DNA marker analysis for pyramided of Fusarium head blight (FHB) resistance QTLs from different germplasm

A pyramided FHB resistance line of wheat (WSY) was previously developed from three FHB resistant cultivars (Sumai 3, Wangshuibai, and Nobeokabouzu) in the Jiangsu Academy of Agricultural Sciences, China. In the present study, we analyzed the genetic relationship between WSY and the three parental cultivars using DNA markers in order to clarify how many and which resistance genes had accumulated in WSY. We analyzed 282 DNA markers from the 21 wheat chromosomes. WSY was found to include different chromosome regions that harbored putative FHB QTLs of the three parental germplasm. Haplotypes of DNA markers on these QTL regions revealed that the 1BL, 2BL, 5AS, and 7AL QTL regions were from Sumai 3, the 2AS, 2DS, 3AS, and 6BS QTL regions were from Wangshuibai, and the 3BS QTL region was from Nobeokabouzu. This study showed that different resistance genes from the different resistant germplasm had indeed accumulated in WSY. WSY is a potential resistant resource for FHB resistance in wheat breeding programs.     

Mapping of main and epistatic effect QTLs associated to grain protein and gluten strength using a RIL population of durum wheat

Quality, specifically protein content and gluten strength are among the main objectives of a durum wheat breeding program. The aim of this work was to validate quantitative trait loci (QTLs) associated with grain protein content (GPC) and gluten strength measured by SDS sedimentation volume (SV) and to find additional QTLs expressed in Argentinean environments. Also, epistatic QTL and QTL x environmental interactions were analyzed. A mapping population of 93 RILs derived from the cross UC1113 x Kofa showing extreme values in gluten quality was used. Phenotypic data were collected along six environments (three locations, two years). Main effect QTLs associated with GPC were found in equivalent positions in two environments on chromosomes 3BS (R² = 21.0-21.6%) and 7BL (R² = 12.1-13%), and in one environment on chromosomes 1BS, 2AL, 2BS, 3BL, 4AL, 5AS, 5BL and 7AS. The most important and stable QTL affecting SV was located on chromosome 1BL (Glu-B1) consistently detected over the six environments (R² = 20.9- 54.2%). Additional QTLs were found in three environments on chromosomes 6AL (R² = 6.4-12.5%), and in two environments on chromosomes 6BL (R² = 11.5-12.1%), 7AS (R² = 8.2-10.2%) and 4BS (R² = 11–16.4%). In addition, pleiotropic effects were found affecting grain yield, test weight, thousand-kernel- weight and days to heading in some of these QTLs. Epistatic QTLs and QTL x environment interactions were found for both quality traits, mostly for GPC. The flanking markers of the QTLs detected in this work could be efficient tools to select superior genotypes for the mentioned traits.     

Identification of novel QTLs for seedling and adult plant leaf rust resistance in a wheat doubled haploid population

Pyramiding of genes that confer partial resistance is a method for developing wheat (Triticum aestivum L.) cultivars with durable resistance to leaf rust caused by Puccinia triticina. In this research, a doubled haploid population derived from the cross between the synthetic hexaploid wheat (SHW) (×Aegilotriticum spp.) line TA4152-60 and the North Dakota breeding line ND495 was used for identifying genes conferring partial resistance to leaf rust in both the adult plant and seedling stages. Five QTLs located on chromosome arms 3AL, 3BL, 4DL, 5BL and 6BL were associated with adult plant resistance with the latter four representing novel leaf rust resistance QTLs. Resistance effects of the 4DL QTL were contributed by ND495 and the effects of the other QTLs were contributed by the SHW line. The QTL on chromosome arm 3AL had large effects and also conferred seedling resistance to leaf rust races MJBJ, TDBG and MFPS. The other major QTL, which was on chromosome arm 3BL, conferred seedling resistance to race MFPS and was involved in a significant interaction with a locus on chromosome arm 5DS. The QTLs and the associated molecular markers identified in this research can be used to develop wheat cultivars with potentially durable leaf rust resistance.     

Molecular mapping of genomic regions associated with wheat seedling growth under osmotic stress

A quantitative trait loci (QTL) approach was applied to dissect the genetic control of the common wheat seedling response to osmotic stress. A set of 114 recombinant inbred lines was subjected to osmotic stress from the onset of germination to the 8^th day of seedling development, induced by the presence of 12 % polyethylene glycol. Root, coleoptile and shoot length, and root/shoot length ratio were compared under stress and control conditions. In all, 35 QTL mapping to ten chromosomes, were identified. Sixteen QTL were detected in controls, 17 under stressed conditions, and two tolerance index QTL were determined. The majority of the QTL were not stress-specific. In regions on five chromosome arms (1AS, 1BL, 2DS, 5BL and 6BL) the QTL identified under stress co-mapped with QTL affecting the same trait in controls, and these were classified as seedling vigour QTL, in addition to those expressed in controls. Tolerance-related QTL were detected on four chromosome arms. A broad region on chromosome 1AL, including five QTL, with a major impact of the gene Glu-A1 (LOD 3.93) and marker locus Xksuh9d (LOD 2.91), positively affected root length under stress and tolerance index for root length, respectively. A major QTL (LOD 3.60), associated with marker locus Xcdo456a (distal part of chromosome arm 2BS) determined a tolerance index for shoot length. Three minor QTL (LOD < 3.0) for root length and root/shoot length ratio under osmotic stress were identified in the distal parts of chromosome arms 6DL (marker locus Xksud27a) and 7DL (marker locus Xksue3b). Selecting for the favourable alleles at marker loci associated with the detected QTL for growth traits may represent an efficient approach to enhance the plants’ ability to maintain the growth of roots, coleoptile and shoots in drought-prone soils at the critical early developmental stages.     

Identification of novel tan spot resistance loci beyond the known host-selective toxin insensitivity genes in wheat

Tan spot, caused by Pyrenophora tritici-repentis, is a destructive foliar disease of wheat causing significant yield reduction in major wheat growing areas throughout the world. The objective of this study was to identify quantitative trait loci (QTL) conferring resistance to tan spot in the synthetic hexaploid wheat (SHW) line TA4152-60. A doubled haploid (DH) mapping population derived from TA4152-60 × ND495 was inoculated with conidia produced by isolates of each of four virulent races of P. tritici-repentis found in North America. QTL analysis revealed a total of five genomic regions significantly associated with tan spot resistance, all of which were contributed by the SHW line. Among them, two novel QTLs located on chromosome arms 2AS and 5BL conferred resistance to all isolates tested. Another novel QTL on chromosome arm 5AL conferred resistance to isolates of races 1, 2 and 5, and a QTL specific to a race 3 isolate was detected on chromosome arm 4AL. None of these QTLs corresponded to known host selective toxin (HST) insensitivity loci, but a second QTL on chromosome arm 5BL conferred resistance to the Ptr ToxA producing isolates of races 1 and 2 and corresponded to the Tsn1 (Ptr ToxA sensitivity) locus. This indicates that the wheat-P. tritici-repentis pathosystem is much more complex than previously thought and that selecting for toxin insensitivity alone will not necessarily lead to tan spot resistance. The markers associated with the QTLs identified in this work will be useful for deploying the SHW line as a tan spot resistance source in wheat breeding. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.     

QTL analysis of resistance to Fusarium head blight in Swiss winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Fusarium head blight (FHB) of wheat is a widespread and destructive disease which occurs in humid and semi-humid areas. FHB epidemics can cause serious yield and quality losses under favorable climatic conditions, but the major concern is the contamination of grains with mycotoxins. Resistance to FHB is quantitatively inherited and greatly influenced by the environment. Its evaluation is costly and time-consuming. The genetic basis of FHB resistance has mainly been studied in spring wheat. The objective of this study was to map quantitative trait loci (QTLs) for resistance to FHB in a population of 240 recombinant inbred lines (RILs) derived from a cross between the two Swiss winter wheat cultivars Arina (resistant) and Forno (susceptible). The RILs were genotyped with microsatellite and RFLP markers. The resulting genetic map comprises 380 loci and spans 3,086 cM. The 240 RILs were evaluated for resistance to FHB in six field trials over 3 years. Composite interval mapping (CIM) analyses carried out on FHB AUDPC (i.e. mean values across six environments) revealed eight QTLs which altogether explained 47% of the phenotypic variance. The three main QTLs were mapped on the long arms of chromosomes 6D (R²=22%), 5B (R²=14%) and 4A (R²=10%). The QTL detected on 5B originated from the susceptible parent Forno. Other QTLs with smaller effects on FHB resistance were detected on chromosomes 2AL, 3AL, 3BL, 3DS and 5AL.Communicated by H.C. Becker     

Association mapping for pre-harvest sprouting resistance in white winter wheat

Association mapping identified quantitative trait loci (QTLs) and the markers linked to pre-harvest sprouting (PHS) resistance in an elite association mapping panel of white winter wheat comprising 198 genotypes. A total of 1,166 marker loci including DArT and SSR markers representing all 21 chromosomes of wheat were used in the analysis. General and mixed linear models were used to analyze PHS data collected over 4 years. Association analysis identified eight QTLs linked with 13 markers mapped on seven chromosomes. A QTL was detected on each arm of chromosome 2B and one each on chromosome arms 1BS, 2DS, 4AL, 6DL, 7BS and 7DS. All except the QTL on 7BS are located in a location similar to previous reports and, if verified, the QTL on 7BS is likely to be novel. Principal components and the kinship matrix were used to account for the presence of population structure but had only a minor effect on the results. Although, none of the QTLs was highly significant across all environments, a QTL on the long arm of chromosome 4A was detected in three different environments and also using the best linear unbiased predictions over years. Although previous reports have identified this as a major QTL, its effects were minor in our biparental mapping populations. The results of this study highlight the benefits of association mapping and the value of using elite material in association mapping for plant breeding programs.     

GW3-1和IND109标记对普通小麦粒重的QTL定位分析

应用一个由115个系组成的W7984/Opata85的重组自交系(RIL)群体,建立了一个由394个(292个RFLP、94个SSR和8个特殊的基因杂交探针)DNA分子标记组成的遗传连锁图,对小麦千粒重进行了单个标记的回归分析和复合区间作图的QTL定位,在单个标记的回归分析中检测到11个千粒重的QTLs(P<0.01);复合区间作图分析结果表明,其中4个标记bcd348a、GW3-1、IND109和Rz2的遗传效应较大,其贡献率分别为9.1%、19.0%、8.07%和8.14%,分别位于小麦的2BS、4AL、5BL和7DS上,特别是在水稻第3条染色体上控制籽粒大小的GW3-1和IND109分别位于小麦4A和5B染色体的长臂端.研究结果对小麦应用分子标记辅助选择或分子克隆基因有重要的参考价值.     

Mapping of FHB resistance QTLs in tetraploid wheat.

Triticum turgidum L var. durum is known to be particularly susceptible to infection by Fusarium graminearum, the causal agent for Fusarium head blight (FHB), which results in severe yield losses and grain contaminated with mycotoxins. This research was aimed at identifying FHB resistance in tetraploid wheat and mapping the location of FHB resistance genes. A tetraploid cross of durum wheat ('Strongfield') x Triticum carthlicum ('Blackbird') was used to generate a doubled-haploid (DH) population. This population was evaluated for type II resistance to F. graminearum in replicated greenhouse trials, in which heads were innoculated and the percent of infected spikelets was determined 21 days later. The population was also genotyped with microsatellite markers to construct a map of 424 loci, covering 2 052 cM. The FHB reaction and genotypic data were used to identify FHB resistance quantitative trait loci (QTLs). It was determined that 2 intervals on chromosomes 2BL and 6BS controlled FHB resistance in this tetraploid cross. The FHB resistance allele on chromosome 2BL (r2=0.26, logarithm of odds (LOD)=8.5) was derived from 'Strongfield', and the FHB resistance allele on chromosome 6BS (r2=0.23, LOD=6.6) was derived from 'Blackbird'. Two other loci, on chromosomes 5AS and 2AL, were shown to regulate FHB infection and to have an epistatic effect on the FHB resistance QTL on chromosome 6BS. Further, the FHB resistance QTL peak on chromosome 6BS was clearly coincident with the known FHB resistance gene Fhb2, derived from Sumai 3. The results show that FHB resistance can be expressed in durum wheat, and that T. carthlicum and Triticum aestivum likely share a common FHB resistance gene on chromosome 6BS.     

Molecular mapping of Fusarium head blight resistance in the winter wheat population Dream/Lynx

Fusarium head blight (FHB), mainly caused by Fusarium graminearum and F. culmorum, can significantly reduce the grain quality of wheat (Triticum aestivum L.) due to mycotoxin contamination. The objective of this study was to identify quantitative trait loci (QTLs) for FHB resistance in a winter wheat population developed by crossing the resistant German cultivar Dream with the susceptible British cultivar Lynx. A total of 145 recombinant inbred lines (RILs) were evaluated following spray inoculation with a F. culmorum suspension in field trials in 2002 in four environments across Germany. Based on amplified fragment length polymorphism and simple sequence repeat marker data, a 1,734 cM linkage map was established assuming that the majority of the polymorphic parts of the genome were covered. The area under disease progress curve (AUDPC) was calculated based on the visually scored FHB symptoms. The population segregated quantitatively for FHB severity. Composite interval mapping analysis for means across the environments identified four FHB resistance QTLs on chromosomes 6AL, 1B, 2BL and 7BS. Individually the QTLs explained 19%, 12%, 11% and 21% of the phenotypic variance, respectively, and together accounted for 41%. The QTL alleles conferring resistance on 6AL, 2BL and 7BS originated from cv. Dream. The resistance QTL on chromosome 6AL partly overlapped with a QTL for plant height. The FHB resistance QTL on 7BS coincided with a QTL for heading date, but the additive effect on heading date was of minor importance. The resistance QTL on chromosome 1B was associated with the T1BL.1RS wheat-rye translocation of Lynx.     

Quantitative Trait Loci for Aluminum Resistance in Wheat Cultivar Chinese Spring

Aluminum (Al) toxicity is one of the major constrains for wheat production in many wheat growing areas worldwide. Further understanding of inheritance of Al resistance may facilitate improvement of Al resistance of wheat cultivars (Triticum aestivum L.). A set of ditelosomic lines derived from the moderately Al-resistant wheat cultivar Chinese Spring was assessed for Al resistance. The root growth of ditelosomic lines DT5AL, DT7AL, DT2DS and DT4DS was significantly lower than that of euploid Chinese Spring under Al stress, suggesting that Al-resistance genes might exist on the missing chromosome arms of 5AS, 7AS, 2DL and 4DL of Chinese Spring. A population of recombinant inbred lines (RILs) from the cross Annong 8455 × Chinese Spring-Sumai 3 7A substitution line was used to determine the effects of these chromosome arms on Al resistance. A genetic linkage map consisting of 381 amplified fragment length polymorphism (AFLP) markers and 168 simple sequence repeat (SSR) markers was constructed to determine the genetic effect of the quantitative trait loci (QTLs) for Al resistance in Chinese Spring. Three QTLs, Qalt.pser-4D, Qalt.pser-5A and Qalt.pser-2D, were identified that enhanced root growth under Al stress, suggesting that inheritance of Al resistance in Chinese Spring is polygenic. The QTL with the largest effect was flanked by the markers of Xcfd23 and Xwmc331 on chromosome 4DL and most probably is multi-allelic to the major QTL identified in Atlas 66. Two additional QTLs, Qalt.pser-5A and Qalt.pser-2D on chromosome 5AS and 2DL, respectively, were also detected with marginal significance in the population. Some SSR markers identified in this study would be useful for marker-assisted pyramiding of different QTLs for Al resistance in wheat cultivars.     

Identification of chromosome arms influencing expression of the HMW glutenins in wheat

The high-molecular-weight (HMW) glutenin genes, located on the group 1L chromosome arms, are a major determinant for baking quality in wheat ( Triticum aestivum L.). In addition, the HMW glutenin genes provide a valuable model system for studying the evolution and regulation of orthologous and paralogous genes in polyploid species. The goal of this study was to identify loci that modify the expression of the HMW glutenins, and to map them to specific chromosome arms. Comparisons were made between endosperms with zero versus three (or three versus six) doses for each of the 42 chromosome arms of wheat. SDS-PAGE and scanning densitometry were used to quantify the protein expression levels of the four HMW glutenin genes in cv. Chinese Spring, for each of the dosage comparisons. Fifteen chromosome arms were found to have significant effects on Glu-B1-1, excluding the structural gene dosage effect: eight positive effects on 1AL, 2AS, 2BL, 2DS, 5DS, 6AL, 6DL, and 7AL and seven negative effects on 1BS, 1DS, 1DL, 4DL, 6BS, 6DS, and 7AS. Nineteen chromosome arms had significant effects on Glu-B1-2, excluding the structural gene dosage effect: eight positive effects on 1AL, 2AS, 2BS, 3AL, 4BL, 6DS, 7BL and 7DS and 11 negative effects on 1AS, 1BS, 1DS, 1DL, 2AL, 2BL, 3DS, 4BS, 4DL, 5BL, and 6BS. Twenty chromosome arms had significant effects on Glu-D1-1, excluding the structural gene dosage effect: 11 positive effects on 1AL, 1BL, 2BS, 2DS, 5BS, 5DS, 6AL, 6DS, 6DL, 7AL, and 7BL and nine negative effects on 1AS, 1BS, 1DS, 2BL, 4DL, 5BL, 5DL, 6BL, and 7DS. Twenty-five chromosome arms had significant effects on Glu-D1-2, excluding the structural gene dosage effect: 17 positive effects on 1BL, 2AS, 2BS, 2DS, 2DL, 3AS, 3AL, 3BS, 5AS, 5BS, 5DL, 6AL, 6DL, 7AL, 7BS, 7BL, and 7DL and eight negative effects on 1DS, 4DL, 5AL, 5BL, 6BS, 6BL, 6DS and 7DS. Of the 164 gene-chromosome arm tests performed, about 52% (85/164) showed no significant effects, and 48% (79/164) showed significant effects, excluding the structural gene dosage effects. Of the significant effects, 56% (44/79) were positive effects, and 44% (35/79) were negative effects. Comparisons of dosage effects on orthologous loci (both x-type or both y-type HMW glutenins) showed that orthologous HMW glutenin genes are largely influenced by the same regulatory systems. Less correlation was found for comparisons between paralogous genes, although considerable conservation was observed at this level as well. These observations suggest that after polyploidization, many of the duplicated orthologous regulatory loci were inactivated by mutation, thus consolidating control over the HMW glutenin genes. Possible candidates for orthologous regulatory genes were identified in maize and barley. This study represents the first comprehensive search of the wheat genome for regulators of the HMW glutenins.     

Genetic characterization of QTL associated with resistance to Fusarium head blight in a doubled-haploid spring wheat population.

Fusarium head blight (FHB) is one of the most important fungal wheat diseases worldwide. Understanding the genetics of FHB resistance is the key to facilitating the introgression of different FHB resistance genes into adapted wheat. The objectives of the present study were to detect and map quantitative trait loci (QTL) associated with FHB resistance genes and characterize the genetic components of the QTL in a doubled-haploid (DH) spring wheat population using both single-locus and two-locus analysis. A mapping population, consisting of 174 DH lines from the cross between DH181 (resistant) and AC Foremost (susceptible), was evaluated for type I resistance to initial infection during a 2-year period in spray-inoculated field trials, for Type II resistance to fungal spread within the spike in 3 greenhouse experiments using single-floret inoculation, and for resistance to kernel infection in a 2001 field trial. One-locus QTL analysis revealed 7 QTL for type I resistance on chromosome arms 2DS, 3AS, 3BS, 3BC (centromeric), 4DL, 5AS, and 6BS, 4 QTL for type II resistance on chromosomes 2DS, 3BS, 6BS, and 7BL, and 6 QTL for resistance to kernel infection on chromosomes 1DL, 2DS, 3BS, 3BC, 4DL, and 6BS. Two-locus QTL analysis detected 8 QTL with main effects and 4 additive by additive epistatic interactions for FHB resistance and identified novel FHB resistance genes for the first time on chromosomes 1DL, 4AL, and 4DL. Neither significant QTL by environment interactions nor epistatic QTL by environment interactions were found for either type I or type II resistance. The additive effects of QTL explained most of the phenotypic variance for FHB resistance. Marker-assisted selection for the favored alleles at multiple genomic regions appears to be a promising tool to accelerate the introgression and pyramiding of different FHB resistance genes into adapted wheat genetic backgrounds.     

Chromosomal locations of the genes for histones and a histone gene-binding protein family HBP-1 in common wheat

The chromosomal locations of the genes in common wheat that encode the five histones and five members of the HBP (histone gene-binding protein)-1 family were determined by hybridizing their cloned DNAs to genomic DNAs of nullitetrasomic and telosomic lines of common wheat, Triticum aestivum cv. Chinese Spring. The H1 and H2a genes are located on different sets of homoeologous chromosomes or chromosome arms, namely, 5A, 5B and 5D, and 2AS, 2BS and 2DS, respectively. Genes for the other histones, H2b, H3 and H4, are found in high copy number and are dispersed among a large number of chromosomes. The genes for all members of the HBP-1 family are present in small copy numbers. Those for HBP-1a(1) are located on six chromosome arms, 3BL, 5AL, 5DL, 6AL, 6BS and 7DL, whereas those for each HBP-1a(c14), 1a(17), 1b(c1), and 1b(c38) are on a single set of homoeologous chromosome arms; 4AS, 4BL, 4DL; 6AS, 6BS, 6DS; 3AL, 3BL, 3DL; and 3AS, 3BS, 3DS, respectively. The genes for histones H1 and H2a, and for all members of the HBP-1 family except HBP-1a(1) are assumed to have different phylogenetic origins. The genes for histone 2a and HBP-1a(17) are located in the RFLP maps of chromosomes 2B and 6A, respectively. Gene symbols are proposed for all genes whose chromosomal locations have been determined.     

Mapping QTLs for alpha-amylase activity in rye grain

Genetic control of alpha-amylase activity in rye grain was investigated by QTL mapping based on DS2 x RXL10 intercross consisting of 99 F5-6 families propagated at one location during four vegetation seasons. A wide range of variation in alpha-amylase activity and transgression effects were found among families and parental lines. This variation was shown to be determined in 40.1% by 7 significant (LOD score not less than 2.5) and 2 putative QTLs (2 < LOD < 2.5) distributed on all rye chromosomes except 4R. Two significant QTLs located on 3RL and 5RL chromosome arms were expressed each year. The third significant QTL was detected in three years (1RL). The other four significant QTLs (2RL, 5RS, 6RL, 7RL) were found in one year of study. The number and composition of QTLs were specific for a given year varying from three to six. QTLs were not correlated with isoenzyme polymorphisms at the structural alpha-Amy1 loci. A QTL associated with a region containing the alpha-Amy3 locus was detected on chromosome 5RL. Both high- and low-activity QTL alleles were found in each parental line, which explains the appearance of transgressive recombinants in the segregating population.     

Mapping of loci affecting copper tolerance in wheat—The possible impact of the vernalization gene Vrn-A1

Pot experiments with copper-treated soil and a control were performed in a greenhouse to determine QTLs for copper tolerance in wheat, using deletion, introgression and single chromosome recombinant lines. Genetic and physical mapping identified loci for copper tolerance on the long arm of chromosomes 5A and 5D, while loci with minor effects were found on the long and short arms of chromosome 5B. Tests on ‘Chinese Spring’–Aegilops tauschii introgression lines revealed a locus influencing copper tolerance on chromosome 3DS. QTLs for copper tolerance on chromosome 5A were mapped genetically and physically to exactly the same position as the gene for vernalization requirement (Vrn-A1). It is therefore suggested that Vrn-A1 may have a pleiotropic effect on copper tolerance may be due to the control of Cbf genes.     

Quantitative trait loci for aluminum resistance in wheat

Quantitative trait loci (QTL) for wheat resistance to aluminum (Al) toxicity were analyzed using simple sequence repeats (SSRs) in a population of 192 F₆ recombinant inbred lines (RILs) derived from a cross between an Al-resistant cultivar, Atlas 66 and an Al-sensitive cultivar, Chisholm. Wheat reaction to Al was measured by relative root growth and root response to hematoxylin stain in nutrient-solution culture. After screening 1,028 SSR markers for polymorphisms between the parents and bulks, we identified two QTLs for Al resistance in Atlas 66. One major QTL was mapped on chromosome 4D that co-segregated with the Al-activated malate transporter gene (ALMT1). Another minor QTL was located on chromosome 3BL. Together, these two QTLs accounted for about 57% of the phenotypic variation in hematoxylin staining score and 50% of the variation in net root growth (NRG). Expression of the minor QTL on 3BL was suppressed by the major QTL on 4DL. The two QTLs for Al resistance in Atlas 66 were also verified in an additional RIL population derived from Atlas 66/Century. Several SSR markers closely linked to the QTLs were identified and have potential to be used for marker-assisted selection (MAS) to improve Al-resistance of wheat cultivars in breeding programs.     

Genetic analysis of resistance to septoria tritici blotch in the French winter wheat cultivars Balance and Apache

The ascomycete Mycosphaerella graminicola is the causal agent of septoria tritici blotch (STB), one of the most destructive foliar diseases of bread and durum wheat globally, particularly in temperate humid areas. A screening of the French bread wheat cultivars Apache and Balance with 30 M. graminicola isolates revealed a pattern of resistant responses that suggested the presence of new genes for STB resistance. Quantitative trait loci (QTL) analysis of a doubled haploid (DH) population with five M. graminicola isolates in the seedling stage identified four QTLs on chromosomes 3AS, 1BS, 6DS and 7DS, and occasionally on 7DL. The QTL on chromosome 6DS flanked by SSR markers Xgpw5176 and Xgpw3087 is a novel QTL that now can be designated as Stb18. The QTLs on chromosomes 3AS and 1BS most likely represent Stb6 and Stb11, respectively, and the QTLs on chromosome 7DS are most probably identical with Stb4 and Stb5. However, the QTL identified on chromosome 7DL is expected to be a new Stb gene that still needs further characterization. Multiple isolates were used and show that not all isolates identify all QTLs, which clearly demonstrates the specificity in the M. graminicola–wheat pathosystem. QTL analyses were performed with various disease parameters. The development of asexual fructifications (pycnidia) in the characteristic necrotic blotches of STB, designated as parameter P, identified the maximum number of QTLs. All other parameters identified fewer but not different QTLs. The segregation of multiple QTLs in the Apache/Balance DH population enabled the identification of DH lines with single QTLs and multiple QTL combinations. Analyses of the marker data of these DH lines clearly demonstrated the positive effect of pyramiding QTLs to broaden resistance spectra as well as epistatic and additive interactions between these QTLs. Phenotyping of the Apache/Balance DH population in the field confirmed the presence of the QTLs that were identified in the seedling stage, but Stb18 was inconsistently expressed and might be particularly effective in young plants. In contrast, an additional QTL for STB resistance was identified on chromosome 2DS that is exclusively and consistently expressed in mature plants over locations and time, but it was also strongly related with earliness, tallness as well as resistance to Fusarium head blight. Although to date no Stb gene has been reported on chromosome 2D, the data provide evidence that this QTL is only indirectly related to STB resistance. This study shows that detailed genetic analysis of contemporary commercial bread wheat cultivars can unveil novel Stb genes that can be readily applied in marker-assisted breeding programs.     

Analysis and mapping of Rhizoctonia root rot resistance traits from the synthetic wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) line SYN-172

The prevalence of root disease after planting in cold spring soils has hindered the adoption of reduced or no-tillage cereal cropping systems in the Pacific Northwest. In particular, R. solani AG8, a necrotrophic root pathogen, can cause significant damage to wheat stands under these conditions. In previous efforts to find root rot resistance, a CIMMYT synthetic wheat line, SYN-172, was found to have little to no seedling stunting from disease and lower root disease scores. To identify trait-maker associations, a population consisting of 150 BC₁F₅ recombinant inbred lines from a cross of “Louise,” a typically susceptible Pacific Northwest (PNW) cultivar, and SYN-172 was created. A total of 689 polymorphic markers were used to identify trait-marker associations for seedling stunting in field green bridge and growth chamber environments. In total, five quantitative trait loci (QTL) were found on chromosome arms 1AL, 2AL, 5BL, 7DS, and 7DL. One QTL, on chromosome 2AL, was consistently detected in all four of the environments tested, and originated from SYN-172. A second QTL on 7DL, originating from the susceptible parent Louise, was found consistently in all three of the field environments, but not in soils artificially infested with R. solani AG8. These QTL have not been previously reported and will be useful root rot resistance genes when transferred into the PNW spring wheat germplasm.     

Genetic analysis of resistance to yellow rust in hexaploid wheat using a mixture model for multiple crosses

DNA-based molecular markers have been used in numerous studies for tagging specific genes in wheat for subsequent use in marker-assisted selection. Usually in plant breeding, procedures for mapping genes are based on analysis of a single segregating population. However, breeding programmes routinely evaluate large numbers of progeny derived from multiple-related crosses with some parental lines shared. In most such related crosses, the number of progeny is quite small. Thus, statistical techniques for detecting quantitative trait loci (QTLs) using data from conventional multi-cross breeding programmes are interesting. The objective of this study is to present a mixture model for QTL mapping in crosses of multiple inbred varieties with non-normal phenotype distributions and to use this model to map QTLs for yellow rust resistance in elite wheat breeding material. Three doubled haploid populations consisting of 41, 42 and 55 lines, respectively, originating from four parental varieties were studied. Multi-cross QTL analysis with three specific pathogen isolates of Puccinia striiformis f. sp. tritici and a mixture of the isolates revealed QTLs for resistance at four different genomic locations. These QTLs were found on chromosome 2AL, 2AS, 2BL and 6BL and explained between 21 and 41% of the phenotypic variation. Two of these QTLs, one on the long arm of chromosome 2A and one on the short arm of chromosome 2A were identical to the known yellow rust resistance genes Yr32 and Yr17, respectively, whereas the QTLs located on the long arms of chromosomes 2B and 6B may reflect types of resistance to yellow rust, which have not previously been mapped.