosome 1D of Aegilops tauschii and wheat. At least two unrelated resistance-gene analogs (RGAs) mapped close to known leaf rust resistance genes (Lr21 and Lr40) located distal to seed storage protein genes on chromosome 1DS. One of the two RGA clones, which was previously shown to be part of a candidate gene for stripe rust resistance (Yr10) located within the homoeologous region on 1BS, identified at least three gene family members on chromosome 1DS of Ae. tauschii. One of the gene members co-segregated with the leaf rust resistance genes, Lr21 and Lr40, in Ae. tauschii and wheat segregating families. Hence, a RGA clone derived from a candidate gene for stripe rust resistance located on chromosome 1BS detected candidate genes for leaf rust resistance located in the corresponding region on 1DS of wheat. Received: 10 January 2000 / Accepted: 25 March 2000     

Transient transformation of the rust fungus Puccinia graminis f. sp. tritici

The biotrophic rust fungus Puccinia graminis f. sp. tritici (Pgt) was transformed by particle bombardment. The promoter from the Pgt translation elongation factor 1alpha (EF-1alpha) gene was fused to the bacterial marker genes hygromycin B phosphotransferase (hpt) and beta-glucuronidase (GUS). Transformation constructs were introduced into uredospores of Pgt, an obligate pathogen of wheat, by biolistic bombardment. Uredospores transformed with the construct containing the hpt gene germinated and initiated branching on selective medium, indicating that they had acquired resistance to hygromycin B. However, transformants stopped growing 5 days after bombardment. GUS activity in uredospores and germlings was histochemically detected 4-16 h after bombardment. GUS expression was also obtained using the INF24 promoter from the bean rust fungus Uromyces appendiculatus, demonstrating that heterologous genes can be expressed in P. graminis under the control of regulatory sequences from closely related organisms.     

Resistance gene-analog polymorphism markers co-segregating with the <Emphasis Type="SmallCaps">YR5</Emphasis> gene for resistance to wheat stripe rust

The Yr5 gene confers resistance to all races of the stripe rust pathogen ( Puccinia striiformis f. sp. tritici) of wheat in the United States. To develop molecular markers for Yr5, a BC(7):F(3) population was developed by backcrossing the Yr5 donor ' Triticum spelta album' (TSA) with the recurrent parent 'Avocet Susceptible' (AVS). Seedlings of the Yr5 near-isogenic lines (AVS/6* Yr5), AVS, TSA, and the BC(7):F(3) lines were tested with North American races of P. striiformis f. sp. tritici under controlled greenhouse conditions. The single gene was confirmed by a 1:2:1 segregation ratio for homozygous-resistant, heterozygous and homozygous-susceptible BC(7):F(3) lines. Genomic DNA was extracted from the parents (the Yr5 near-isogenic line and AVS) and 202 BC(7):F(3) lines. The resistance gene-analog polymorphism (RGAP) technique was used to identify molecular markers. The parents and the homozygous-resistant and homozygous-susceptible BC(7):F(3) bulks were used to identify putative RGAP markers for Yr5. Association of the markers with Yr5 was determined using segregation analysis with DNA from the individual BC(7):F(3) lines. Of 16 RGAP markers confirmed by segregation analysis with 109 BC(7):F(3) lines, and nine of the markers confirmed with an additional 93 BC(7):F(3) lines, three markers co-segregated with the resistance allele and three markers co-segregated with the susceptibility allele at the Yr5 locus. The other four markers were tightly linked to the locus. Analysis of a set of Chinese Spring nulli-tetrasomic lines with three markers that co-segregated with, or were linked to, the susceptibility allele confirmed that the Yr5 locus is on chromosome 2B. Of five RGAP markers that were cloned and sequenced, markers Xwgp-17 and Xwgp-18 that co-segregated with the Yr5 locus were co-dominant and had 98% homology with each other in both DNA and translated amino-acid sequences. The two markers had 97% homology with a resistance gene-like sequence from Aegilops ventricosa and had significant homology with many known plant resistance genes, resistance gene analogs and expressed sequence tags (ESTs) from wheat and other plant species. The markers Xwgp-17 and Xwgp-18 also had significant homology with the NB-ARC domain that is in several genes for plant resistance to diseases, nematode cell death and human apoptotic signaling. These markers should be useful to clone Yr5 and combine Yr5 with other genes for durable and superior resistance for the control of stripe rust.