Comparative genetic maps reveal extreme crossover localization in the Aegilops speltoides chromosomes

A total of 137 loci were mapped in Aegilops speltoides, the closest extant relative of the wheat B genome, using two F₂ mapping populations and a set of wheat-Ae. speltoides disomic addition (DA) lines. Comparisons of Ae. speltoides genetic maps with those of Triticum monococcum indicated that Ae. speltoides conserved the gross chromosome structure observed across the tribe Triticeae. A putative inversion involving the short arm of chromosome 2 was detected in Ae. speltoides. A translocation between chromosomes 2 and 6, present in the wheat B genome, was absent. The ligustica/aucheri spike dimorphism behaved as allelic variation at a single locus, which was mapped in the centromeric region of chromosome 3. The genetic length of each chromosome arm was about 50 cM, irrespective of its physical length. Compared to T. monococcum genetic maps, recombination was virtually eliminated from the proximal 50–100 cM and was localized in short distal regions, which were often expanded compared to the T. monococcum maps. The wheat B genome and the genome of Ae. longissima, a close relative of Ae. speltoides, do not show the extreme localization of crossovers observed in Ae. speltoides.     

A “one-marker-for-two-genes” approach for efficient molecular discrimination of Pm12 and Pm21 conferring resistance to powdery mildew in wheat

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is one of the most important wheat diseases worldwide. Pyramiding different resistance genes into single cultivar has been proposed as one remedy to provide durable resistance. Powdery mildew resistance genes Pm12 (T6BS-6SS.6SL), transferred from Aegilops speltoides to wheat cv. Wembley, and Pm21 (T6VS.6AL), introduced from Dasypyrum villosum to wheat cv. Yangmai5, conferred broad-spectrum resistance to B. graminis f. sp. tritici. Both Pm12 and Pm21 genes are located on the short arms of homologous group six involved translocated chromosomes 6SS.6BL and 6VS.6AL, respectively. Simple sequence repeat motifs of wheat simple sequence repeat (SSR) and expressed sequence tag (EST) sequences on the short arm of homologous group six chromosomes were analyzed to develop molecular markers for discriminating chromosome arms 6AS, 6BS, 6DS, 6VS, and 6SS. One EST–SSR marker, Xcau127, was polymorphic, and therefore can be used to distinguish the two resistance genes and the respective susceptible alleles. This marker allowed us to develop an efficient “one-marker-for-two-genes” procedure for identifying powdery mildew resistance genes Pm12 and Pm21 for marker-assisted selection and gene pyramiding in wheat breeding programs. Wei Song and Chaojie Xie contributed equally to this work     

Pm34: a new powdery mildew resistance gene transferred from Aegilops tauschii Coss. to common wheat (Triticum aestivum L.)

Powdery mildew is a major fungal disease in wheat growing areas worldwide. A novel source of resistance to wheat powdery mildew present in the germplasm line NC97BGTD7 was genetically characterized as a monogenic trait in greenhouse and field trials using F₂ derived lines from a NC97BGTD7 X Saluda cross. Microsatellite markers were used to map and tag this resistance gene, now designated Pm34. Three co-dominant microsatellite markers linked to Pm34 were identified and their most likely order was established as: Xbarc177-5D, 5.4cM, Pm34, 2.6cM, Xbarc144-5D, 14cM, Xgwm272-5D. These microsatellite markers were previously mapped to the long arm of the 5D chromosome and their positions were confirmed using Chinese Spring nullitetrasomic Nulli5D-tetra5A and ditelosomic Dt5DL lines. Pm2, the only other known Pm gene on chromosome 5D, has been mapped to the short arm and its specificity is different from that of Pm34.     

H9, H10, and H11 compose a cluster of Hessian fly-resistance genes in the distal gene-rich region of wheat chromosome 1AS

H9, H10, and H11 are major dominant resistance genes in wheat, expressing antibiosis against Hessian fly [(Hf) Mayetiola destructor (Say)] larvae. Previously, H9 and H10 were assigned to chromosome 5A and H11 to 1A. The objectives of this study were to identify simple-sequence-repeat (SSR) markers for fine mapping of these genes and for marker-assisted selection in wheat breeding. Contrary to previous results, H9 and H10 did not show linkage with SSR markers on chromosome 5A. Instead, H9, H10, and H11 are linked with SSR markers on the short arm of chromosome 1A. Both H9 and H10 are tightly linked to flanking markers Xbarc263 and Xcfa2153 within a genetic distance of 0.3–0.5 cM. H11 is tightly linked to flanking markers Xcfa2153 and Xbarc263 at genetic distances of 0.3 cM and 1.7 cM. Deletion bin mapping assigned these markers and genes to the distal 14% of chromosome arm 1AS, where another Hf-resistance gene, Hdic (derived from emmer wheat), was also mapped previously. Marker polymorphism results indicated that a small terminal segment of chromosome 1AS containing H9 or H10 was transferred from the donor parent to the wheat lines Iris or Joy, and a small intercalary fragment carrying H11 was transferred from the resistant donor to the wheat line Karen. Our results suggest that H9, H10, H11, Hdic, and the previously identified H9- or H11-linked genes (H3, H5, H6, H12, H14, H15, H16, H17, H19, H28, and H29) may compose a cluster (or family) of Hf-resistance genes in the distal gene-rich region of wheat chromosome 1AS; and H10 most likely is the same gene as H9.Mention of commercial or proprietary product does not constitute an endorsement by the USDA.     

A chromosome-specific sequence common to the B genome of polyploid wheat and Aegilops searsii

A low-copy, non-coding chromosome-specific DNA sequence, isolated from common wheat, was physically mapped to the distal 19% region of the long arm of chromosome 3B (3BL) of common wheat. This sequence, designated WPG118, was then characterized by Southern hybridization, PCR amplification and sequence comparison using a large collection of polyploid wheats and diploid Triticum and Aegilops species. The data show that the sequence exists in all polyploid wheats containing the B genome and absent from those containing the G genome. At the diploid level, it exists only in Ae. searsii, a diploid species of section Sitopsis, and not in other diploids including Ae. speltoides, the closest extant relative to the donor of the B genome of polyploid wheat. This finding may support the hypothesis that the B-genome of polyploid wheat is of a polyphyletic origin, i.e. it is a recombined genome derived from two or more diploid Aegilops species.     

High-density genetic and physical bin mapping of wheat chromosome 1D reveals that the powdery mildew resistance gene <Emphasis Type="Italic">Pm24</Emphasis> is located in a highly recombinogenic region

Genetic maps of wheat chromosome 1D consisting of 57 microsatellite marker loci were constructed using Chinese Spring (CS) × Chiyacao F₂ and the International Triticeae Mapping Initiative (ITMI) recombinant inbred lines (RILs) mapping populations. Marker order was consistent, but genetic distances of neighboring markers were different in two populations. Physical bin map of 57 microsatellite marker loci was generated by means of 10 CS 1D deletion lines. The physical bin mapping indicated that microsatellite marker loci were not randomly distributed on chromosome 1D. Nineteen of the 24 (79.2%) microsatellite markers were mapped in the distal 30% genomic region of 1DS, whereas 25 of the 33 (75.8%) markers were assigned to the distal 59% region of 1DL. The powdery mildew resistance gene Pm24, originating from the Chinese wheat landrace Chiyacao, was previously mapped in the vicinity of the centromere on the short arm of chromosome 1D. A high density genetic map of chromosome 1D was constructed, consisting of 36 markers and Pm24, with a total map length of 292.7 cM. Twelve marker loci were found to be closely linked to Pm24. Pm24 was flanked by Xgwm789 (Xgwm603) and Xbarc229 with genetic distances of 2.4 and 3.6 cM, respectively, whereas a microsatellite marker Xgwm1291 co-segregated with Pm24. The microsatellite marker Xgwm1291 was assigned to the bin 1DS5-0.70-1.00 of the chromosome arm 1DS. It could be concluded that Pm24 is located in the ‘1S0.8 gene-rich region’, a highly recombinogenic region of wheat. The results presented here would provide a start point for the map-based cloning of Pm24.     

Tomato chromosome 6: a high resolution map of the long arm and construction of a composite integrated marker-order map

Integration of molecular and classical genetic maps is an essential requirement for marker-assisted breeding, quantitative trait locus mapping and map-based cloning. With respects to tomato, such maps are only available for the top part of chromosome 1, for chromosome 3 and for the short arm and the centromere proximal part of the long arm of chromosome 6. Employing an L. esculentum line carrying an L. hirsutum introgression we constructed an integrated linkage map for the telomere proximal part of the long arm of tomato chromosome 6, thereby completing the integrated map published previously. With an average map distance of only 0.6 cM the map provides detailed information on the relative position of molecular markers and several traits of economical importance, such as the fruit color marker B. Furthermore, two additional crosses using lines containing L. pennellii introgressions were performed to address the question as to how the recombination frequency in a marked interval on the long arm of chromosome 6 is affected by introgressed segments from different origins. It is concluded that recombination is not merely affected by the local level of homology but also by surrounding sequences. Combination of all the linkage data generated in various crosses described in this and other studies enabled the construction of the first integrated map of an entire tomato chromosome. This map carries 42 loci and shows the position of 15 classical genes relative to 59 molecular markers.     

Construction and study of leaf rust-resistant common wheat lines with translocations of <Emphasis Type="Italic">Aegilops speltoides</Emphasis> Tausch. Genetic material

Genotyping was performed for the leaf rust-resistant line 73/00i (Triticum aestivum × Aegilops speltoides). Fluorescence in situ hybridization (FISH) with probes Spelt1 and pSc119.2 in combination with microsatellite analysis were used to determine the locations and sizes of the Ae. speltoides genetic fragments integrated into the line genome. Translocations were identified in the long arms of chromosomes 5B and 6B and in the short arm of chromosome 1B. The Spelt1 and pSc119.2 molecular cytological markers made it possible to rapidly establish lines with single translocation in the long arms of chromosomes 5B and 6B. The line carrying the T5BS · 5BL-5SL translocation was highly resistant to leaf rust, and the lines carrying the T6BS · 6BL-6SL translocation displayed moderate resistance. The translocations differed in chromosomal location from known leaf resistance genes transferred into common wheat from Ae. speltoides. Hence, it was assumed that new genes were introduced into the common wheat genome from Ae. speltoides. The locus that determined high resistance to leaf rust and was transferred into the common wheat genome from the long arm of Ae. speltoides chromosome 5S by the T5BS · 5BL-5SL translocation was preliminarily designated as LrAsp5.     

Identification of alien chromatin specifying resistance to wheat streak mosaic and greenbug in wheat germ plasm by C-banding and in situ hybridization

Summary The chromosome constitutions of eight wheat streak mosaic virus (WSMV)-resistant lines, three of which are also greenbug resistant, derived from wheat/ Agropyron intermedium/Aegilops speltoides crosses were analyzed by C-banding and in situ hybridization. All lines could be traced back to CI15092 in which chromosome 4A is substituted for by an Ag. intermedium chromosome designated 4Ai-2, and the derived lines carry either 4Ai-2 or a part of it. Two (CI17881, CI17886) were 4Ai-2 addition lines. CI17882 and CI17885 were 4Ai-2-(4D) substitution lines. CI17883 was a translocation substitution line with a pair of 6AL.4Ai-2S and a pair of 6AS.4Ai-2L chromosomes substituting for chromosome pairs 4D and 6A of wheat. CI17884 carried a 4DL.4Ai-2S translocation which substituted for chromosome 4D. CI17766 carried a 4AL.4Ai-2S translocation substituting for chromosome 4A. The results show that the 4Ai-2 chromosome is related to homoeologous group 4 and that the resistance gene(s) against WSMV is located on the short arm of 4Ai-2. In addition, CI17882, CI17884, and CI17885 contained Ae. speltoides chromosome 7S substituting for chromosome 7A of wheat. The greenbug resistance gene Gb5 was located on chromosome 7S.Contribution No. 90-515-J from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan, Kan., USA     

Genetic characterization of powdery mildew resistance in U.S. hard winter wheat

Powdery mildew significantly affects grain yield and end-use quality of winter wheat in the southern Great Plains. Employing resistance resources in locally adapted cultivars is the most effective means to control powdery mildew. Two types of powdery mildew resistance exist in wheat cultivars, i.e., qualitative and quantitative. Qualitative resistance is controlled by major genes, is race-specific, is not durable, and is effective in seedlings and in adult plants. Quantitative resistance is controlled by minor genes, is non-race-specific, is durable, and is predominantly effective in adult plants. In this study, we found that the segregation of powdery mildew resistance in a population of recombinant inbred lines developed from a cross between the susceptible cultivar Jagger and the resistant cultivar 2174 was controlled by a major QTL on the short arm of chromosome 1A and modified by four minor QTLs on chromosomes 1B, 3B, 4A, and 6D. The major QTL was mapped to the genomic region where the Pm3 gene resides. Using specific PCR markers for seven Pm3 alleles, 2174 was found to carry the Pm3a allele. Pm3a explained 61% of the total phenotypic variation in disease reaction observed among seedlings inoculated in the greenhouse and adult plants grown in the field and subjected to natural disease pressure. The resistant Pm3a allele was present among 4 of 31 cultivars currently being produced in the southern Great Plains. The genetic effects of several minor loci varied with different developmental stages and environments. Molecular markers associated with these genetic loci would facilitate incorporating genetic resistance to powdery mildew into improved winter wheat cultivars.     

Genetic mapping of the powdery mildew resistance gene Pm6 in wheat by RFLP analysis

Pm6 in bread wheat (Triticum aestivum L.), which was transferred from Triticum. timopheevii L., is a gene conferring resistance to the powdery mildew disease caused by Erysiphe graminis f. sp. tritici. Six near-isogenic lines ( NILs ) of Pm6 in a cultivar ’Prins’ background were analyzed to map this gene using restriction fragment length polymorphism (RFLP). Each of the six NILs possessed a T. timopheevii-derived segment, varying in length, and associated with powdery mildew resistance. Lines IGV1–465 (FAO163b/ 7*Prins) and IGV1–467 (Idaed 59B/7*Prins) had the shortest introgressed segments, which were detected only by DNA probes BCD135 and PSR934, respectively. The polymorphic loci detected by both probes were mapped to the long arm of chromosome 2B. Lines IGV1–458 (CI13250/7*Prins) and IGV1–456 (CI12559/8*Prins) contained the longest T. timopheevii segments involving both arms of donor chromosome 2G across the centromere. All these introgressed segments had an overlapping region flanked by the loci xpsr934 and xbcd135 on 2BL. Thus, Pm6 was located in this region since the powdery mildew resistance in all the NILs resulted from the introgressed fragments. Using the F₂ mapping population from a cross of IGV1–463 (PI170914/7*Prins)×Prins, Pm6 was shown to be closely linked to the loci xbcd135 and xbcd266 at a genetic distance of 1.6 cM and 4.8 cM, respectively. BCD135 was successfully used in detecting the presence of Pm6 in different genetic backgrounds. Received: 29 June 1999 / Accepted: 6 July 1999     

Suppressed recombination rate in 6VS/6AL translocation region carrying the Pm21 locus introgressed from Haynaldia villosa into hexaploid wheat

Pm21 is an effective gene for powdery mildew resistance transferred from Haynaldia villosa into common wheat cultivars. No virulence against this gene has been detected so far. A set of 42 powdery mildew isolates collected in Israel and tested in the current study also revealed no virulence against this gene. Pm21 was previously reported to be located on the short arm of 6VS/6AL translocation chromosome. We constructed a high-density genetic map of chromosome 6A, consisting of 28 PCR markers and the Pm21 gene. A comparison with previously published genetic maps of wheat chromosome 6A revealed that the recombination rate in the 6VS/6AL translocation region was poor. We assume that suppressed recombination caused by the alien H. villosa genetic material is the most reasonable explanation for the tight genetic linkage and the inadequacy between the Pm21 genetic map and the Pm21 physical map of 6A. A large number of sequence-tag sites (STS) and simple sequence repeat markers, which co-segregate with or are closely linked to the Pm21 gene, and the conversion of three resistance gene analog markers into new STS markers, provide a reliable and easy-to-use molecular tool for marker-assisted selection of Pm21 in wheat breeding programs. An additional gene, Pm31, previously reported to be derived from Triticum dicoccoides, was mapped into a similar genomic location to Pm21. Screening of the parental lines and the mapping population with Pm21 diagnostic markers clearly confirmed that the donor line of Pm31 is H. villosa and not T. dicoccoides. Therefore, we conclude that Pm21 and Pm31 refer to the same gene, derived from H. villosa, and that the designation of Pm31 as a new Pm gene was erroneous.     

Genetic linkage between C-bands and storage protein genes in chromosome 1B of tetraploid wheat

Summary Genetic mapping of polymorphic C-bands allows direct comparisons between genetic and physical maps. Eleven C-bands and two seed storage protein genes on chromosome 1B, polymorphic between Langdon durum and four accessions of T. dicoccoides, were used to study the distribution of recombination along the entire length of the chromosome. Recombination in the short arm was almost completely restricted to the satellite, two-thirds of the arm's length from the centromere; the Gli-B1 gene was found to be tightly linked to the telomeric C-band. In the long arm, the distal 51.4% of the arm accounted for 88% of recombination; the proximal half of the arm accounted for the remaining 12%. While the amount of crossing-over differed significantly between the four T. dicoccoides 1B chromosomes, there were no significant differences in the relative distributions of crossing-over along the chromosome. Consequently, the genetic maps obtained from the four individual T. dicoccoides chromosomes were combined to yield a consensus map of 14 markers (including the centromere) for the chromosome.     

A consensus map of rye integrating mapping data from five mapping populations

A consensus map of rye (Secale cereale L.) was constructed using JoinMap 2.0 based on mapping data from five different mapping populations, including ‘UC90’ × ‘E-line’, ‘P87’ × ‘P105’, ‘I_0.1-line’ × ‘I_0.1-line’, ‘E-line’ × ‘R-line’, and ‘Ds2’ × ‘RxL10’. The integration of the five mapping populations resulted in a 779-cM map containing 501 markers with the number of markers per chromosome ranging from 57 on 1R to 86 on 4R. The linkage sizes ranged from 71.5 cM on 2R to 148.7 cM on 4R. A comparison of the individual maps to the consensus map revealed that the linear locus order was generally in good agreement between the various populations, but the 4R orientations were not consistent among the five individual maps. The 4R short arm and long arm assignments were switched between the two population maps involving the ‘E-line’ parent and the other three individual maps. Map comparisons also indicated that marker order variations exist among the five individual maps. However, the chromosome 5R showed very little marker order variation among the five maps. The consensus map not only integrated the linkage data from different maps, but also greatly increased the map resolution, thus, facilitating molecular breeding activities involving rye and triticale.     

Mapping genes Lr53 and Yr35 on the short arm of chromosome 6B of common wheat with microsatellite markers and studies of their association with Lr36

The rust resistance genes Lr53 and Yr35, transferred to common wheat from Triticum dicoccoides, were reported previously to be completely linked on chromosome 6B. Four F ₃ families were produced from a cross between a line carrying Lr53 and Yr35 (98M71) and the leaf rust and stripe rust susceptible genotype Avocet “S” and were rust tested using Puccinina triticina pathotype 53-1,(6),(7),10,11 and Puccinia striiformis f. sp. tritici pathotype 110 E143 A+. The homozygous resistant lines produced infection types of “;1−” and “;N” to these pathotypes, respectively. The Chi-squared tests indicated goodness-of-fit of the data for one leaf rust gene and one stripe rust gene segregation. Linkage analysis using this population demonstrated recombination of 3% between the genes. Microsatellite markers located on the short arm of chromosome 6B were used to map the genes, with the markers cfd1 and gwm508 being mapped approximately 1.1 and 4.5 cM, respectively, proximal to Lr53. Additional studies of the relationship between Lr36, also located on the short arm of chromosome 6B, and Lr53 indicated that the two genes were independent.     

Precise mapping of a locus affecting grain protein content in durum wheat

Grain protein content (GPC) is an important factor in pasta and breadmaking quality, and in human nutrition. It is also an important trait for wheat growers because premium prices are frequently paid for wheat with high GPC. A promising source for alleles to increase GPC was detected on chromosome 6B of Triticum turgidum var. dicoccoides accession FA-15-3 (DIC). Two previous quantitative trait locus (QTL) studies found that the positive effect of DIC-6B was associated to a single locus located between the centromere and the Nor-B2 locus on the short arm of chromosome 6B. Microsatellite markers Xgwm508 and Xgwm193 flanking the QTL region were used in this study to develop 20 new homozygous recombinant substitution lines (RSLs) with crossovers between these markers. These 20 RSLs, plus nine RSLs developed in previous studies were characterized with four new RFLP markers located within this chromosome segment. Grain protein content was determined in three field experiments organized as randomized complete block designs with ten replications each. The QTL peaks for protein content were located in the central region of a 2.7-cM interval between RFLP markers Xcdo365 and Xucw67 in the three experiments. Statistical analyses showed that almost all lines could be classified unequivocally within low- and high- protein groups, facilitating the mapping of this trait as a single Mendelian locus designated Gpc-6B1. The Gpc-6B1 locus was mapped 1.5-cM proximal to Xcdo365 and 1.2-cM distal to Xucw67. These new markers can be used to reduce the size of the DIC chromosome segment selected in marker-assisted selection programs. Markers Nor-B2 and Xucw66 flanking the previous two markers can be used to select against the DIC segment and reduce the linkage drag during the transfer of Gpc-6B1 into commercial bread and pasta wheat varieties. The precise mapping of the high GPC gene, the high frequency of recombinants recovered in the targeted region, and the recent development of a tetraploid BAC library including the Gpc-6B1 DIC allele are the first steps towards the map-based cloning of this gene.Communicated by J. Dvorak     

Molecular mapping of the wheat powdery mildew resistance gene Pm24 and marker validation for molecular breeding

Molecular markers were identified in common wheat for the Pm24 locus conferring resistance to different isolates of the powdery mildew pathogen, Erysiphe graminis DM f. sp. tritici (Em. Marchal). Bulked segregant analysis was used to identify amplified fragment length polymorphism (AFLP) markers and microsatellite markers linked to the gene Pm24 in an F₂ progeny from the cross Chinese Spring (susceptible)× Chiyacao (resistant). Two AFLP markers XACA/CTA-407 and XACA/CCG-420, and three microsatellite markers Xgwm106, Xgwm337 and Xgwm458, were mapped in coupling phase to the Pm24 locus. The AFLP marker locus XACA/CTA-407 co-segregated with the Pm24 gene, and XACA/CCG-420 mapped 4.5 cM from this gene. Another AFLP marker locus XAAT/CCA-346 co- segregated in repulsion phase with the Pm24 locus. Pm24 was mapped close to the centromere on the short arm of chromosome 1D, contrary to the previously reported location on chromosome 6D. Pm24 segregated independently of gene Pm22, also located on chromosome 1D. An allele of microsatellite locus Xgwm337 located 2.4±1.2 cM from Pm24 was shown to be diagnostic and therefore potentially useful for pyramiding two or more genes for powdery mildew resistance in a single genotype. Received: 25 August 1999 / Accepted: 16 December 1999     

Molecular cytogenetic analysis of Agropyron elongatum chromatin in wheat germplasm specifying resistance to wheat streak mosaic virus

Summary Three lines derived from wheat (6x) x Agropyron elongatum (10x) that are resistant to wheat streak mosaic virus (WSMV) were analyzed by chromosome pairing, banding, and in situ hybridization. Line CI15321 was identified as a disomic substitution line where wheat chromosome 1D is replaced by Ag. elongatum chromosome 1Ae-1. Line 87-94-1 is a wheat-Ag. elongatum ditelosomic addition 1Ae-1L. Line CI15322 contains an Ag. elongatum chromosome, 1Ae-2, that substitutes for chromosome 1D. The short arm of 1Ae-2 paired with the short arm of 1Ae-1 at metaphase I (MI) in 82% of the pollen mother cells (PMCs). However, the long arms of these two chromosomes did not pair with each other. In CI15322, the long arm of chromosome 4D has an Agropyron chromosome segment which was derived from the distal part of 1Ae-1L. This translocation chromosome is designated as T4DS·4DL-1L. T4DS·4DL-1Ae-1L has a 0.73 m distal part of the long arm of 4D replaced by a 1.31 m distal segment from 1Ae-1L. The major WSMV resistance gene(s) in these lines is located on the distal part of 1Ae-1L.Contribution No. 92-599-J from the Kansas Agricutural Experiment Station, Kansas State University, Manhattan, Kansas, USA     

A storage-protein marker associated with the suppressor of Pm8 for powdery mildew resistance in wheat

A suppressor of resistance to powdery mildew conferred by Pm8 showed complete association with the presence of a storage-protein marker resolved by electrophoresis on SDS-PAGE gels. This marker was identified as the product of the gliadin allele Gli-A1a. The mildewresponse phenotypes of wheats possessing the 1BL.1RS translocation were completely predictable from electrophoretograms. The suppressor, designated SuPm8, was located on chromosome 1AS. It was specific in its suppression of Pm8, and did not affect the rye-derived resistance phenotypes of wheat lines with Pm17, also located in 1RS, or of lines with Pm7.     

Characterization and chromosomal location of Pm40 in common wheat: a new gene for resistance to powdery mildew derived from Elytrigia intermedium

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is a very destructive wheat (Triticum aestivum) disease. Resistance was transferred from Elytrigia intermedium to common wheat by crossing and backcrossing, and line GRY19, that was subsequently selected, possessed a single dominant gene for seedling resistance. Five polymorphic microsatellite markers, Xgwm297, Xwmc335, Xwmc364, Xwmc426 and Xwmc476, on chromosome arm 7BS, were mapped relative to the powdery mildew resistance locus in an F₂ population of Mianyang 11/GRY19. The loci order Xwmc426–Xwmc335–Pm40–Xgwm297–Xwmc364–Xwmc476, with 5.9, 0.2, 0.7, 1.2 and 2.9 cM genetic distances, was consistent with published maps. The resistance gene transferred from Elytrigia intermedium into wheat line GRY19 was novel, and was designated Pm40. The close flanking markers will enable marker assisted transfer of this gene into wheat breeding populations. P.G. Luo and H.Y. Luo contributed equally to the work.