Toward an Integrated Linkage Map of Common Bean. III. Mapping Genetic Factors Controlling Host-Bacteria Interactions

Restriction fragment length polymorphism (RFLP)-based genetic linkage maps allow us to dissect the genetic control of quantitative traits (QT) by locating individual quantitative trait loci (QTLs) on the linkage map and determining their type of gene action and the magnitude of their contribution to the phenotype of the QT. We have performed such an analysis for two traits in common bean, involving interactions between the plant host and bacteria, namely Rhizobium nodule number (NN) and resistance to common bacterial blight (CBB) caused by Xanthomonas campestris pv. phaseoli. Analyses were conducted in the progeny of a cross between BAT93 (fewer nodules; moderately resistant to CBB) and Jalo EEP558 (more nodules; susceptible to CBB). An RFLP-based linkage map for common bean based on 152 markers had previously been derived in the F(2) of this cross. Seventy F(2)-derived F(3) families were inoculated in separate greenhouse experiments with Rhizobium tropici strain UMR1899 or X. c. pv. phaseoli isolate isolate W18. Regression and interval mapping analyses were used to identify genomic regions involved in the genetic control of these traits. These two methods identified the same genomic regions for each trait, with a few exceptions. For each trait, at least four putative QTLs were identified, which accounted for approximately 50% and 75% of the phenotypic variation in NN and CBB resistance, respectively. A chromosome region on linkage group D7 carried factor(s) influencing both traits. In all other cases, the putative QTLs affecting NN and CBB were located in different linkage groups or in the same linkage group, but far apart (more than 50 cM). Both BAT93 and Jalo EEP558 contributed alleles associated with higher NN, whereas CBB resistance was always associated with BAT93 alleles. Further investigations are needed to determine whether the QTLs for NN and CBB on linkage group D7 represent linked genes or the same gene with pleiotropic effects. Identification of the QTLs raises the possibility of initiating map-based cloning and marker-assisted selection for these traits.     

Fine mapping a major QTL <Emphasis Type="Italic">qFCC7</Emphasis><Subscript><Emphasis Type="Italic">L</Emphasis></Subscript> for chlorophyll content in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cv. PA64s

Chlorophyll (Chl) content is an important agronomic trait directly affecting the photosynthetic rate. Using a high-density genetic map of 132 recombinant inbred lines (RILs) derived from the cross between 93-11 and PA64s, we detected the quantitative trait loci (QTLs) for Chl content of the top three leaves under two nitrogen (N) conditions at two developmental stages. A total of 32 main-effect QTLs located on chromosomes 1, 4, 5, 6, 7, 8, and 12 were identified, and these QTLs individually accounted for 6.0–20.8?% of the total phenotypic variation. A major QTL qFCC7 _L affecting the Chl content under low N condition was identified, and its positive allele came from PA64s. This QTL might be associated with the ability to tolerate low-N stress in rice. The chromosomal segment substitution line (CSSL) with the corresponding segment from PA64s had a higher SPAD value and photosynthetic rate than 93-11 and showed a lower specific leaf area (SLA). We performed a fine-mapping using a BC₄F₂ population via marker-assisted backcross and finally mapped this QTL to a 124.5 kb interval on the long arm of chromosome 7. Candidate gene analysis showed that there were sequence variations and expression differences in the predicted candidate gene between the two parents. These results suggest that the QTL qFCC7 _L may be useful for breeding the rice varieties with higher photosynthetic rate and grain yield.     

Resistance Gene Analog Polymorphism (RGAP) Markers Co-Localize with Disease Resistance Genes and QTL in Common Bean

Resistance (R) genes containing nucleotide-binding site (NBS)-leucine rich repeats (LRR) are the most prevalent types of R gene in plants. The objective of this study was to develop PCR-based R-gene analog polymorphism (RGAP) markers for common bean (Phaseolus vulgaris L). Twenty degenerate primers were designed from the conserved kinase-1a (GVGKTT) and hydrophobic domains (GLPLAL) of known NBS-LRR type R-genes and from EST databases. Sixty-six of the 100 primer combinations tested yielded polymorphism. Thirty-two RGAP markers were mapped in the BAT 93/Jalo EEP558 core mapping population for common bean. The markers mapped to 10 of 11 linkage groups with a strong tendency for clustering. In addition, the RGAP markers co-located, on six linkage groups, with 15 resistance gene analogs (RGAs) that were previously mapped in other populations of common bean. The distance between the priming sites in NBS-LRR type R-genes is around 500 bp. Of the 32 RGAP markers, 19 had sizes larger and 13 less than 500 bp. RGAP markers mapped close to known R-genes on B11, and to QTLs for resistance on B1, B2, B6, B7, B8, B10, and B11. RGAP appears to provide a useful marker technique for tagging and mapping R-genes in segregating common bean populations, discovery of candidate genes underlying resistance QTL, and future cloning of R-genes in common bean.     

Extension of the core map of common bean with EST-SSR,RGA, AFLP,and putative functional markers

Microsatellites and gene-derived markers are still underrepresented in the core molecular linkage map of common bean compared to other types of markers. In order to increase the density of the core map, a set of new markers were developed and mapped onto the RIL population derived from the ‘BAT93’ × ‘Jalo EEP558’ cross. The EST-SSR markers were first characterized using a set of 24 bean inbred lines. On average, the polymorphism information content was 0.40 and the mean number of alleles per locus was 2.7. In addition, AFLP and RGA markers based on the NBS-profiling method were developed and a subset of the mapped RGA was sequenced. With the integration of 282 new markers into the common bean core map, we were able to place markers with putative known function in some existing gaps including regions with QTL for resistance to anthracnose and rust. The distribution of the markers over 11 linkage groups is discussed and a newer version of the common bean core linkage map is proposed.     

QTL × N-level interaction for plant height in rice (Oriza Sativa L.)

Quantitative trait loci (QTLs) for plant height in rice were mapped onto the molecular marker linkage map of a double-haploid (DH) population derived from a cross between IR64 and Azucena under low and high N levels, in both nutrient solution and soil culture experiments. Two QTLs, one on chromosome 1 and the other on chromosome 8, were detected at high N levels in soil and/or nutrient solution culture experiments. A total of 8 QTLs were identified at low N level in the soil and/or nutrient solution culture experiments, which located on chromosomes 1, 2, 3, 4, 5 and 6, respectively. The QTL flanked by the molecular markers RZ730 RZ801 on chromosome 1 was identified in all experimental conditions. This position corresponds to sd-1, a semi-dwarfing gene. The QTLs on chromosomes 2, 3, 4, 5 and 6 were only detected at low N level and the QTL on chromosome 8 was only detected at high N level in the nutrient solution culture experiment. Based on the differing responses to low N stress between two parents and the QTL×N-level interaction observed in this study, it is suggested that the expression of several QTLs associated with plant height could be induced by low N stress.     

Marker-based introduction of three quantitative-trait loci conferring resistance to <Emphasis Type="Italic">Fusarium</Emphasis> head blight into an independent elite winter wheat breeding population

Fusarium head blight (FHB) is one of the most important wheat diseases that causes yield and quality losses as well as contamination with deoxynivalenol (DON). This study aimed for marker-based introduction of three previously mapped QTLs from two German winter wheat resistance sources into an elite background unrelated to the mapping population. A double cross (DC) served as initial population that combined two resistance donor-QTL alleles from "Dream" (Qfhs.lfl-6AL, Qfhs.lfl-7BS) and one donor-QTL allele from "G16-92" on chromosome 2BL with two high yielding, susceptible elite winter wheats ("Brando", "LP235.1"). The initial population of 600 DC-derived F(1) lines was selected with SSR markers for the respective QTLs. After two marker-selection steps, each of eight marker classes was represented by 9-22 lines possessing the respective donor-QTL allele or all possible combinations thereof in the homozygous state. The effect of the QTLs was estimated by field tests at four locations inoculated with Fusarium culmorum. Resistance was measured as the mean of multiple FHB ratings (0-100%). Marker classes incorporating only one QTL were not significantly more resistant than the class without any QTL, the combination of two donor-QTL alleles reduced FHB significantly. On average, lines with Qfhs.lfl-6AL were significantly taller than lines without this QTL. A considerable variation for FHB resistance was found in all marker classes. Marker-based introduction of two QTLs enhanced mean FHB rating by about 40 percentage points, the selected plants, however, were, on average, significantly taller. Both findings strongly support a phenotypic selection following after marker-based introduction of effective QTLs.     

QTL analysis of cadmium and zinc accumulation in the heavy metal hyperaccumulator Thlaspi caerulescens

Thlaspi caerulescens (Tc; 2n = 14) is a natural Zn, Cd and Ni hyperaccumulator species belonging to the Brassicaceae family. It shares 88% DNA identity in the coding regions with Arabidopsis thaliana (At) (Rigola et al. 2006). Although the physiology of heavy metal (hyper)accumulation has been intensively studied, the molecular genetics are still largely unexplored. We address this topic by constructing a genetic map based on AFLP markers and expressed sequence tags (ESTs). To establish a genetic map, an F(2) population of 129 individuals was generated from a cross between a plant from a Pb/Cd/Zn-contaminated site near La Calamine, Belgium, and a plant from a comparable site near Ganges (GA), France. These two accessions show different degrees of Zn and, particularly, Cd accumulation. We analyzed 181 AFLP markers (of which 4 co-dominant) and 13 co-dominant EST sequences-based markers and mapped them to seven linkage groups (LGs), presumably corresponding to the seven chromosomes of T. caerulescens. The total length of the genetic map is 496 cM with an average density of one marker every 2.5 cM. This map was used for Quantitative Trait Locus (QTL) mapping in the F(2). For Zn as well as Cd concentration in root we mapped two QTLs. Three QTLs and one QTL were mapped for Zn and Cd concentration in shoot, respectively. These QTLs explain 23.8-60.4% of the total variance of the traits measured. We found only one common locus (LG6) for Zn and Cd (concentration in root) and one common locus for shoot and root concentrations of Zn (LG1) and of Cd (LG3). For all QTLs, the GA allele increased the trait value except for two QTLs for Zn accumulation in shoot (LG1 and LG4) and one for Zn concentration in root (LG1).     

Quantitative trait loci that regulate plasma lipid concentration in hereditary obese KK and KK-Ay mice

To identify quantitative trait loci (QTLs) responsible for regulating plasma lipid concentration associated with obesity, linkage analysis was carried out on the 190 F2 progeny of a cross between C57BL/6J female and KK-Ay (Ay allele at the agouti locus congenic) male. In F2 a/a (agouti locus genotype) mice, two QTLs were identified on chromosome 1 and a QTL on chromosome 3 for total-cholesterol. A QTL for HDL-cholesterol was identified on chromosome 1 and a QTL for NEFA on chromosome 9. In F2 Ay/a mice, two QTLs for HDL-cholesterol were found on chromosome 1. Loci for other lipids with suggestive linkage were also identified. In both F2 mice, one QTL on chromosome 1 for total- and HDL-cholesterol was mapped near D1Mit150, in the vicinity of the apolipoprotein A-II (Apoa2) locus. Seven nucleotide substitutions out of 309 nucleotide apolipoprotein A-II cDNA sequences were identified between KK and C57BL/6J. The Ay allele may be an indication of the plasma lipid levels, but its influence was less apparent than in the case of weight control. The loci for lipids were not on identical chromosomes with those previously identified for obesity, suggesting that hyperlipidemia in KK does not coincidentally occur with obesity.     

陆地棉产量性状QTLs的分子标记及定位

用我国的高产栽培品种泗棉3号和美国栽培品种TM-1为材料,构建F₂和F_2∶3作图群体,应用301对SSR引物和1040个RAPD引物,对产量性状QTLs进行了分子标记筛选,结果共筛选出了37对SSR多态性引物和10个RAPD多态性引物的49个位点,鉴定出了控制产量性状变异的主效QTLs。定位于第9染色体的连锁群,分别具有控制铃重、衣分和籽指的主效QTLs,铃重的2个QTLs分别解释F_2∶3群体表型变异的18.2%和21.0%;在F₂群体检测到的1个衣分QTL解释表型变异的25%,另一个衣分QTL在F₂群体和F_2∶3群体都检测到,解释F₂群体衣分的24.9%的表型变异,解释F_2∶3群体衣分的5.9%的表型变异;在F_2∶3群体铃重的一个QTL的同一位置同时检测到一个籽指QTL,它解释15.6%的表型变异,是一因多效或是紧密连锁的两个QTLs,有待进一步研究。本研究标记的产量性状主效QTLs可用于棉花产量性状的标记辅助选择。     

Detection of Obesity QTLs on Mouse Chromosomes 1 and 7 by Selective DNA Pooling

The inheritance of obesity has been analyzed in an intercross between the lean 129/Sv mouse strain and the obesity-prone EL/Suz mouse strain. The weights of three major fat pads were determined on 4-month-old mice, and the sum of these weights, divided by body weight, was used as an adiposity index. The strategy of selective DNA pooling was used as a primary screen to identify putative quantitative trait loci (QTLs) affecting adiposity index. DNA pools representing the leanest 15% and fattest 15% of the F2 progeny were compared for differential allelic enrichment using widely dispersed microsatellite variants. To evaluate putative QTLs, individual genotyping and interval mapping were employed to estimate QTL effects and assess statistical significance. One QTL affecting adiposity index, which accounted for 12.3% of phenotypic variance in gender-merged data, was mapped to the central region of Chromosome (Chr) 7. The QTL allele inherited from EL conferred increased adiposity. A second QTL that accounts for 6.3% of phenotypic variance was identified on Chr 1 nearD1Mit211.At both QTLs, the data are consistent with dominant inheritance of the allele contributing to obesity. The possible relationships between these QTLs and previously described obesity QTLs, major obesity mutations, and candidate genes are discussed.     

Integrated analysis of leaf morphological and color traits in different populations of Chinese cabbage (<Emphasis Type="Italic">Brassica rapa</Emphasis> ssp. <Emphasis Type="Italic">pekinensis</Emphasis>)

Key message

QTLs and candidate gene markers associated with leaf morphological and color traits were identified in two immortalized populations of Brassica rapa, which will provide genetic information for marker-assisted breeding.

Abstract

Brassica rapa is an important leafy vegetable consumed worldwide and morphology is a key character for its breeding. To enhance genetic control, quantitative trait loci (QTLs) for leaf color and plant architecture were identified using two immortalized populations with replications of 2 and 4 years. Overall, 158 and 80 QTLs associated with 23 and 14 traits were detected in the DH and RIL populations, respectively. Among them, 23 common robust-QTLs belonging to 12 traits were detected in common loci over the replications. Through comparative analysis, five crucifer genetic blocks corresponding to morphology trait (R, J&U, F and E) and color trait (F, E) were identified in three major linkage groups (A2, A3 and A7). These might be key conserved genomic regions involved with the respective traits. Through synteny analysis with Arabidopsis, 64 candidate genes involved in chlorophyll biosynthesis, cell proliferation and elongation were co-localized within QTL intervals. Among them, SCO3, ABI3, FLU, HCF153, HEMB1, CAB3 were mapped within QTLs for leaf color; and CYCD3;1, CYCB2;4, AN3, ULT1 and ANT were co-localized in QTL regions for leaf size. These robust QTLs and their candidate genes provide useful information for further research into leaf architecture with crop breeding.

     

QTL mapping of flag leaf-related traits in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Key message

QTL controlling flag leaf length, flag leaf width, flag leaf area and flag leaf angle were mapped in wheat.

Abstract

This study aimed to advance our understanding of the genetic mechanisms underlying morphological traits of the flag leaves of wheat (Triticum aestivum L.). A recombinant inbred line (RIL) population derived from ND3331 and the Tibetan semi-wild wheat Zang1817 was used to identify quantitative trait loci (QTLs) controlling flag leaf length (FLL), flag leaf width (FLW), flag leaf area (FLA), and flag leaf angle (FLANG). Using an available simple sequence repeat genetic linkage map, 23 putative QTLs for FLL, FLW, FLA, and FLANG were detected on chromosomes 1B, 2B, 3A, 3D, 4B, 5A, 6B, 7B, and 7D. Individual QTL explained 4.3–68.52% of the phenotypic variance in different environments. Four QTLs for FLL, two for FLW, four for FLA, and five for FLANG were detected in at least two environments. Positive alleles of 17 QTLs for flag leaf-related traits originated from ND3331 and 6 originated from Zang1817. QTLs with pleiotropic effects or multiple linked QTL were also identified on chromosomes 1B, 4B, and 5A; these are potential target regions for fine-mapping and marker-assisted selection in wheat breeding programs.

     

QTLs for days to silking in a recombinant inbred line maize population subjected to high and low nitrogen regimes

Days to silking (DTS) is one of the most important traits in maize (Zea mays). To investigate its genetic basis, a recombinant inbred line population was subjected to high and low nitrogen (N) regimes to detect quantitative trait loci (QTLs) associated with DTS. Three QTLs were identified under the high N regime; these explained 25.4% of the phenotypic variance. Due to additive effects, the QTL on chromosome 6 increased DTS up to 0.66 days; while the other two QTLs mapped on chromosome 9 (one linked with Phi061 and the other linked with Nc134) decreased DTS 0.89 and 0.91 days, respectively. Under low N regime, two QTLs were mapped on chromosomes 6 and 9, which accounted for 25.9% of the phenotypic variance. Owing to additive effects, the QTL on chromosome 6 increased DTS 0.67 days, while the other QTL on chromosome 9 decreased it 1.48 days. The QTL on chromosome 6, flanked by microsatellite markers Bnlg1600 and Phi077, was detected under both N regimes. In conclusion, we identified four QTLs, one on chromosome 6 and three on chromosome 9. These results contribute to our understanding of the genetic basis of DTS and will be useful for developing marker-assisted selection in maize breeding programs.     

Genetic analysis of the psychostimulant effects of nicotine in chromosome substitution strains and F2 crosses derived from A/J and C57BL/6J progenitors

Previous research utilizing the AcB/BcA recombinant congenic strains (RCS) of mice mapped provisional quantitative trait loci (QTLs) for the psychostimulant effects of nicotine to multiple regions on chromosomes 7, 11, 12, 14, 16, and 17. The current study was designed to confirm these QTLs in an A/J (A) × C57Bl/6J (B6) F2 cross and a panel of B6.A chromosome substitution strains (CSS). The panel of B6.A CSS consists of 21 strains, each carrying a different A/J chromosome on a B6 background. The A × B6 F2, CSS, A, and B6 mice were tested for sensitivity to the effects of nicotine on locomotor activity using a computerized open-field apparatus. In A × B6 F2 mice two QTLs were identified which confirm those previously observed in the AcB/BcA RCS. Significant differences in the expression of nicotine-induced activity were associated with loci on chromosome 11 (D11Mit62) and chromosome 16 (D16Mit131) in the A × B6 F2. At the chromosome 11 QTL, an A allele was associated with lower nicotine-induced activity scores relative to the B6. In contrast, the A allele was associated with greater relative nicotine activity values for the chromosome 16 QTL. A survey of the CSS panel confirmed the presence of QTLs for nicotine activation on chromosomes 2, 14, 16, and 17 previously identified in the AcB/BcA RCS. In the informative CSS strains, A alleles were consistently associated with greater nicotine-induced activity scores compared to the B6. The results of the present study are the first to validate QTLs for sensitivity to the effects of nicotine across multiple strains of mice. QTLs on chromosomes 2, 11, 14, 16, and 17 were confirmed in CSS and/or F2 mice. Significantly, the identification of a QTL on chromosome 16 has now been replicated in three crosses derived from the A and B6 progenitors.     

Interaction of quantitative trait loci for resistance to common bacterial blight and pathogen isolates in <Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.

Common bacterial blight (CBB) is a major disease of common bean (Phaseolus vulgaris L.) worldwide. Genetic resistance is the most effective and environmentally safe approach for controlling CBB, and identification of resistance quantitative trait loci (QTL) can improve response to selection when breeding for CBB resistance. Interactions of CBB resistance QTL and pathogen isolates with different levels of aggressiveness were studied using an F _4:5 recombinant inbreed line (RIL) population, derived from a cross between the susceptible cultivar “Sanilac” and the resistant breeding line “OAC 09-3.” Disease phenotyping was performed under field and growth room conditions using multiple bacterial isolates with differential levels of aggressiveness. QTL analysis was performed with 237 molecular markers. The effect of pathogen isolate on the average phenotypic value in the RIL population and the interaction of RILs and the pathogen isolates were highly significant. Two QTL underlying CBB resistance were detected on Pv08 and Pv03. A major QTL (R ² _p between 15 and 56%) was identified in a 5-cM (380 kbp) interval in the distal end of the long arm of Pv08. This genomic region was significantly associated with multiple disease evaluation traits in field and growth room assays and against different isolates of the pathogen, which included the previously known CBB marker SU91. A new QTL on Pv03 (Xa3.3^SO), associated with the PvSNP85p745405 allele from the susceptible parent, Sanilac, appeared to be an isolate-specific QTL against the aggressive fuscans isolate ISO118. Interaction between the SU91 and Xa3.3^SO QTL resulted in a significant reduction in mean disease severity for almost all disease evaluation traits after plants were challenged with the isolate ISO118. The 7.92 and 7.79% diseased areas in RILs with both QTL, compared with 14.92 and 13.81% in RILs without either in test1 and in test2 quantified by image analysis, showed a 44 and 47% reduction of percent diseased areas, indicating that the two QTL interact to limit the expansion of CBB symptoms after infection by ISO118. The information obtained in this study indicates that while the broad-spectrum SU91 QTL is useful in breeding programs, isolate-specific QTL, such as Xa3.3^SO, will aid in breeding bean varieties with enhanced resistance against aggressive regional isolates.     

Mapping QTLs for submergence tolerance in rice using a population fixed for <Emphasis Type="Italic">SUB1A</Emphasis> tolerant allele

Submergence is a widespread problem of rice production, especially in low-lying areas in South and Southeast Asia. Despite the success of Sub1 mega varieties, repeated instances of prolonged and severe flooding in stress-prone areas suggests that the SUB1 gene is no longer sufficient in those regions and requires improved varieties with increased tolerance. A study was conducted to identify quantitative trait loci (QTLs) associated with submergence tolerance using 115 F₇ recombinant inbred lines (RILs) derived from the cross of Ciherang-Sub1, a popular Indonesian cultivar carrying the SUB1 gene that has relatively higher tolerance to submergence compared to the performance of most other Sub1 lines and the submergence and stagnant flooding tolerant IR10F365. As the tolerant allele at SUB1A on chromosome 9 was fixed in this mapping population, additional QTLs responsible for submergence tolerance were expected to be revealed. Genotyping with an Infinium 6K SNP chip resulted in 469 polymorphic markers that were then used for QTL mapping. Phenotyping was performed under complete submergence with two replicates. A major QTL for submergence derived from Ciherang-Sub1, named qSUB8.1, was detected on chromosome 8 with a LOD score of 10.3 and phenotypic variance of 27.5%. Additionally, a smaller QTL, also derived from Ciherang-Sub1, was detected on chromosome 2 with a LOD score of 3.5 and phenotypic variance of 12.7%. There was no digenic interaction detected between these QTLs suggesting their independent action. The QTLs detected in this study can be used in marker-assisted selection to further improve the tolerance of other Sub1 varieties.     

RFLP-facilitated investigation of the quantitative resistance of rice to brown planthopper ( Nilaparvata lugens)

Quantitative trait loci (QTLs), conferring quantitative resistance to rice brown planthopper (BPH), were investigated using 160 F₁₁ recombinant inbred lines (RILs) from the Lemont/Teqing cross, a complete RFLP map, and replicated phenotyping of seedbox inoculation. The paternal indica parent, Teqing, was more-resistant to BPH than the maternal japonica parent, Lemont. The RILs showed transgressive segregation for resistance to BPH. Seven main-effect QTLs and many epistatic QTL pairs were identified and mapped on the 12 rice chromosomes. Collectively, the main-effect and epistatic QTLs accounted for over 70% of the total variation in damage scores. Teqing has the resistance allele at four main-effect QTLs, and the Lemont allele resulted in resistance at the other three. Of the main-effect QTLs identified, QBphr5b was mapped to the vicinity of gl1, a major gene controlling leaf and stem pubescence. The Teqing allele controlling leaf and stem pubescence was associated with resistance, while the Lemont allele for glabrous stem and leaves was associated with susceptibility, indicating that this gene may have contributed to resistance through antixenosis. Similar to the reported BPH resistance genes, the other six detected main-effect QTLs were all mapped to regions where major disease resistance genes locate, suggesting they might have contributed either to antibiosis or tolerance. Our results indicated that marker-aided pyramiding of major resistance genes and QTLs should provide effective and stable control over this devastating pest. Received: 10 December 2000 / Accepted: 7 May 2001     

QTLs for powdery mildew resistance in peach ×Prunus davidiana crosses: consistency across generations and environments

Powdery mildew, caused by Sphaerotheca pannosa var. persicae is one of the most important diseases in European peach orchards. Quantitative trait loci controlling powdery mildew resistance were detected using three related F1, F2 and BC2 populations derived from the cross between the resistant parent P. davidiana clone P1908 and the susceptible peach cultivar Summergrand. Powdery mildew resistance of each population was evaluated under natural exposure, in several locations and over several years. Thirteen QTLs were detected. For nine of them, the favourable allele came from the resistant parent. Five QTLs were consistently detected across the three populations. The F1 hybrid used to produce F2 and BC2 populations had not inherited the favourable allele from P1908 for QTL detected on LG3 and LG8 in F1 population. QTLs were not detected in the corresponding regions in F2 and BC2 populations. In two other genomic areas, significant substitution effects between P1908 alleles were evidenced in the F1 population, but the favourable allele came from Summergrand in the F2 and BC2 populations. Analysis of phenotypic data suggested an important qualitative change in the distribution of powdery mildew resistance after 1996, confirmed by QTL analysis. Indeed, a dramatic decrease of the effect of the major QTL previously detected on LG6 was observed after 1996, while the QTL on LG8 was increasingly involved in the control of powdery mildew resistance. Consequences for peach breeding strategies to improve powdery mildew resistance are discussed.