Evidence of natural selection for disease resistance in Composite Cross Five (CCV) of barley

Cambridge Composite Cross Five (CCV) of barley was studied utilising hordeins, restriction fragment length polymorphisms (RFLPs) and reaction to powdery mildew with a view to understanding the genetic changes occurring in the population. Changes in the frequency of individual hordein patterns as well as pattern combinations showed directional trends in successive generations in three parallel populations maintained as discrete populations since 1977 in Cambridge. Certain hordein pattern combinations were more common in the resistance classes and there was a strong association between hordein patterns and mildew reaction. RFLP analysis revealed that 80% of a random sample taken from generation F₂₄ of Population I had the same restriction pattern as that of the cultivar Algerian, which was one of the original 30 parental lines of CCV. This cultivar is the source of the Mla1 allele in barley improvement programmes in Europe. We argue, based on supporting evidence from hordein analysis and tests of reaction to selected mildew isolates of known virulence isolates together with UK virulence surveys, that selection for Mla1 in Cambridge has been the predominant evolutionary force in CCV in Cambridge.     

Sources of powdery mildew resistance in barley landraces from morocco

A total of forty eight accessions of barley landraces from Morocco were screened for resistance to powdery mildew. Twenty two (46%) of tested landraces showed resistance reactions and thirty four single plant lines were selected. Eleven of these lines were tested in seedling stage with seventeen and another twenty three lines with twenty three isolates of powdery mildew respectively. The isolates were chosen according to the virulence spectra observed on the ‘Pallas’ isolines differential set. Line 229–2–2 was identified with resistance to all prevalent in Europe powdery mildew virulence genes. Lines 230–1–1, 248–1–3 showed susceptible reaction for only one and lines 221–3–2, 227–1–1, 244–3–4 for only two isolates respectively. Three different resistance alleles (Mlat, Mla6, and MLA14) were postulated to be present in tested lines alone or in combination. In thirty (88%) tested lines it was impossible to determine which specific gene or genes for resistance were present. Most probably these lines possessed alleles not represented in the ‘Pallas’ isolines differential set. The distribution of reaction type indicated that about 71% of all reaction types observed were classified as powdery mildew resistance (scores 0, 1 and 2). Majority (79%) of resistance reaction types observed in tested lines was intermediate resistance reaction type two and twenty three lines (68%) showed this reaction for inoculation with more than 50% isolates used. The use of new effective sources of resistance from Moroccan barley landraces for diversification of resistance genes for powdery mildew in barley cultivars was discussed.     

Virulence analysis of wheat powdery mildew population (Erysiphe graminis f.sp.tritici) from the region of Slovakia and Hungary

In the year 1992 a total of 163 isolates of wheat powdery mildew were tested. The samples of mildew isolates were obtained by means of a mobile spore catching apparatus. The populations from 4 regions of Slovakia and 3 regions of Hungary were analyzed. The resistance due toPm5, Pm8 andMl-i genes at the observed locations has already been overcome. The resistance genesPm1, Pm2 and a gene combinationPm2+Pm6 ensure the protection only against a part of the patho-types of powdery mildew population. Virulence corresponding to thePm4b gene has been low so far. The regional patterns of pathogen virulence are in good agreement with the gene resistance spectrum by the cultivars grown regionally. Little differences in virulence among the populations from the regions of Slovakia and Hungary indicate that this part of Eastern Europe should be considered as an epidemiologic unit.     

Powdery Mildew Resistance in Barley Landraces from Morocco

Nineteen barley landraces collected from Morocco were screened for resistance to powdery mildew. The landraces originated from the collection at the Polish Gene Bank, IHAR Radzików, Poland. The fifteen landraces tested showed powdery mildew resistance reactions and 35 single plant lines were selected. Twenty-one of these lines were tested in the seedling stage with 30, four lines with 17 and another 10 lines with 23 differential isolates of powdery mildew, respectively. The isolates were chosen according to their virulence spectra observed on the Pallas isolines differential set. Nine lines (E 1029-1-1, E 1042-2-2, E 1050-1-1, E 1054-5-1, E 1056-2-5, E 1056-3-1, E 1061-1-1, E 1061-1-3 and E 1067-1-2) which originated from seven landraces showed resistance to all prevalent European powdery mildew virulence genes. The most frequent score was 2 and 16 lines showed this reaction for inoculation with most isolates used. The distribution of reaction type indicated that about 77% of all reaction types observed were classified as powdery mildew resistance (scores 0, 1 and 2). In all lines the presence of unknown genes alone or in combinations with specific ones was postulated. Four different resistance alleles ( Mlat , Mla6 , Mla14 and Mla12 ) were postulated to be present in 10 tested lines alone or in combination. Alleles Mlat , Mla6 and Mla14 were postulated to be present in four and Mla12 in two tested lines, respectively. The value of barley landraces for diversification of resistance genes for powdery mildew is discussed.     

Virulence Gene Frequency Change in Erysiphe graminis f. sp. hordei Due to Selection by Non-Corresponding Barley Mildew Resistance Genes and Hitchhiking

Individual isolates (in total 4040) of Erysiphe graminis f. sp. hordei were collected between 1989 and 1991 in fields of spring barley varieties with resistance genes Mla9, Mla12 and Ml(La) in the local air spora, i.e. remote from barley fields, in Hesse, Germany. Their virulence pattern was determined on a 13-partite differential set. In the air spora. virulence complexity (the number of virulence genes per isolate) increased by 30% (4.2 vs. 5.5) between 1989 and 1991. This was mainly due to an increase of pathotypes with virulence genes Va7, Val3, Vk and V(La). In each mildew field population, frequencies of several non-corresponding virulence genes, e.g. V(La) on Mla9 variety, greatly increased during the period. Such change can be caused by hitchhiking selection, i.e. indirect selection resulting from asexual reproduction, and direct selection by non-corresponding resistance genes. To separate the two effects, the aerial barley mildew population of 1991 was sampled on cv. Pallas and near-isogenic lines of Pallas. Frequencies of non-corresponding virulence genes as observed in samples from the near-isogenic lines were compared to expected non-corresponding virulence frequencies which were obtained by subdividing the sample on Pallas into 12 subsamples, comprising all isolates virulent to line 1 through 12, respectively. Among 90 resistance/virulence gene combinations selection for non-cor-responding virulence was significant in five cases (e.g. Va9 on Mla7 host) and against non-corresponding virulence in nine cases (e.g. Vk on Ml(La) host). Hitchhiking selection was significant in eight cases. In 11 cases the two types of selection were significant in combination but not individually. It is suggested to monitor the stability of selection against non-corresponding virulence genes in race surveys and utilize it in diversified barley crops such as variety mixtures in order to retard the evolution of complex races.     

Effect of three suppressors on the expression of powdery mildew resistance genes in barley

Three recessive mutagen-induced alleles that partially suppress the phenotypic expression of the semidominant powdery mildew resistance gene Mla12 have been studied. When each suppressor is present in homozygous condition, the infection type 0, conferred by gene Mla12 when homozygous, is changed to intermediate infection types. The three suppressor lines were crossed with seven near-isogenic lines with different powdery mildew resistance genes and one, M100, was crossed with nine additional lines. Seedlings of parents and from the F1and F2 generations were tested with powdery mildew isolates that possessed the appropriate avirulence and virulence genes. The segregation of phenotypes in the F2 generation disclosed that the three suppressors affected the phenotypic expression of three resistance genes, whereas that of four resistance genes remained unaffected. The suppressor in mutant M100 affected the phenotypic expression of 9 of the 10 additional resistance genes present. It is suggested that the three suppressors are mutationally modified genes involved in host defence processes. This implies that different resistance genes employ different, but overlapping, spectra of defence processes, or signal transduction pathways. Key words : barley, Hordeum vulgare, powdery mildew, Erysiphe graminis hordei, mutation, resistance, suppressor.     

Characterisation of powdery mildew resistance in a segregating diploid rose population

Powdery mildew (Podosphaera pannoso) is one of the most serious fungal diseases on both greenhouse and field grown roses. Improvement of disease resistance is a major selection aim for garden rose breeders. For rose cultivars, being mostly tetraptoid, it is complicated to develop molecular markers for resistance. Hence, a segregating diploid population was established from a cross between 'Yesterday', a commercial available rose variety susceptible to powdery mildew, and R. wichurana, a rose species with resistance to certain isolates of powdery mildew. A progeny of 94 seedlings was planted in the field. The segregation of powdery mildew resistance was studied in this population by means of a bioassay with two different monoconidial isolates of powdery mildew. Based on the response to these inoculations different groups were selected: a first group of genotypes was susceptible to both isolates, other groups were susceptible to one of both isolates and a last group was resistant to both tested isolates. The disease resistance inherits for both isolates in a quantitative way. A genetic map based on AFLP and SSR markers was established and will be used for QTL analysis of powdery mildew resistance.     

Genetic analysis of the obligate parasitic barley powdery mildew fungus based on RFLP and virulence loci

Summary Genome organization of the biotrophic barley powdery mildew fungus was studied using restriction fragment length polymorphism (RFLP). Genomic DNA clones containing either low-or multiple-copy sequences appeared to be the best RFLP markers, as they frequently revealed polymorphisms that could be readily detected. A total of 31 loci were identified using 11 genomic DNA clones as probes. Linkage analysis of the 31 RFLP loci and five virulence loci resulted in the construction of seven groups of linked loci. Two of these contained both RFLP markers and virulence genes. RFLP markers were found to be very efficient in characterizing mildew isolates, as only three markers were necessary to differentiate 28 isolates. The DNA of the barley powdery mildew fungus appeared to contain a considerable number of repetitive sequences dispersed throughout the genome.     

Molecular marker analyses of powdery mildew resistance in barley

Powdery mildew, caused byEryisphe graminis f. sp.hordei, is one of the most important diseases of barley (Hordeum vulgare). A number of loci conditioning resistance to this disease have been reported previously. The objective of this study was to use molecular markers to identify chromosomal regions containing genes for powdery mildew resistance and to estimate the resistance effect of each locus. A set of 28 F₁ hybrids and eight parental lines from a barley diallel study was inoculated with each of five isolates ofE. graminis. The parents were surveyed for restriction fragment length polymorphisms (RFLPs) at 84 marker loci that cover about 1100 cM of the barley genome. The RFLP genotypes of the F₁s were deduced from those of the parents. A total of 27 loci, distributed on six of the seven barley chromosomes, detected significant resistance effects to at least one of the five isolates. Almost all the chromosomal regions previously reported to carry genes for powdery mildew resistance were detected, plus the possible existence of 1 additional locus on chromosome 7. The analysis indicated that additive genetic effects are the most important component in conditioning powdery mildew resistance. However, there is also a considerable amount of dominance effects at most loci, and even overdominance is likely to be present at a number of loci. These results suggest that quantitative differences are likely to exist among alleles even at loci which are considered to carry major genes for resistance, and minor effects may be prevalent in cultivars that are not known to carry major genes for resistance.     

Mapping multiple disease resistance genes using a barley mapping population evaluated in Peru, Mexico, and the USA

We used a well-characterized barley mapping population (BCD 47 × Baronesse) to determine if barley stripe rust (BSR) resistance quantitative trait loci (QTL) mapped in Mexico and the USA were effective against a reported new race in Peru. Essentially the same resistance QTL were detected using data from each of the three environments, indicating that these resistance alleles are effective against the spectrum of naturally occurring races at these sites. In addition to the mapping population, we evaluated a germplasm array consisting of lines with different numbers of mapped BSR resistance alleles. A higher BSR disease severity on CI10587, which has a single qualitative resistance gene, in Peru versus Mexico suggests there are differences in pathogen virulence between the two locations. Confirmation of a new race in Peru will require characterization using a standard set of differentials, an experiment that is underway. The highest levels of resistance in Peru were observed when the qualitative resistance gene was pyramided with quantitative resistance alleles. We also used the mapping population to locate QTL conferring resistance to barley leaf rust and barley powdery mildew. For mildew, we identified resistance QTL under field conditions in Peru that are distinct from the Mla resistance that we mapped using specific isolates under controlled conditions. These results demonstrate the long-term utility of a reference mapping population and a well-characterized germplasm array for locating and validating genes conferring quantitative and qualitative resistance to multiple pathogens.     

Quantitative trait loci controlling barley powdery mildew and scald resistances in two different barley doubled haploid populations

Powdery mildew and scald can cause significant yield loss in barley. In order to identify new resistance genes for powdery mildew and scald in barley, two barley doubled haploid (DH) populations were screened for adult plant resistance in the field and glasshouse under natural infection. The mapping populations included 92 DH lines from the cross of TX9425 × Franklin and 177 DH lines from the cross of Yerong × Franklin. Two quantitative trait loci (QTL) for resistance to powdery mildew were identified in the TX9425 × Franklin population. These QTL were mapped to chromosomes 7H and 5H, respectively. The phenotypic variation explained by the two QTL detected in this population was 22 and 17%, respectively. Three significant QTL were identified from the Yerong × Franklin population for the resistance to powdery mildew; the major one, detected on the short arm of chromosome 1H, explained 66% of phenotypic variation. The major QTL for scald resistance, identified from two different populations which shared a common parent, Franklin, were mapped in the similar position on 3H. However, the Franklin allele provided resistance to one population but susceptibility to the other population. The Yerong allele on 3H showed much better resistance to scald than the Franklin allele, which has not been reported before. Using high-density maps for both populations, some markers which were very close to the resistance genes were identified. Transgression beyond the parents in disease resistances of the DH populations indicates that both small-effect QTLs and genetic background may also have significant contributions towards the resistance.     

Isozyme Variation and Genetic Distances of Erysiphe graminis DC. Formae Speciales

Abstract Isozymes of ten different enzymes and unspecific stained proteins were used as biochemical genetic markers to study genetic variation within and between E. graminis ff. sp. hordei, avenae, secalis and tritici. In addition, grainproteins of the corresponding host species were examined. In each forma specialis, one genotype proved to be predominant. 131 distinct isozyme and 93 protein bands were distinguishable in these genotypes. However, divergent banding patterns differed only in 8 bands from the predominant banding patterns found within the formae speciales avena, secalis and tritici. The genetic relationships between powdery mildew formae speciales and host species were computed by cluster analysis from similarity (F) and dissimilarity (D) coefficients and illustrated by phylogenetic trees. Marked correspondence was found between E. graminis ff. sp. secalis and tritici (F: 82–90%). Lower homologies were obtained from the comparison ofthese formae speciales respectively with E. graminis ff. sp. hordei (F: 28–34%) and avenae (F: 24–32%). All phylogenetic trees constructed revealed the same arrangement classification of the formae speciales with similar graduation. The comparison of the host species revealed the highest similarity between S. cereale and T. aestivum (F: 74%). Regression analysis confirmed significant correlation between the genetic relationships within host species and powdery mildew formae speciales (r²= 0.81).     

Identification and mapping of a new powdery mildew resistance gene on chromosome 6D of common wheat

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is one of the most serious wheat diseases. The rapid evolution of the pathogen's virulence, due to the heavy use of resistance genes, necessitates the expansion of resistance gene diversity. The common wheat line D57 is highly resistant to powdery mildew. A genetic analysis using an F(2) population derived from the cross of D57 with the susceptible cultivar Yangmai 158 and the derived F(2:3) lines indicated that D57 carries two dominant powdery mildew resistance genes. Based on mapping information of polymorphic markers identified by bulk segregant analysis, these two genes were assigned to chromosomes 5DS and 6DS. Using the F(2:3) lines that segregated in a single-gene mode, closely linked PCR-based markers were identified for both genes, and their chromosome assignments were confirmed through linkage mapping. The gene on chromosome 5DS was flanked by Xgwm205 and Xmag6176, with a genetic distance of 8.3 cM and 2.8 cM, respectively. This gene was 3.3 cM from a locus mapped by the STS marker MAG6137, converted from the RFLP marker BCD1871, which was 3.5 cM from Pm2. An evaluation with 15 pathogen isolates indicated that this gene and Pm2 were similar in their resistance spectra. The gene on chromosome 6DS was flanked by co-segregating Xcfd80 and Xmag6139 on one side and Xmag6140 on the other, with a genetic distance of 0.7 cM and 2.7 cM, respectively. This is the first powdery mildew resistance gene identified on chromosome 6DS, and plants that carried this gene were highly resistant to all of the 15 tested pathogen isolates. This gene was designated Pm45. The new resistance gene in D57 could easily be transferred to elite cultivars due to its common wheat origin and the availability of closely linked molecular markers.     

Postulation of resistance genes to barley diseases in heterogeneous varieties

Resistance to causal agents of diseases is an important varietal characteristic that influences the management practice of crop plants and thus production costs of commodities. At present, almost all European barley varieties possess at least one major gene for resistance to powdery mildew. After hybridizing selected parental varieties, resistance genes often segregate in subsequent generations and, therefore, some varieties comprise lines that differ in the number or combinations of resistance genes. The objective of this research was to describe the various methods available for postulating resistance genes to pathogens in heterogeneous varieties using resistance to powdery mildew of barley as an example. Four spring barleys (‘Orbit’, ‘Malva’, ‘Tocada’ and CLE 233), and a six-row variety of winter barley, F 12872, were screened. For postulating resistance genes, several testing procedures and many Blumeria graminis f.sp. hordei isolates were used. Minimum amounts of seed were determined and different methods of obtaining homogeneous seed samples from heterogeneous varieties were compared. It was found that ‘Orbit’ and ‘Malva’ are composed of three and ‘Tocada’, CLE 233 and F 12872 of two lines with different resistances to powdery mildew. Problems of postulating resistance genes in heterogeneous varieties and the advantages of testing leaf segments instead of young plants are discussed.     

Screening for Spontaneous Virulent Mutants of Erysiphe graminis D.C. f.sp. hordei on Barley lines with Resistance Genes Ml-a1, Ml-a6, Ml-a12 and Ml-g

Seedlings of 4 barley lines with powdery mildew resistance genes Ml-a1, Ml-a6 Ml-a12 and Ml-g were inoculated with powdery mildew culture CR3 which is avirulent to the 4 host lines. The inoculation density was 1.2 infectious conidia per mm², and in total 50 million conidia were screened for the occurrence of virulent mutans. During 30 cycles of screening, 43 putative virulent mutants were selected, multiplied and tested. They could be grouped in 5 different genotypes according to virulence spectrum. Based on the virulence spectre, mating type, biochemical tests and analyses of test crosses, 3 of the types were rejected as being of mutational origin, and the verification of the remaining 2 were not consistent with the expectations deduced from a gene-for-gene interaction. Provided that none of the genotypes found were of mutational origin, the spontaneous mutation frequency from avirulence to virulence in barley powdery mildew is therefore below 2 × 10^–8. A reconstructation experiment showed that the density of avirulent inoculum did not reduce the survival rate of rate virulent genotypes     

The transfer of a powdery mildew resistance gene from Hordeum bulbosum L to barley (H. vulgare L.) chromosome 2 (2I)

Hordeum bulbosum L. is a source of disease resistance genes that would be worthwhile transferring to barley (H. vulgare L.). To achieve this objective, selfed seed from a tetraploid H. vulgare x H. bulbosum hybrid was irradiated. Subsequently, a powdery mildew-resistant selection of barley phenotype (81882/83) was identified among field-grown progeny. Using molecular analyses, we have established that the H. bulbosum DNA containing the powdery mildew resistance gene had been introgressed into 81882/83 and is located on chromosome 2 (2I). Resistant plants have been backcrossed to barley to remove the adverse effects of a linked factor conditioning triploid seed formation, but there remains an association between powdery mildew resistance and non-pathogenic necrotic leaf blotching. The dominant resistance gene is allelic to a gene transferred from H. bulbosum by co-workers in Germany, but non-allelic to all other known powdery mildew resistance genes in barley. We propose Mlhb as a gene symbol for this resistance.     

Localisation of genes for resistance against Blumeria graminis f.sp. hordei and Puccinia graminis in a cross between a barley cultivar and a wild barley (Hordeum vulgare ssp. spontaneum) line

The aims of this investigation have been to map new (quantitative) resistance genes against powdery mildew, caused by Blumeria graminis f.sp. hordei L., and leaf rust, caused by Puccinia hordei L., in a cross between the barley ( Hordeum vulgare ssp. vulgare) cultivar "Vada" and the wild barley ( Hordeum vulgare ssp. spontaneum) line "1B-87" originating from Israel. The population consisted of 121 recombinant inbred lines. Resistance against leaf rust and powdery mildew was tested on detached leaves. The leaf rust isolate "I-80" and the powdery mildew isolate "Va-4", respectively, were used for the infection in this experiment. Moreover, powdery mildew disease severity was observed in the field at two different epidemic stages. In addition to other DNA markers, the map included 13 RGA (resistance gene analog) loci. The structure of the data demanded a non-parametric QTL-analysis. For each of the four observations, two QTLs with very high significance were localised. QTLs for resistance against powdery mildew were detected on chromosome 1H, 2H, 3H, 4H and 7H. QTLs for resistance against leaf rust were localised on 2H and 6H. Only one QTL was common for two of the powdery mildew related traits. Three of the seven QTLs were localised at the positions of the RGA-loci. Three of the five powdery mildew related QTLs are sharing their chromosomal position with known qualitative resistance genes. All detected QTLs behaved additively. Possible sources of the distorted segregation observed, the differences between the results for the different powdery mildew related traits and the relation between qualitative and quantitative resistance are discussed.     

CAPS and DHPLC Analysis of a Single Nucleotide Polymorphism in the Cytochrome b Gene Conferring Resistance to Strobilurins in Field Isolates of Blumeria graminis f. sp. hordei

A single nucleotide polymorphism in the wheat powdery mildew (Blumeria graminis f. sp. tritici) cytochrome b gene is responsible for resistance to inhibitors of the quinol outer binding site of the cytochrome bc₁ complex (QoI) fungicides. Analysis of a partial sequence of the cytochrome b gene from field isolates resistant and sensitive to QoI fungicides revealed the same point mutation in barley powdery mildew (B. graminis f. sp. hordei). Analysis of 118 and 40 barley powdery mildew isolates using a cleaved amplified polymorphic sequence assay and denaturing high performance liquid chromatography, respectively, confirmed that this single nucleotide polymorphism also confers resistance to QoI fungicides in barley powdery mildew.     

一粒小麦抗白粉病和条锈病基因的分析

一粒小麦是普通小麦抗性改良的宝贵资源.本研究对24份一粒小麦分别进行了白粉病和条锈病混合菌种苗期接种鉴定,进一步分别用一套白粉病菌菌株(15个)对2份乌拉尔图小麦和条锈病菌小种(21个)对1份栽培一粒小麦进行接种鉴定,其中乌拉尔图小麦UR206能抵抗所有供试白粉菌菌株,UR204除对白粉菌菌株E11感病外,对其余菌株表现抗性;栽培一粒小麦MO205对不同条锈菌小种表现出不同的抗性反应,研究表明乌拉尔图小麦UR206、UR204和栽培一粒小麦MO205分别含有与已知抗白粉病和抗条锈病基因不同的新基因.对乌拉尔图小麦UR204、UR206和栽培一粒小麦MO205分别进行抗白粉和条锈病基因的遗传分析,结果表明乌拉尔图小麦UR204和UR206分别含有一对显性抗白粉病基因,栽培一粒小麦MO205含有两对独立遗传的显性抗条锈病基因.     

兼抗抗白粉病和黄矮病小麦育种材料的创造与鉴定

白粉病和黄矮病是小麦生产上的重要病害,近几年来这两种病害经常在我国一些小麦产区同时发生。为解决该问题,本研究通过杂交、回交方法将抗黄矮病的Bdv2基因（源自于YW642）和抗白粉病的Pm21基因（源自于CB037）聚合在一起,育成了兼抗黄矮病和白粉病的小麦新材料。通过田间抗病性鉴定与分子标记辅助选择相结合,得到聚合了Bdv2基因和Pm21基因的BC1代小麦22株,F2代小麦51株。农艺性状调查显示,这些含Pm21和Bdv2基因的双抗白粉病和黄矮病小麦新材料的农艺性状优于感病植株和原先的亲本,可以在小麦白粉病和黄矮病兼性抗病育种中作为优异种质资源加以利用。