Development of an STS marker tightly linked to <Emphasis Type="Italic">Yr26</Emphasis> against wheat stripe rust using the resistance gene-analog polymorphism (RGAP) technique

The gene Yr26 confers resistance to all races of Puccinia striiformis f. sp. tritici (PST), the casual pathogen of wheat stripe rust in China. Here, we report development of a molecular marker closely linked to Yr26 using a resistance gene-analog polymorphism (RGAP) technique. A total of 787 F₂ plants and 165 F₃ lines derived from the cross Chuanmai 42/Taichung 29 were used for linkage analysis. Eighteen near-isogenic lines (NILs) and 18 Chinese wheat cultivars and advanced lines with different genes for stripe rust resistance were employed for the validation of STS markers. A total of 1,711 RGAP primer combinations were used to test the parents and resistant and susceptible bulks. Five polymorphic RGAP markers were used for genotyping all F₂ plants. Linkage analysis showed that the five RGAP markers were closely linked to Yr26 with genetic distances ranging from 0.5 to 2.9 cM. These markers were then converted into STS markers, one, CYS-5, of which was located 0.5 cM to Yr26 and was closely associated with the resistance gene when validated over 18 NILs and 18 Chinese wheat cultivars and lines. The results indicated that CYS-5 can be used in marker-assisted selection targeted at pyramiding Yr26 and other genes for stripe rust resistance.     

Identification of molecular markers for the detection of the yellow rust resistance gene Yr17 in wheat

The Yr17 gene, which is present in many European wheat cultivars, displays yellow rust resistance at the seedling stage. The gene introduced into chromosome 2A from Aegilops ventricosa was previously found to be closely linked (0.5 cM) to leaf and stem rust resistance genes Lr37 and Sr38, respectively. The objective of this study was to identify molecular markers linked to the Yr17 gene. We screened with RAPD primers, for polymorphism, the DNAs of cv. Thatcher and the leaf rust-resistant near-isogenic line (NIL) RL 6081 of cv. Thatcher carrying the Lr37 gene. Using a F2 progeny of the cross between VPM1 (resistant) and Thésée (susceptible), the RAPD marker OP-Y15580 was found to be closely linked to the Yr17 gene. We converted the OP- Y15580 RAPD marker into a sequence characterized amplified region (SCAR). This SCAR marker (SC-Y15) was linked at 0.8 ± 0.7 cM to the Yr17 resistance gene. We tested the SC-Y15 marker over a survey of 37 wheat cultivars in order to verify its consistency in different genetic backgrounds and to explain the resistance of some cultivars against yellow rust. Moreover, we showed that the Xpsr150-2Mv locus marker of Lr gene described by Bonhomme et al. [6] which possesses A. ventricosa introgression on the 2A chromosome was also closely linked to the Yr17 gene. Both the SCAR SC-Y15 and Xpsr150-2Mv markers should be used in breeding programmes in order to detect the cluster of the three genes Yr17, Lr37 and Sr38 in cross progenies. This revised version was published online in June 2006 with corrections to the Cover Date.     

黄淮麦区小麦品种(系)中Yr26基因的SSR检测

选用与Yr26紧密连锁的SSR标记Xgwm11和Xgwm18结合田间抗性鉴定,对239份黄淮麦区小麦品种(系)进行检测,以明确Yr26基因在黄淮麦区小麦品种资源中的分布.结果表明:共有35份品种(系)含有与Yr26紧密连锁的SSR标记Xgwm18或Xgwm11的特征带,占检测样本的14.6%.在这35份材料中,31份田间抗性鉴定表现免疫至中抗,4份表现中感.分子标记检测与田间抗病性检测吻合度较好,该标记可以用于Yr26基因的分子标记辅助选择.综合分子标记和田间鉴定,31份小麦(系)含有Yr26基因,占102份抗病材料的30.39%.     

小麦抗条锈基因Yr69的连锁标记开发

抗条锈病基因Yr69对我国小麦条锈菌(Puccinia striiformis f.sp.tritici)小种具有广谱抗性,在小麦抗条锈病育种中具有重要价值.为提高分子标记辅助选择育种的效率,加快Yr69在小麦抗病育种中的应用,本研究利用条锈菌小种CYR34对包含340个小麦家系的'Taichung29/CH7086'...     

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.     

Identification of molecular markers linked to the Yr15 stripe rust resistance gene of wheat originated in wild emmer wheat, Triticum dicoccoides

 The Yr15 gene of wheat confers resistance to the stripe rust pathogen Puccinia striiformis West., which is one of the most devastating diseases of wheat throughout the world. In the present study, molecular markers flanking the Yr15 gene of wheat have been identified using the near-isogenic-lines approach. RFLP screening of 76 probe-enzyme combinations revealed one polymorphic marker (Nor/TaqI) between the susceptible and the resistant lines. In addition, out of 340 RAPD primers tested, six produced polymorphic RAPD bands between the susceptible and the resistant lines. The genetic linkage of the polymorphic markers was tested on segregating F₂ population (123 plants) derived from crosses between stripe rust-susceptible Triticum durum wheat, cv D447, and a BC₃F₉ resistant line carrying Yr15 in a D447 background. A 2.8-kb fragment produced by the Nor RFLP probe and a 1420-bp PCR product generated by the RAPD primer OPB13 showed linkage, in coupling, with the Yr15 gene. Employing the standard maximum-likelihood technique it was found that the order OPB13 ₁₄₂₀ –Yr15–Nor1 on chromosome 1B appeared to be no less than 1000-times more probable than the closest alternative. The map distances between OPB13 ₁₄₂₀ –Yr15–Nor1 are 27.1 cM and 11.0 cM for the first and second intervals, respectively. The application of marker-assisted selection for the breeding of new wheat cultivars with the stripe rust resistance gene is discussed. Received: 27 February 1997/Accepted: 7 March 1997     

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.     

Restriction fragment length polymorphisms linked to genes for resistance to crown rust (Puccinia coronata) in near-isogenic lines of hexaploid oat (Avena sativa).

Crown rust, perhaps the most important fungal disease of oat, is caused by Puccinia coronata. An examination of near-isogenic lines (NILs) of hexaploid oat (Avena sativa) was conducted to identify markers linked to genes for resistance to crown rust. These lines were created such that a unique resistance gene is present in each of the two recurrent parent backgrounds. The six NILs of the current study, X434-II, X466-I, and Y345 (recurrent parent C237-89) and D486, D494, and D526 (recurrent parent Lang), thus provide a pair of lines to study each of three resistance genes. Restriction fragment length polymorphisms and resistance loci were mapped using BC1F2 populations. Three markers were found linked to a locus for resistance to crown rust race 203, the closest at 1.9 cM in line D494 and 3.8 cM in line X466-I. In lines D526 and Y345 a marker was placed 1.0 and 1.9 cM, respectively, from the locus conferring resistance to crown rust race 345, and in D486 and X434-II a marker mapped at 8.0 and 10.2 cM from the locus for resistance to rust race 264B.     

Molecular mapping of a gene for stripe rust resistance in spring wheat cultivar IDO377s

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases of wheat worldwide. The best strategy to control stripe rust is to grow resistant cultivars. One such cultivar resistant to most races in North America is ‘IDO377s’. To study the genetics of its resistance this spring wheat cultivar was crossed with ‘Avocet Susceptible’ (AvS). Seedlings of the parents, F₂ plants, and F₃ lines were tested under controlled greenhouse conditions with races PST-43 and PST-45 of P. striiformis f. sp. tritici. IDO377s carries a single dominant gene for resistance. Resistance gene analog polymorphism (RGAP) and simple sequence repeat (SSR) techniques were used to identify molecular markers linked to the resistance gene. A total of ten markers were identified, two of which flanked the locus at 4.4 and 5.5 cM. These flanking RGAP markers were located on chromosome 2B with nulli-tetrasomic lines of ‘Chinese Spring’. Their presence in the ditelosomic 2BL line localized them to the long arm. The chromosomal location of the resistance gene was further confirmed with two 2BL-specific SSR markers and a sequence tagged site (STS) marker previously mapped to 2BL. Based on the chromosomal location, reactions to various races of the pathogen and tests of allelism, the IDO377s gene is different from all previously designated genes for stripe rust resistance, and is therefore designated Yr43. A total of 108 wheat breeding lines and cultivars with IDO377s or related cultivars in their parentage were assayed to assess the status of the closest flanking markers and to select lines carrying Yr43. The results showed that the flanking markers were reliable for assisting selection of breeding lines carrying the resistance gene. A linked stripe rust resistance gene, previously identified as YrZak, in cultivar Zak was designated Yr44.     

New slow-rusting leaf rust and stripe rust resistance genes <Emphasis Type="Italic">Lr67</Emphasis> and <Emphasis Type="Italic">Yr46</Emphasis> in wheat are pleiotropic or closely linked

The common wheat genotype ‘RL6077’ was believed to carry the gene Lr34/Yr18 that confers slow-rusting adult plant resistance (APR) to leaf rust and stripe rust but located to a different chromosome through inter-chromosomal reciprocal translocation. However, haplotyping using the cloned Lr34/Yr18 diagnostic marker and the complete sequencing of the gene indicated Lr34/Yr18 is absent in RL6077. We crossed RL6077 with the susceptible parent ‘Avocet’ and developed F₃, F₄ and F₆ populations from photoperiod-insensitive F₃ lines that were segregating for resistance to leaf rust and stripe rust. The populations were characterized for leaf rust resistance at two Mexican sites, Cd. Obregon during the 2008–2009 and 2009–2010 crop seasons, and El Batan during 2009, and for stripe rust resistance at Toluca, a third Mexican site, during 2009. The F₃ population was also evaluated for stripe rust resistance at Cobbitty, Australia, during 2009. Most lines had correlated responses to leaf rust and stripe rust, indicating that either the same gene, or closely linked genes, confers resistance to both diseases. Molecular mapping using microsatellites led to the identification of five markers (Xgwm165, Xgwm192, Xcfd71, Xbarc98 and Xcfd23) on chromosome 4DL that are associated with this gene(s), with the closest markers being located at 0.4 cM. In a parallel study in Canada using a Thatcher × RL6077 F₃ population, the same leaf rust resistance gene was designated as Lr67 and mapped to the same chromosomal region. The pleiotropic, or closely linked, gene derived from RL6077 that conferred stripe rust resistance in this study was designated as Yr46. The slow-rusting gene(s) Lr67/Yr46 can be utilized in combination with other slow-rusting genes to develop high levels of durable APR to leaf rust and stripe rust in wheat.     

Characterization and molecular mapping of Yr52 for high-temperature adult-plant resistance to stripe rust in spring wheat germplasm PI 183527

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most destructive diseases of wheat worldwide. Resistance is the best approach to control the disease. High-temperature adult-plant (HTAP) stripe rust resistance has proven to be race non-specific and durable. However, genes conferring high-levels of HTAP resistance are limited in number and new genes are urgently needed for breeding programs to develop cultivars with durable high-level resistance to stripe rust. Spring wheat germplasm PI 183527 showed a high-level of HTAP resistance against stripe rust in our germplasm evaluations over several years. To elucidate the genetic basis of resistance, we crossed PI 183527 and susceptible wheat line Avocet S. Adult plants of parents, F(1), F(2) and F(2:3) progeny were tested with selected races under the controlled greenhouse conditions and in fields under natural infection. PI 183527 has a single dominant gene conferring HTAP resistance. Resistance gene analog polymorphism (RGAP) and simple sequence repeat (SSR) markers in combination with bulked segregant analysis (BSA) were used to identify markers linked to the resistance gene. A linkage map consisting of 4 RGAP and 7 SSR markers was constructed for the resistance gene using data from 175 F(2) plants and their derived F(2:3) lines. Amplification of nulli-tetrasomic, ditelosomic and deletion lines of Chinese Spring with three RGAP markers mapped the gene to the distal region (0.86-1.0) of chromosome 7BL. The molecular map spanned a genetic distance of 27.3?cM, and the resistance gene was narrowed to a 2.3-cM interval flanked by markers Xbarc182 and Xwgp5258. The polymorphism rates of the flanking markers in 74 wheat lines were 74 and 30?%, respectively; and the two markers in combination could distinguish the alleles at the resistance locus in 82?% of tested genotypes. To determine the genetic relationship between this resistance gene and Yr39, a gene also on 7BL conferring HTAP resistance in Alpowa, a cross was made between PI 183527 and Alpowa. F(2) segregation indicated that the genes were 36.5?±?6.75?cM apart. The gene in PI 183527 was therefore designed as Yr52. This new gene and flanking markers should be useful in developing wheat cultivars with high-level and possible durable resistance to stripe rust.     

Evaluation of Pakistan wheat germplasms for stripe rust resistance using molecular markers

Wheat production in Pakistan is seriously constrained due to rust diseases and stripe rust (yellow) caused by Puccinia striiformis f. sp. tritici, which could limit yields. Thus development and cultivation of genetically diverse and resistant varieties is the most sustainable solution to overcome these diseases. The first objective of the present study was to evaluate 100 Pakistan wheat cultivars that have been grown over the past 60 years. These cultivars were inoculated at the seedling stage with two virulent stripe rust isolates from the United States and two from Pakistan. None of the wheat cultivars were resistant to all tested stripe rust isolates, and 16% of cultivars were susceptible to the four isolates at the seedling stage. The data indicated that none of the Pakistan wheat cultivars contained either Yr5 or Yr15 genes that were considered to be effective against most P. striiformis f. sp. tritici isolates from around the world. Several Pakistan wheat cultivars may have gene Yr10, which is effective against isolate PST-127 but ineffective against PST-116. It is also possible that these cultivars may have other previously unidentified genes or gene combinations. The second objective was to evaluate the 100 Pakistan wheat cultivars for stripe rust resistance during natural epidemics in Pakistan and Washington State, USA. It was found that a higher frequency of resistance was present under field conditions compared with greenhouse conditions. Thirty genotypes (30% of germplasms) were found to have a potentially high temperature adult plant (HTAP) resistance. The third objective was to determine the genetic diversity in Pakistan wheat germplasms using molecular markers. This study was based on DNA fingerprinting using resistance gene analog polymorphism (RGAP) marker analysis. The highest polymorphism detected with RGAP primer pairs was 40%, 50% and 57% with a mean polymorphism of 36%. A total of 22 RGAP markers were obtained in this study. RGAP, simple sequence repeat (SSR) and sequence tagged site (STS) markers were used to determine the presence and absence of some important stripe rust resistance genes, such as Yr5, Yr8, Yr9, Yr15 and Yr18. Of the 60 cultivars analyzed, 17% of cultivars showed a RGAP marker band for Yr9 and 12% of cultivars exhibited the Yr18 marker band. No marker band was detected for Yr5, Yr8 and Yr15, indicating a likely absence of these genes in the tested Pakistan wheat cultivars. Cluster analysis based on molecular and stripe rust reaction data is useful in identifying considerable genetic diversity among Pakistan wheat cultivars. The resistant germplasms identified with 22 RGAP markers and from the resistance evaluations should be useful in developing new wheat cultivars with stripe rust resistance.     

Molecular markers for wheat leaf rust resistance gene Lr41

Leaf rust, caused by Puccinia triticina Eriks., is an important foliar disease of common wheat (Triticum aestivum L.) worldwide. Pyramiding several major rust-resistance genes into one adapted cultivar is one strategy for obtaining more durable resistance. Molecular markers linked to these genes are essential tools for gene pyramiding. The rust-resistance gene Lr41 from T. tauschii has been introgressed into chromosome 2D of several wheat cultivars that are currently under commercial production. To discover molecular markers closely linked to Lr41, a set of near-isogenic lines (NILs) of the hard winter wheat cultivar Century were developed through backcrossing. A population of 95 BC₃F_2:6 NILs were evaluated for leaf rust resistance at both seedling and adult plant stages and analyzed with simple sequence repeat (SSR) markers using bulked segregant analysis. Four markers closely linked to Lr41 were identified on chromosome 2DS; the closest marker, Xbarc124, was about 1 cM from Lr41. Physical mapping using Chinese Spring nullitetrasomic and ditelosomic genetic stocks confirmed that markers linked to Lr41 were on chromosome arm 2DS. Marker analysis in a diverse set of wheat germplasm indicated that primers BARC124, GWM210, and GDM35 amplified polymorphic bands between most resistant and susceptible accessions and can be used for marker-assisted selection in breeding programs.     

Development of SCAR markers linked to the gene Or5 conferring resistance to broomrape (Orobanche cumana Wallr.) in sunflower

A consensus molecular linkage map of 61.9 cM containing the Or5 gene, which confers resistance to race E of broomrape orobanche cumana, five SCAR markers (three dominant, two codominant) and one RAPD marker were identified based on segregation data scored from two F₂ populations of susceptible×resistant sunflower line crosses. Bulked segregant analysis was carried out to generate the five SCAR markers, while the single RAPD marker in the group was identified from 61 segregating RAPD markers that were directly screened on one of the two F₂ populations. The five SCAR markers, RTS05, RTS28, RTS40, RTS29 and RTS41, were significantly (LOD≥4.0) linked to the Or5 gene and mapped separately at 5.6, 13.6, 14.1, 21.4 and 39.4 cM from the Or5 locus on one side, while the RAPD marker, UBC120_660, was found at 22.5 cM (LOD=1.4) on the opposite side. These markers should facilitate the efficient transfer of the resistance gene among sunflower breeding lines. As the first report on molecular markers linked to a broomrape resistance gene, the present work provides a starting point to study other genes and to examine the hypothesis of the clustering of broomrape resistance genes in sunflower. Received: 16 September 1998 / Accepted: 22 June 1999     

Microsatellite mapping of adult-plant leaf rust resistance gene <Emphasis Type="Italic">Lr22a</Emphasis> in wheat

This study was conducted to identify microsatellite markers (SSR) linked to the adult-plant leaf rust resistance gene Lr22a and examine their cross-applicability for marker-assisted selection in different genetic backgrounds. Lr22a was previously introgressed from Aegilops tauschii Coss. to wheat (Triticum aestivum L.) and located to chromosome 2DS. Comparing SSR alleles from the donor of Lr22a to two backcross lines and their recurrent parents showed that between two and five SSR markers were co-introgressed with Lr22a and the size range of the Ae. tauschii introgression was 9-20 cM. An F(2) population from the cross of 98B34-T4B x 98B26-N1C01 confirmed linkage between the introgressed markers and Lr22a on chromosome 2DS. The closest marker, GWM296, was 2.9 cM from Lr22a. One hundred and eighteen cultivars and breeding lines of different geographical origins were tested with GWM296. In total 14 alleles were amplified, however, only those lines predicted or known to carry Lr22a had the unique Ae. tauschii allele at GWM296 with fragments of 121 and 131 bp. Thus, GWM296 is useful for selecting Lr22a in diverse genetic backgrounds. Genotypes carrying Lr22a showed strong resistance to leaf rust in the field from 2002 to 2006. Lr22a is an ideal candidate to be included in a stack of leaf rust resistance genes because of its strong adult-plant resistance, low frequency of commercial deployment, and the availability of a unique marker.     

Inheritance and QTL analysis of durable resistance to stripe and leaf rusts in an Australian cultivar, Triticum aestivum 'Cook'.

An F4-derived F6 recombinant inbred line population (n = 148) of a cross between the durable stripe (yellow) rust (caused by Puccinia striiformis) and leaf (brown) rust (caused by Puccinia triticina) resistant cultivar, Triticum aestivum 'Cook', and susceptible genotype Avocet-YrA was phenotyped at several locations in Canada and Mexico under artificial epidemics of leaf or stripe rusts and genotyped using amplified fragment length polymorphism (AFLP) and microsatellite markers. Durable adult plant resistance to stripe and leaf rusts in 'Cook' is inherited quantitatively and was based on the additive interaction of linked and (or) pleiotropic slow-rusting genes Lr34 and Yr18 and the temperature-sensitive stripe rust resistance gene, YrCK, with additional genetic factors. Identified QTLs accounted for 18% to 31% of the phenotypic variation in leaf and stripe rust reactions, respectively. In accordance with the high phenotypic associations between leaf and stripe rust resistance, some of the identified QTLs appeared to be linked and (or) pleiotropic for both rusts across tests. Although a QTL was identified on chromosome 7D with significant effects on both rusts at some testing locations, it was not possible to refine the location of Lr34 or Yr18 because of the scarcity of markers in this region. The temperature-sensitive stripe rust resistance response, conditioned by the YrCK gene, significantly contributed to overall resistance to both rusts, indicating that this gene also had pleiotropic effects.     

Molecular Detection of Stripe Rust Resistance Gene(s) in 115 Wheat Cultivars (lines) from the Yellow and Huai River Valley Wheat Region

Stripe rust (yellow rust), caused by Puccinia striiformis f.sp. tritici (Pst), is a serious disease of wheat worldwide, including China. Growing resistant cultivars is the most cost‐effective and environmentally friendly approach to control the disease. To assess the stripe rust resistance in commercial wheat cultivars and advanced lines in the Yellow and Huai River Valley Wheat Region, 115 wheat cultivars (lines) collected from 13 provinces in this region were evaluated with the most prevalent Chinese Pst races CYR32, CYR33 and the new race V26 at seedling stage. In addition, these wheat entries were inoculated with the mixed races of CYR32 and CYR33 at the adult‐plant stage in the field. The results indicated that 53 (46.1%) cultivars (lines) had all‐stage resistance to all the three races, and 16 (13.9%) cultivars (lines) showed adult‐plant resistance. The possible stripe rust resistance genes in these entries were postulated by the closely linked markers of all‐stage resistance genes Yr5, Yr9, Yr10, Yr15 and Yr26 and adult‐plant resistance gene Yr18. Molecular analysis indicated that resistance genes Yr5, Yr9, Yr10, Yr18 and Yr26 were found in 5 (4.3%), 38 (33.0%), 1 (0.9%), 2 (1.7%) and 8 (7.0%) entries, respectively. No entry was found to carry the Yr15 gene. In future breeding programs, Yr5, Yr15 and Yr18 should be used to pyramid with other effective genes to develop wheat cultivars with high‐level and durable resistance to stripe rust, whereas Yr9, Yr10 and Yr26 should not be used or used in a limited way due to the virulent races present in China.     

Molecular mapping of stem and leaf rust resistance in wheat

Stem rust caused by Puccinia graminis f. sp. tritici Eriks and Henn and leaf rust caused by Puccinia triticina Rob. ex Desm. are major constraints to wheat production worldwide. In the present study, F₄-derived SSD population, developed from a cross between Australian cultivars ‘Schomburgk’ and ‘Yarralinka’, was used to identify molecular markers linked to rust resistance genes Lr3a and Sr22. A total of 1,330 RAPD and 100 ISSR primers and 33 SSR primer pairs selected ob the basis of chromosomal locations of these genes were used. The ISSR marker UBC 840₅₄₀ was found to be linked with Lr3a in repulsion at a distance of 6.0 cM. Markers cfa2019 and cfa2123 flanked Sr22 at a distance of 5.9 cM (distal) and 6.0 cM (proximal), respectively. The use of these markers in combination would predict the presence or absence of Sr22 in breeding populations. A previously identified PCR-based diagnostic marker STS638 linked to Lr20 was validated in this population. This marker showed a recombination value of 7.1 cM with Lr20.     

Identification and validation of molecular markers linked to the leaf rust resistance gene Lr19 in wheat

A leaf rust resistance gene Lr19 on the chromosome 7DL of wheat derived from Agropyron elongatum was tagged with random amplified polymorphic DNA (RAPD) and microsatellite markers. The F₂ population of 340 plants derived from a cross between the leaf rust resistant near-isogenic line (NIL) of Thatcher (Tc + Lr19) and leaf rust susceptible line Agra Local that segregated for dominant monogenic leaf rust resistance was utilized for generating the mapping population. The molecular markers were mapped in the F₂ derived F₃ homozygous population of 140 seedlings. Sixteen RAPD markers were identified as linked to the alien gene Lr19 among which eight were in a coupling phase linkage. Twelve RAPD markers co-segregated with Lr19 locus. Nine microsatellite markers located on the long arm of chromosome 7D were also mapped as linked to the gene Lr19, including 7 markers which co-segregated with Lr19 locus, thus generating a saturated region carrying 25 molecular markers linked to the gene Lr19 within 10.2 ± 0.062 cM on either side of the locus. Two RAPD markers S265₅₁₂ and S253₇₃₇ which flanked the locus Lr19 were converted to sequence characterized amplified region markers SCS265₅₁₂ and SCS253₇₃₆, respectively. The marker SCS265₅₁₂ was linked with Lr19 in a coupling phase and the marker SCS253₇₃₆ was linked in a repulsion phase, which when used together mimicked one co-dominant marker capable of distinguishing the heterozygous resistant seedlings from the homozygous resistant. The molecular markers were validated on NILs mostly in Thatcher background isogenic for 44 different Lr genes belonging to both native and alien origin. The validation for polymorphism in common leaf rust susceptible cultivars also confirmed the utility of these tightly linked markers to the gene Lr19 in marker-assisted selection.     

Mapping a stripe rust resistance gene YrC591 in wheat variety C591 with SSR and AFLP markers

Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici (PST), is one of the most destructive diseases of common wheat (Triticum aestivum L.). To determine inheritance of stripe rust resistance and map the resistance gene(s) in wheat variety C591, F₁, F_2, and F₃ progenies derived from the Taichung 29 × C591 cross were inoculated with Chinese PST race CY32 in the greenhouse. Genetic analysis identified a single dominant gene, temporarily designated YrC591. A total of 178 SSR and 130 AFLP markers were used to test the parents and resistant and susceptible bulks. From the bulk segregant analysis, seven polymorphic SSR and two AFLP markers were selected for genotyping the F₂ population. SSR marker Xcfa2040-7B, and SCAR marker SC-P35M48 derived from AFLP marker P35M48 ₃₇₃ were identified to be closely linked to the resistance gene with genetic distances of 8.0 and 11.7 cM, respectively. The SSR markers mapped the resistance gene on chromosome arm 7BL. In the seedling test with five PST races, the reaction patterns of C591 were different from wheat cultivars or lines carrying Yr2 or Yr6 that also are found on chromosome 7B. The results indicate that YrC591 is probably a novel stripe rust resistance gene.