Mapping QTL involved in powdery mildew resistance of the apple clone U 211

Scab and powdery mildew, caused by Venturia inaequalis (Cke.) Wint. and Podosphaera leucotricha (Ellis et Ev.) Salm. are the most important apple diseases. The apple clone U 211 is resistant to scab and is also highly resistant to powdery mildew under field conditions. The interval mapping method was applied for the identification of genomic regions conferring U 211 resistance to powdery mildew. The genetic maps of the ‘Idared’ and U 211 genome sectors were constructed using amplified fragment lenght polymorphism and simple sequence repeat markers and 98 individuals from the progeny of the cross ‘Idared’× U 211. On the basis of the phenotypic and molecular marker data 10 powdery mildew resistance quantitative trait loci (QTL) were identified in U 211 and ‘Idared’. One of the QTL in the clone U 211 explained 48‐72% of the phenotypic variation and its effect was stable over years.     

Development of apple powdery mildew on sources of resistance to Podosphaera leucotricha, exposed to an inoculum virulent against the major resistance gene Pl-2

The major resistance gene Pl-2 in apple has recently been shown to be overcome by a virulent population of powdery mildew, Podosphaera leucotricha , in genotypes of the fourth generation after introgression of Pl-2 from crab apple, Malus zumi . Tests of pathogenicity in controlled conditions and observations in an orchard showed that the initial progenitor of Pl-2 , MAL 68/1, was overcome by a virulent inoculum. As a consequence, all selections derived from MAL 68/1 may lose their resistance to powdery mildew, if Pl-2 is used alone. Sources of resistance other than Pl-2 remained efficient to the virulent population: M. hupehensis , M. mandshurica , M. robusta , M. sargentii , M. sieboldii , D12, Mildew Immune Selection and 'White Angel'. The combination of Pl-2 with quantitative resistance genes resulted in a high level of resistance.     

Genetics and histology of powdery mildew resistance in apple

Summary Eight scab-resistant cultivars and selections along with eight commercial apple cultivars were evaluated for powdery mildew resistance in greenhouse and nursery tests. Dayton, Liberty, Delicious and Tolman Sweet were rated moderately resistant to infection in both greenhouse and nursery tests. Segregation of seedlings among 14 progenies for mildew reaction indicated that mildew resistance is polygenically controlled in this material with additive gene effects. Recovery of mildew resistant seedlings from crosses involving a scab-resistant parent(s) suggested that this material can be useful in developing scab- and mildew-resistant apple cultivars. Histological investigations were conducted to describe mildew symptoms of infected leaves.     

Identification of PCR-based markers linked to the powdery-mildew-resistance gene Pl1 from Malus robusta in cultivated apple

The availability of molecular markers linked to mildew resistance genes would enhance the efficiency of apple-breeding programmes. This investigation focuses on the identification of random amplified polymorphic DNA (RAPD) markers linked to the Pl₁ gene for mildew resistance, which has introgressed from Malus robusta into cultivated apples. The RAPD marker technique was combined with a modified ‘bulked seg-regant analysis’ mapping strategy. About 850 random decamer primers used as single primers or in combinations were tested by PCR analysis on the basis of resistant and susceptible DNA pools. Selected primers producing RAPD fragments were applied in an additional selection step to M. robusta and genotypes representing intermediate breeding stages of the breeding population 93/9, for which a 1:1 segregation could be observed for the resistance trait. Seven RAPD markers, all representing introgressed DNA sequences from M. robusta, were identified and arranged with the Pl₁ locus in a common linkage group. The two most tightly-linked RAPD markers, OPAT20₄₅₀ and OPD2₁₀₀₀ were mapped with a genetic distance of 4.5 and 5 cM, respectively, from the Pl₁ gene. Both markers are suitable for marker-assisted selection in apple breeding. The polymorphic DNA fragment OPAT20₄₅₀ was cloned and sequenced, and longer primers for the generation of a sequence-characterized amplified region (SCAR) marker have been constructed; this marker was easier to score than the original RAPD marker.     

A specific PCR assay for resistance to biotypes 1 and 2 of the rosy leaf curling aphid in apple based on an RFLP marker closely linked to the Sd1 gene

This report describes the conversion of a restriction fragment length polymorphism (RFLP) marker (the 2B12a locus). linked to the Sd₁ aphid resistance gene, to a polymerase chain reaction (PCR) based marker. A section of the 2BI2 probe was sequenced and two primers were designed lo amplify this sequence in the cultivars‘Prima’and‘Fiesta’: all the amplification products were the same size. After sequencing. two specific 24-mer oligonueleotides were synthesized (DdARM-5¹ and DdAR.M-3²) to exploit a single base-pair difference. These primers were used to screen 44 plants from the‘Prima’x‘Fiesta’family and generated a single amplification product (196bp). in approximately half of the seedlings, which was linked to the resistance gene Sd₁,. The DdARM primer combination was used to evaluate a range of apple cultivars and selections, including some varieties derived from‘Cox’and alternative sources of resistance reported in the literature. In parallel with this work, the phenotypic response of the same genotypes was either confirmed or determined in replicated glasshouse tests. The sequence characterized amplified regions (.SCAR) marker was amplified in all the resistant plants, with the exception of‘Northern Spy’and 3760 (the sources of Sd₂ and Sd₃ resistance, respectively), but never in the susceptible plants. The possible role of this marker in a marker-assisted breeding strategy, and its compatibility with a SCAR marker linked to the I, gene for resistance to apple scab. is discussed.     

小麦新种质CH1357抗白粉病遗传分析及染色体定位

白粉病是影响小麦产量和品质的一种主要病害。小偃麦衍生品系CH1357对白粉病具有较好的成株抗性,苗期对27个菌株表现为免疫或高抗,是一个高抗白粉病的优异抗源。为了明确其抗白粉病基因在染色体上的位置,对台长29/CH1357和绵阳11/CH1357的F_1、BC_1及F_(2:3)家系进行了遗传分析,并利用分离群体分组分析法(bulked segregantanalysis,BSA)将其初步定位。CH1357的白粉病抗性受1对显性核基因控制,位于染色体5DS,暂命名为PmCH1357。其侧翼连锁标记为Xcfd81和Xbwm8,在2个作图群体台长29/CH1357和绵阳11/CH1357中的遗传距离分别为2.0 cM/11.3 cM和1.5 cM/8.9 cM。PmCH1357与5DS染色体上已报道的其他抗白粉病基因抗谱不同,可能是一个新的抗源。     

Molecular mapping of powdery mildew resistance genes in wheat: A review

Powdery mildew, caused by Blumeria graminis f. sp. tritici (syn. Erysiphe graminis f. sp. tritici), is one of the most important diseases of common wheat (Triticum aestivum L.) worldwide. Molecular mapping and cloning of genes for resistance to powdery mildew in hexaploid wheat will facilitate the study of molecular mechanisms underlying resistance to powdery mildew diseases and help understand the structure and function of powdery mildew resistance genes, and permit marker-assisted selection in breeding programs. So far, 48 genes/alleles for resistance to powdery mildew at 32 loci have been identified and located on 16 different chromosomes, of which 21 resistance genes/alleles have been tagged by restriction fragment length polymorphisms (RFLPs), random-amplified polymorphic DNAs (RAPDs), amplified fragment length polymorphisms (AFLPs), sequence characterized amplified regions (SCARs), sequence-tagged sites (STS) or simple sequence repeats (SSRs). Several quantitative trait loci (QTLs) for adult plant resistance (APR) to powdery mildew have been associated with molecular markers. The detailed information on chromosomal location and molecular mapping of these genes has been reviewed. Isolation of powdery mildew resistance genes and development of valid molecular markers for pyramiding resistance genes in breeding programs is also discussed.     

Identification of a novel recessive gene for resistance to powdery mildew (Blumeria graminis f. sp. hordei) in barley (Hordeum vulgare)

One of the most important diseases of barley (Hordeum vulgare) is powdery mildew, caused by Blumeria graminis f. sp. hordei. Spring barley line 173-1-2 was selected from a Moroccan landrace and revealed broad-spectrum resistance to powdery mildew. The objective of this study was to map and characterize the gene for seedling powdery mildew resistance in this line. After crossing with the susceptible cultivar ‘Manchuria’, genetic analysis of F₂ and F₃ families at the seedling stage revealed powdery mildew resistance in line 173-1-2 conditioned by a single recessive gene. Molecular analysis of non-segregating homozygous resistant and homozygous susceptible F₂ plants conducted on the DArTseq platform (Diversity Arrays Technology Pty Ltd) identified significant markers which were converted to allele-specific PCR markers and tested among 94 F₂ individuals. The new resistance gene was mapped on the long arm of chromosome 6H. No other powdery mildew recessive resistance gene has been located on 6H so far. Therefore, we concluded that the 173-1-2 barley line carries a novel recessive resistance gene designated as mlmr.     

Mapping of powdery mildew and leaf rust resistance genes on the wheat-rye translocated chromosome T1BL·1RS using molecular and biochemical markers

Powdery mildew and leaf rust resistance genes on the 1RS arm of the T1BL·1RS translocated chromosome were mapped in relation to the Sec‐1 locus and AFLP and restriction fragment length polymorphism markers, respectively, employing segregating F₃ populations. Integration of molecular markers indicated that Pm17 lies between the Lr26 and Sec‐1 loci, with both resistance genes allocated distally to the Sec‐1 locus in the satellite of the 1RS arm.     

Development of SCAR markers linked to the Pm21 gene conferring resistance to powdery mildew in common wheat

Powdery mildew is an important disease in most of the wheat production areas of the world. The resistance gene Pm21 (6AL/6VS trans-location) derived from Haynaldia villosa confers resistance to all available isolates of Erysiphe (Blumeria) graminis f. sp. tritici in China and Europe. The objective of this study was to develop fast and reliable sequence characterized amplified region (SCAR) markers linked to the Pm21 gene. A random amplified polymorphic DNA (RAPD) marker for Pm21, OPH17₁₄₀₀, was converted to SCAR markers after sequencing the two ends of the polymorphic DNA fragment. Two SCAR markers, SCAR₁₂₆₅ and SCAR₁₄₀₀, were developed to detect the Pm21 gene in different genetic backgrounds. The specific SCAR₁₂₆₅ marker enable large-scale accurate screening for the presence/absence of Pm21 allele.     

Development of reliable PCR markers for the selection of the Vf gene conferring scab resistance in apple

Early selection of scab-resistant apple seedlings can be enhanced by the use of markers tightly linked to the Vf resistance gene. Two sequence characterized amplified regions (SCAR) markers have been obtained from previously described random amplified polymorphic DNA (RAPD) markers. AM19-SCAR is a codominant marker, while AM19-SCAR is dominant, as is the RAPD from which it was derived. A highly detailed map in the vicinity of the Vf gene was built through the cumulative analysis of about 600 seedlings from six different controlled crosses. The usefulness of these and other SCAR markers will be discussed in relation to combining the traditional phenotypic selection with MAS. The availability of two codominant, tightly linked markers flanking both sides of the resistance gene (AL07-SCAR and M18-CAPS) also makes it easy to identify the seedlings homozygous for the resistance gene.     

Resistance to powdery mildew in a doubled haploid barley population and its association with marker loci

The genetic basis of resistance to powdery mildew (Erysiphe graminis DC. f.sp. hordei Marchal) was analyzed using doubled haploid barley (Hordeum vulgare L.) lines from the cross Harrington/TR306. Based on infection types observed after inoculation with defined single-conidium isolates, the lines were classified into four groups. The observed phenotypic ratio fit a two-locus model. The two putative loci were mapped relative to molecular markers. One coincided with the previously mapped dMlg locus on chromosome 4. Based on the observed infection types, Harrington carries the Mlg resistance allele, and TR306 carries a second locus on chromosome 7 (5H); this was tentatively designated Ml(TR). It is the first reported race-specific powdery mildew resistance gene located on that chromosome. These two loci were also detected by simple interval mapping of disease severity data from naturally infected field plots. Composite interval mapping with the first two resistance loci as co-factors detected an additional locus on chromosome 6, with a minor effect on resistance. Finally, superimposing the race-specific classification onto the field data provided evidence for a minor-effect locus on chromosome 7 (5H). The Mlg locus had the largest effect, the Ml(TR) locus had an intermediate effect and the other two loci had very small effects. This study demonstrates the effectiveness of an integrated approach to identifying and mapping resistance loci using classification data from inoculated experiments and quantitative data from field experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mapping of leaf and neck blast resistance genes with resistance gene analog, RAPD and RFLP in rice

An F₈ recombinant inbred population was constructed using a commercial indica rice variety Zhong 156 as the female parent and a semidwarf indica variety Gumei 2 with durable resistance to rice blast as the male parent. Zhong 156 is resistant to the fungus race ZC₁₅ at the seedling stage but susceptible to the same race at the flowering stage. Gumei 2 is resistant to ZC₁₅ at both stages. The blast resistance of 148 recombinant inbred lines was evaluated using the blast race ZC₁₅. Genetic analysis indicated that the resistance to leaf blast was controlled by three genes and the presence of resistant alleles at any loci would result in resistance. One of the three genes did not have effects at the flowering stage. Two genes, tentatively assigned as Pi24(t) and Pi25(t), were mapped onto chromosome 12 and 6,respectively, based on RGA (resistance gene analog), RFLP and RAPD markers. Pi24(t) conferred resistance to leaf blast only, and its resistance allele was from Zhong 156. Pi25(t) conferred resistance to both leaf and neck blast, and its resistance allele was from Gumei 2. In a natural infection test in a blast hot-spot, Pi25(t) exhibited high resistance to neck blast, while Pi24(t) showed little effect. This revised version was published online in August 2006 with corrections to the Cover Date.     

Development of molecular markers linked to the wheat powdery mildew resistance gene Pm4b and marker validation for molecular breeding

Powdery mildew, caused by Blumeria graminis (DC.) E.O. Speer f. sp. tritici, is an important disease in wheat (Triticum aestivum L.). Bulk segregant analysis (BSA) was employed to identify SRAP (sequence‐related amplified polymorphism), sequence tagged site (STS) and simple sequence repeat (SSR) markers linked to the Pm4b gene, which confers good resistance to powdery mildew in wheat. Out of 240 SRAP primer combinations tested, primer combinations Me8/Em7 and Me12/Em7 yielded 220‐bp and 205‐bp band, respectively, each of them associated with Pm4b. STS‐₂₄₁ also linked to Pm4b with a genetic distance of 4.9 cM. Among the eight SSR markers located on wheat chromosome 2AL, Xgwm382 was found to be polymorphic and linked to Pm4b with a genetic distance of 11.8 cM. Further analysis was carried out using the four markers to investigate marker validation for marker‐assisted selection (MAS). The results showed that a combination of the linked markers STS_?241, Me8/Em7_?220 and Xgwm382 could be used for marker‐assisted selection of the resistance gene Pm4b in wheat breeding programmes.     

Molecular mapping determines that Hessian fly resistance gene H9 is located on chromosome 1A of wheat

Hessian fly [Mayetiola destructor (Say)] is one of the major insect pests of wheat (Triticum aestivum L.) worldwide. Hessian fly resistance gene H9 was previously reported to condition resistance to Hessian fly biotype L that is prevalent in many wheat‐growing areas of eastern USA and an RAPD marker, OPO05₁₀₀₀, linked to H9 in wheat was developed using wheat near‐isogenic lines (NILs). However, marker‐assisted selection (MAS) with RAPD markers is not always feasible. One of the objectives in this study was to convert an RAPD marker linked to the gene H9 into a sequence characterized amplified region (SCAR) marker to facilitate MAS and to map H9 in the wheat genome. The RAPD fragment from OPO05₁₀₀₀ was cloned, sequenced, and converted into a SCAR marker SOPO05₉₀₉, whose linkage relationship with H9 was subsequently confirmed in two F₂ populations segregating for H9. Linkage analysis identified one sequence tagged site (STS) marker, STS‐Pm3, and the eight microsatellite markers Xbarc263, Xcfa2153, Xpsp2999, Xgwm136, Xgdm33, Xcnl76, Xcnl117 and Xwmc24 near the H9 locus on the distal region of the short arm of chromosome 1A, contrary to the previously reported location of H9 on chromosome 5A. Locus Xbarc263 was 1.2 cM distal to H9, which itself was 1.7 cM proximal to loci Xcfa2153, Xpsp2999 and Xgwm136. The loci Xgwm136, Xcfa2153 and SOPO05₉₀₉ were shown to be specific to H9 and not diagnostic to several other Hessian fly resistance genes, and therefore should be useful for pyramiding H9 with other Hessian fly resistance genes in a single genotype.     

Mapping Co, a gene controlling the columnar phenotype of apple, with molecular markers

The columnar phenotype is a very valuable genetic resource for apple breeding because of its compact growth form determined by the dominant gene Co. Using bulked segregant analysis combined with several DNA molecular marker techniques to screen the F₁ progeny of Spur Fuji × Telamon (heterozygous for Co), 9 new DNA markers (6 RAPD, 1 AFLP and 2 SSRs) linked to the Co gene were identified. A total of 500 10-mer random primers, 56 pairs of selective AFLP primers and 8 SSR primer pairs were screened. One RAPD marker S1142₆₈₂, and the AFLP marker, E-ACT/M-CTA₃₄₆, were converted into SCAR markers designated SCAR₆₈₂ and SCAR₂₁₆, respectively. These markers will enable early selection in progenies where Co is difficult to identify. The Co gene was located between the SSR markers CH03d11 and COL on linkage group 10 of the apple genetic linkage map. Finally, a local genetic map of the region around the Co gene was constructed by linkage analysis of the nine new markers and three markers developed earlier.     

Comparison of a modified assay method for the endopeptidase marker Ep-D1b with the Sequence Tag Site marker XustSSR2001–7DL for strawbreaker foot rot resistance in wheat

The endopeptidase marker Ep‐D1b and Sequence Tag Site (STS) marker XustSSR2001–7DL were reported to be closely associated with the most effective resistance gene (Pch1) in wheat (Triticum aestivum L.) for strawbreaker foot rot [Pseudocercosporella herpotrichoides (Fron) Deighton]. Our objectives were to: (i) develop an efficient assay method for Ep‐D1b in wheat; (ii) correlate endopeptidase zymograms to strawbreaker foot rot reactions of various wheat genotypes; and (iii) compare the utility of Ep‐D1b and XustSSR2001–7DL for predicting disease response. An improved method of assaying for the Ep‐D1b marker using roots from a single seedling was developed, which is a 2.5‐fold improvement over the previous method. Thirty‐eight wheat genotypes with known reactions to strawbreaker foot rot were analysed for Ep‐D1b and the STS marker. Six distinct endopeptidase zymograms were identified among these 38 genotypes tested, and three of these patterns were novel. The endopeptidase marker was 100% accurate for predicting strawbreaker foot rot disease response, whereas the STS marker predicted the correct phenotype with approximately 90% accuracy. The endopeptidase marker Ep‐D1b was more effective and was more economical for use in marker‐assisted selection strategies for Pch1 in our laboratory compared with the STS marker.     

Linkage relations among eyespot resistance gene Pch2, endopeptidase Ep-A1b, and RFLP marker Xpsr121 on chromosome 7A of wheat

Marker-based selection of Ep-D1b has been used successfully to incorporate Pch1, the gene for eyespot resistance on chromosome 7D, into commercial wheat. However, attempts to transfer resistance conferred by Pch1 (on chromosome 7A) through selection for Ep-A1b have not always been successful. Linkage relations among eyespot resistance gene Pch2, a gene encoding for an isozyme of endopeptidase, Ep-A1b, and RFLP marker Xpsr121 on chromosome 7A were determined using 80 homozygous recombinant substitution lines. The recombinant lines were derived from eyespot susceptible ‘Chinese Spring’ hybridized with a resistant disomic substitution line of ‘Cappelle Desprez’ that has chromosome 7A substituted into ‘Chinese Spring’. Segregations of Pch2, Ep-A1b and Xpsr121 fit an expected 1:1 single-locus ratios based on χ² tests. Linkage analysis revealed that Pch2 was not tightly linked to Ep-Alb (15% recombination). However, close linkage (3.8% recombination) existed between Ep-A1b and Xpsr121. The order of these loci is Pch2-Xpsr121-Ep-A1b. Unlike Pch1 and Ep-D1b, where little or no recombination is found, Pch1 and Ep-A1b showed considerable recombination and therefore linkage cannot be utilized efficiently in marker-based selection.