Molecular mapping of <Emphasis Type="Italic">Rym17</Emphasis>, a dominant and <Emphasis Type="Italic">rym18</Emphasis> a recessive barley yellow mosaic virus (BaYMV) resistance genes derived from <Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.

PK23-2, a line of six-rowed barley (Hordeum vulgare L.) originating from Pakistan, has resistance to Japanese strains I and III of the barley yellow mosaic virus (BaYMV). To identify the source of resistance in this line, reciprocal crosses were made between the susceptible cultivar Daisen-gold and PK23-2. Genetic analyses in the F₁ generation, F₂ generation, and a doubled haploid population (DH45) derived from the F₁ revealed that PK23-2 harbors one dominant and one recessive resistance genes. A linkage map was constructed using 61 lines of DH45 and 127 DNA markers; this map covered 1268.8 cM in 10 linkage groups. One QTL having a LOD score of 4.07 and explaining 26.8% of the phenotypic variance explained (PVE) for resistance to BaYMV was detected at DNA marker ABG070 on chromosome 3H. Another QTL having a LOD score of 3.53 and PVE of 27.2% was located at marker Bmag0490 on chromosome 4H. The resistance gene on chromosome 3H, here named Rym17, showed dominant inheritance, whereas the gene on chromosome 4H, here named rym18, showed recessive inheritance in F₁ populations derived from crosses between several resistant lines of DH45 and Daisen-gold. The BaYMV recessive resistance genes rym1, rym3, and rym5, found in Japanese barley germplasm, were not allelic to rym18. These results revealed that PK23-2 harbors two previously unidentified resistance genes, Rym17 on 3H and rym18 on 4H; Rym17 is the first dominant BaYMV resistance gene to be identified in primary gene pool. These new genes, particularly dominant Rym17, represent a potentially valuable genetic resource against BaYMV disease.     

Strategies for Pyramiding Resistance Genes Against the Barley Yellow Mosaic Virus Complex (BaMMV,BaYMV, BaYMV-2)

Barley Yellow Mosaic Virus disease caused by different strains of BaYMV and BaMMV is a major threat to winter barley cultivation in Europe. Pyramiding of resistance genes may be considered as a promising strategy to avoid the selection of new virus strains and to create more durable resistances. However, this goal cannot be achieved by phenotypic selection due to the lack of differentiating virus strains. For pyramiding of resistance genes rym4, rym5, rym9 and rym11, located on chromosomes 3H and 4H of barley two different strategies have been developed. These strategies are based on doubled haploid lines (DHs) and marker assisted selection procedures. On the one hand F₁ derived DH-plants of single crosses were screened by molecular markers for genotypes being homozygous recessive for both resistance genes. These genotypes were crossed to lines carrying one resistance gene in common and an additional third gene, leading to a DH-population of which 25% carry three resistance genes, 50% have two resistance genes and 25% possess a single resistance gene homozygous recessively. Alternatively, F₁ plants having one resistance gene in common were directly inter-crossed [e.g. (rym4 × rym9) × (rym4 × rym11)] and about 100 seeds were produced per combination. Within these complex cross progenies plants were identified by markers being homozygous at the common resistance locus and heterozygous at the others. From such plants, theoretically present at a frequency of 6.25%, DH-lines were produced, which were screened for the presence of genotypes carrying three or two recessive resistance genes in a homozygous state. Besides DH-plants carrying all possible two-gene combinations, 20 DH-plants out of 107 analysed carrying rym4, rym9, and rym11 and 27 out of 187 tested carrying rym5, rym9, and rym11 homozygously have been detected using the second strategy which is faster but needs co-dominant markers, because in contrast to the first strategy marker selection is carried out on heterozygous genotypes.     

Genomics-based high-resolution mapping of the BaMMV/BaYMV resistance gene rym11 in barley (Hordeum vulgare L.)

Soil-borne barley yellow mosaic virus disease, caused by different strains of Barley yellow mosaic virus (BaYMV) and Barley mild mosaic virus (BaMMV), is one of the most important diseases of winter barley (Hordeum vulgare L.) in Europe and East Asia. The recessive resistance gene rym11 located in the centromeric region of chromosome 4HL is effective against all so far known strains of BaMMV and BaYMV in Germany. In order to isolate this gene, a high-resolution mapping population (10,204 meiotic events) has been constructed. F₂ plants were screened with co-dominant flanking markers and segmental recombinant inbred lines (RILs) were tested for resistance to BaMMV under growth chamber and field conditions. Tightly linked markers were developed by exploiting (1) publicly available barley EST sequences, (2) employing barley synteny to rice, Brachypodium distachyon and sorghum and (3) using next-generation sequencing data of barley. Using this approach, the genetic interval was efficiently narrowed down from the initial 10.72 % recombination to 0.074 % recombination. A marker co-segregating with rym11 was developed providing the basis for gene isolation and efficient marker-assisted selection.     

High-resolution mapping of the Rym4/Rym5 locus conferring resistance to the barley yellow mosaic virus complex (BaMMV, BaYMV, BaYMV-2) in barley (Hordeum vulgare ssp. vulgare L.)

Soil-borne barley yellow mosaic virus disease – caused by a complex of at least three viruses, i.e. Barley mild mosaic virus (BaMMV), Barley yellow mosaic virus (BaYMV) and BaYMV-2 – is one of the most important diseases of winter barley in Europe. The two genes rym4, effective against BaMMV and BaYMV, and rym5, additionally effective against BaYMV-2, comprise a complex locus on chromosome 3HL, which is of special importance to European barley breeding. To provide the genetic basis for positional cloning of the Rym4/Rym5 locus, two high-resolution maps were constructed based on co-dominant flanking markers (MWG838/Y57c10 - MWG010/Bmac29). Mapping at a resolution of about 0.05% rec., rym4 has been located 1.07% recombination distal of marker MWG838 and 1.21% recombination proximal to marker MWG010. Based on a population size of 3,884 F₂ plants (0.013% recombination) the interval harbouring rym5 was delimited to 1.49±0.14% recombination. By testing segmental recombinant inbred lines (RILs) for reaction to the different viruses at a resolution of 0.05% rec. (rym4) and 0.019% rec. (rym5), no segregation concerning the reaction to the different viruses could be observed. AFLP-based marker saturation for rym4, using 932 PstI+2/MseI+3 primer combinations only resulted in three markers with the closest one linked at 0.9% recombination to the gene. Two of these markers detected epialleles arising from the differential cytosine methylation of PstI sites. Regarding rym5, profiling of 1,200 RAPD primers (about 18,000 loci) and 2,048 EcoRI+3/MseI+3 AFLP primer combinations (about 205,000 loci) resulted in one RAPD marker and seven AFLP markers tightly linked to the resistance gene. Flanking markers with the closest linkage to rym5 (0.05% and 0.88% recombination) were converted into STS markers. These markers provide a starting point for chromosomal walking and may be exploited in marker-assisted selection for virus resistance based on rym5.     

Identification and mapping of a new leaf stripe resistance gene in barley (Hordeum vulgare L.)

Pyrenophora graminea is the seed-borne pathogen causal agent of barley leaf stripe disease. Near-isogenic lines (NILs) carrying resistance of the cv ”Thibaut” against the highly virulent isolate Dg2 were obtained by introgressing the resistance into the genetic background of the susceptible cv ”Mirco”. The segregation of the resistance gene was followed in a F₂ population of 128 plants as well as on the F₃ lines derived from the F₂ plants; the segregation fitted the 1:2:1 ratio for a single gene. By using NILs, a RAPD marker associated with the resistance gene was identified; sequence-specific (STS) primers were designed on the basis of the amplicon sequence and a RILs mapping population with an AFLP-based map were used to position this molecular marker to barley chromosome 1 S (7HS). STS and CAPS markers were developed from RFLPs mapped to the telomeric region of barley chromosome 7HS and three polymorphic PCR-based markers were developed. The segregation of these markers was followed in the F₂ population and their map position with respect to the resistance gene was determined. Our results indicate that the Thibaut resistance gene, which we designated as Rdg2a, maps to the telomeric region of barley chromosome 7HS and is flanked by the markers OPQ-9₇₀₀ and MWG 2018 at distances of 3.1 and 2.5 cM respectively. The suitability of the PCR-based marker MWG2018 in selection- assisted barley breeding programs is discussed. Received: 22 June 2000 / Accepted: 16 October 2000     

Effects of barley yellow mosaic disease resistant gene rym1 on the infection by strains of Barley yellow mosaic virus and Barley mild mosaic virus

Although a Chinese landrace of barley, Mokusekko 3, is completely resistant to all strains of Barley yellow mosaic virus (BaYMV) and Barley mild mosaic virus (BaMMV), and is known to have at least two resistant genes, rym1 and rym5, only rym5 has been utilized for BaYMV resistant barley breeding in Japan. In order to clarify the effect of rym1 on BaYMV and BaMMV, and to utilize the gene for resistant barley breeding, the susceptibilities of only rym1 carrying breeding lines against BaYMV and BaMMV were investigated. In the assessment of resistance to BaYMV-I, 341 F(2) populations derived from a cross between the resistant line Y4 with only rym1 and the susceptible cv Haruna Nijo shows that the segregation loosely fits a 1R:3S ratio (0.05 > P > 0.01), suggesting that the resistance is controlled by a single recessive gene, rym1. Further, none of the F(3) lines derived from the nine resistant F(2) plants showed any disease symptoms in the field infected by BaYMV-I. The same nine F(3) lines showed almost the same agronomic characters in the field infected by BaYMV-III as those in the uninfected field, apart from the symptom of showing numerous mosaics. This result indicates that the gene rym1 has an acceptable level of resistance to BaYMV-III. In the assessment of resistance to BaYMV-II, BaMMV-Ka1 and -Na1, an artificial infection method was adopted and the susceptibilities to those viruses were investigated. Although the control varieties, Ko A and Haruna Nijo, were infected with all of them, the rym1 gene carrying BC(2)F(3) lines were completely resistant to all strains. In summary, rym1 is completely resistant to BaYMV-I, -II, BaMMV-Ka1 and -Na1, and has an acceptable level of resistance to BaYMV-III. This study concludes with a discussion of the reason why the important resistance gene rym1 was eliminated along with resistant cultivars during breeding for resistance to BaYMV.     

Fine-mapping of the BaMMV, BaYMV-1 and BaYMV-2 resistance of barley (Hordeum vulgare) accession PI1963

Barley yellow mosaic disease caused by the bymoviruses barley mild mosaic virus (BaMMV) and barley yellow mosaic virus (BaYMV) is one of the economically most important diseases of winter barley in Europe. In European barley breeding programmes, resistance is currently due to only two genes—rym4, which is effective against viruses BaMMV and BaYMV-1, and rym5, which is effective against BaYMV-2. Diversification of resistance is therefore an important task. Because the accession PI1963 confers immunity against all European strains of barley yellow mosaic disease and is not allelic to rym5, we have attempted to develop closely linked markers in order to facilitate the efficient introgression of this resistance into adapted germplasm. By means of restriction fragment length polymorphism analysis, we located a gene locus for resistance to BaMMV, BaYMV-1 and BaYMV-2 of PI1963 on chromosome 4HL using a mapping population (W757) comprising 57 doubled haploid (DH) lines. Subsequent tests for allelism indicated that the BaMMV resistance gene in PI1963 is allelic to rym11. Two DH populations, IPK1 and IPK2, comprising 191 and 161 DH lines, respectively, were derived from the initial mapping population W757 and used for further analysis. As random amplified polymorphic DNA development did not facilitate the identification of more closely linked markers, simple sequence repeat (SSR) analyses were conducted. For population IPK1, the closest SSRs detected were Bmac181 and Bmag353, which flank the gene at 2.1 cM and 2.7 cM, respectively. For the IPK2 population, the SSR markers HVM3 and Bmag353 are located proximally at 2.5 cM and distally at 8.2 cM, respectively. In order to develop markers more tightly linked to rym11, a targeted amplified fragment length polymorphism (AFLP) marker identification approach was adopted using bulks comprising lines carrying recombination events proximal and distal to the target interval. Using this approach we identified six AFLP markers closely linked to rym11, with the two markers, E56M32 and E49M33, co-segregating with rym11 in both populations. The SSRs and AFLPs identified in this study represent useful tools for marker-assisted selection.     

Dissection of resistance to soil-borne yellow-mosaic-inducing viruses of barley (BaMMV,BaYMV, BaYMV-2) in a complex breeders' cross by means of SSRs and simultaneous mapping of BaYMV/BaYMV-2 resistance of var. 'Chikurin Ibaraki 1'

Ninety-three F(1)-derived doubled haploid (DH) lines from a complex breeders' cross involving the Japanese genotype 'Chikurin Ibaraki 1', which is resistant to barley mild mosaic virus (BaMMV) and two strains of barley yellow mosaic virus (BaYMV and BaYMV-2), three susceptible varieties ('Hamu', 'Julia' and a breeding line) and cv. 'Carola', which carries rym4 conferring resistance to BaMMV and BaYMV, were analysed for resistance to BaMMV, BaYMV and BaYMV-2. The DH lines fell into four phenotypic classes. In addition to completely resistant and susceptible genotypes, DHs were observed which were either resistant to BaMMV and BaYMV or to BaYMV and BaYMV-2. For BaMMV and BaYMV-2 resistance, segregation ratios approaching 1r:1s were observed, suggesting the presence of single resistance genes. In contrast, the segregation ratio for BaYMV fits a 3r:1s segregation ratio, suggesting the presence of two independently inherited genes. From the genetic analysis, we conclude that a resistance locus effective against BaYMV and BaYMV-2 originates from Chikurin Ibaraki 1 and segregates independently from the Carola-derived rym4 resistance that is effective against BaYMV and BaMMV. The BaMMV resistance in Chikurin Ibaraki 1 has probably been lost during population development. This hypothesis was tested using a simple-sequence repeat (SSR) marker (Bmac29) linked to rym4. All BaMMV-resistant DH lines supported amplification of the rym4-resistance diagnostic allele. To identify the genetic location of the Chikurin Ibaraki 1-derived resistance against BaYMV/BaYMV-2, bulked DNA samples were constructed from the four resistance classes, and bulked segregant analysis was performed using a genome-wide collection of SSRs. Differentiating alleles were observed at two linked SSRs on chromosome 5H. The location of this BaYMV/BaYMV-2 resistance locus was confirmed and further resolved by linkage analysis on the whole population using a total of five linked SSRs.     

Analysis of bymovirus resistance genes on proximal barley chromosome 4HL provides the basis for precision breeding for BaMMV/BaYMV resistance

Key message

Unlocking allelic diversity of the bymovirus resistance gene rym11 located on proximal barley chromosome 4HL and diagnostic markers provides the basis for precision breeding for BaMMV/BaYMV resistance.

Abstract

The recessive resistance gene rym11 on barley chromosome 4HL confers broad-spectrum and complete resistance to all virulent European isolates of Barley mild mosaic virus and Barley yellow mosaic virus (BaMMV/BaYMV). As previously reported, rym11-based resistance is conferred by a series of alleles of naturally occurring deletions in the gene HvPDIL5-1, encoding a protein disulfide isomerase-like protein. Here, a novel resistance-conferring allele of rym11 is reported that, in contrast to previously identified resistance-conferring variants of the gene HvPDIL5-1, carries a single non-synonymous amino acid substitution. Allelism was confirmed by crossing to genotypes carrying previously known rym11 alleles. Crossing rym11 genotypes with a cultivar carrying the recessive resistance gene rym1, which was reported to reside on the same chromosome arm 4HL like rym11, revealed allelism of both loci. This allelic state was confirmed by re-sequencing HvPDIL5-1 in the rym1 genotype, detecting the haplotype of the rym11-d allele. Diagnostic PCR-based markers were established to differentiate all seven resistance-conferring alleles of the rym11 locus providing precise tools for marker-assisted selection (MAS) of rym11 in barley breeding.     

Genetic analyses of BaMMV/BaYMV resistance in barley accession HOR4224 result in the identification of an allele of the translation initiation factor 4e (Hv-eIF4E) exclusively effective against Barley mild mosaic virus (BaMMV)

Key message

Based on a strategy combining extensive segregation analyses and tests for allelism with allele-specific re-sequencing an Hv-eIF4E allele exclusively effective against BaMMV was identified and closely linked markers for BaYMV resistance were developed.

Abstract

Soil-borne barley yellow mosaic disease is one of the most important diseases of winter barley. In extensive screenings for resistance, accession ‘HOR4224’ being resistant to three strains of Barley mild mosaic virus (BaMMV-ASL1, BaMMV-Sil, and BaMMV-Teik) and two strains of Barley yellow mosaic virus (BaYMV-1 and BaYMV-2) was identified. Analyses using Bmac29, being to some extent diagnostic for the rym4/5 locus, gave hint to the presence of the susceptibility-encoding allele at this locus. Therefore, 107 DH lines derived from the cross ‘HOR4224’ × ‘HOR10714’ (susceptible) were screened for resistance. Genetic analyses revealed an independent inheritance of resistance to BaMMV and BaYMV ( $\chi_{1:1:1:1}^{2}$  = 5.58) both encoded by a single gene (BaMMV $\chi_{1:1}^{2}$  = 0.477; BaYMV $\chi_{1:1}^{2}$  = 0.770). Although Bmac29 indicated the susceptibility-encoding allele, BaMMV resistance of ‘HOR4224’ co-localized with rym4/rym5. The BaYMV resistance was mapped to chromosome 5H in the region of rym3. Sequencing of full length cDNA of the Hv-eIF4E gene displayed an already sequenced allele described to be efficient against BaMMV and BaYMV. However, the F₁ progenies of crosses involving ‘HOR4224’ and rym4/rym5 donors were all resistant to BaMMV but susceptible to BaYMV. Therefore, this is the first report of an allele at the rym4/rym5 locus exclusively efficient against BaMMV. Changes in the specificity are due to one non-synonymous amino acid substitution (I118K). Results obtained elucidate that combining extensive segregation analyses and tests for allelism involving different strains of BaMMV/BaYMV in combination with allele-specific re-sequencing is an efficient strategy for gene and allele detection in complex pathosystems.     

<Emphasis Type="Italic">rym15</Emphasis> from the Japanese cultivar Chikurin Ibaraki 1 is a new barley mild mosaic virus (BaMMV) resistance gene mapped on chromosome 6H

Breeding for resistant cultivars is the only way to prevent high yield loss in barley caused by the soil-borne barley mild mosaic virus (BaMMV) complex. We have characterized the BaMMV resistance of barley cv. Chikurin Ibaraki 1. Doubled haploid lines were obtained from the F₁ between the susceptible six-rowed winter barley cultivar, Plaisant, and Chikurin Ibaraki 1. Each line was tested for reaction to BaMMV by mechanical inoculation followed by DAS-ELISA. Of 44 microsatellites that covered the genome, 22 polymorphic markers were tested on one susceptible and one resistant bulk, each comprising 30 lines. Differential markers and additional microsatellite markers in the same region were then tested on the whole population. A bootstrap analysis was used to compute confidence intervals of distances and to test the orders of the resistance gene and the closest markers. A segregation of 84 resistant/98 susceptible lines fitted a 1:1 ratio (²=1.08, P=0.30), which corresponds to a single gene in this DH lines population. The resistance gene was flanked by two markers near the centromeric region of chromosome 6HS—Bmag0173, at 0.6±1.2 cM, and EBmac0874, at 5.8 ± 3.4 cM. We propose to name this new resistance gene rym15. This resistance gene and associated markers will increase the possibilities to breed efficiently for new cultivars resistant to the barley mosaic disease.Communicated by P. Langridge     

Identification and mapping of two brown planthopper resistance genes in rice

The brown planthopper (BPH) is one of the most serious insect pests of rice. In this study, we conducted a molecular marker-based genetic analysis of the BPH resistance of ’B5’, a highly resistant line that derived its resistant genes from the wild rice Oryza officinalis. Insect resistance was evaluated using 250 F₃ families from a cross between ’B5’ and ’Minghui 63’, based on which the resistance of each F₂ plant was inferred. Two bulks were made by mixing, respectively, DNA samples from highly resistant plants and highly susceptible plants selected from the F₂ population. The bulks were surveyed for restriction fragment length polymorphism using probes representing all 12 chromosomes at regular intervals. The survey revealed two genomic regions on chromosome 3 and chromosome 4 respectively that contained genes for BPH resistance. The existence of the two loci were further assessed by QTL (quantitative trait locus) analysis, which resolved these two loci to a 14.3-cM interval on chromosome 3 and a 0.4-cM interval on chromosome 4. Comparison of the chromosomal locations and reactions to BPH biotypes indicated that these two genes are different from at least nine of the ten previously identified BPH resistance genes. Both of the genes had large effects on BPH resistance and the two loci acted essentially independent of each other in determining t he resistance. These two genes may be a useful BPH resistance resource for rice breeding programs. Received: 6 March 2000 / Accepted: 28 July 2000     

Localization of the rice stripe disease resistance gene, Stv-bi, by graphical genotyping and linkage analyses with molecular markers

 We used graphical genotyping and linkage analyses with molecular markers to determine the chromosomal location of the rice stripe disease resistance gene, Stv-b ⁱ . The stripe resistance gene from the indica rice (Oryza sativa) cv ‘Modan’ was introgressed into several Japanese rice varieties. We found 4 RFLP markers in ‘Modan’, five susceptible parental rice varieties (‘Norin No. 8’, ‘Sachihikari’, ‘Kanto No. 98’, ‘Hokuriku No.103’ and ‘Koganebare’) and four resistant progeny varieties (‘St. No. 1’, ‘Aichi No. 6’, ‘Aoisora’ and ‘Asanohikari’). Graphical genotyping of the resistant progeny revealed a chromosomal segment ascribable to ‘Modan’ and associated with stripe resistance. The chromosomal segment from ‘Modan’ was located at 35.85 cM on chromosome 11. Linkage analysis using 120 F₂ individuals from a cross between ‘Koshihikari’ (susceptible) and ‘Asanohikari’ (resistant) revealed another 8 RFLP markers in the same chromosome. We performed a bioassay for rice stripe resistance in F₃ lines of the F₂ individuals using infective small brown planthoppers and identified an 1.8-cM segment harboring the rice stripe disease resistance gene, Stv-b ⁱ , between XNpb220 and XNpb257/ XNpb254. Furthermore, Stv-b ⁱ was linked by 0.0 cM to a RFLP marker, ST10, which was developed on the basis of the results of RAPD analysis. These DNA markers near the Stv-b ⁱ locus may be useful in marker-assisted selection and map-based cloning of the Stv-b ⁱ gene. Received: 26 September 1997 / Accepted: 4 November 1997     

Molecular mapping of the blast resistance gene, Pi44(t), in a line derived from a durably resistant rice cultivar

 A recombinant inbred line derived from a cross between CO39 and ‘Moroberekan’, RIL276, was found to be resistant to lineage 44 isolates of Pyricularia grisea in the Philippines. One hundred F₂ individuals were obtained from a backcross of RIL276 and CO39. Phenotypic analysis showed that RIL276 carries a single locus, tentatively named Pi44(t), conferring complete resistance to lineage 44 isolates of P. grisea. RFLP probes, STS primers and AFLP markers were applied to identify DNA markers linked to Pi44(t). Neither RFLP nor STS-PCR analysis gave rise to DNA markers linked to the locus. Using bulk segregant AFLP analysis, however, two dominant AFLP markers (AF₃₄₈ and AF₃₄₉) linked to Pi44(t) were identified. AF₃₄₉ and AF₃₄₈ were located at 3.3±1.5 cM and 11±3.5 cM from Pi44(t), respectively. These markers were mapped on chromosome 11 using an F₂ population derived from a cross between ‘Labelle’ and ‘Black Gora’. The location of AF₃₄₈ on chromosome 11 was confirmed using another F₂ mapping population derived from IR40931-26-3-3-5/ PI543851. DNA products at the loci linked to Pi44(t) were amplified from RIL276, ‘Labelle’ and PI543851 using the same primer pairs used to amplify AF₃₄₉ and AF₃₄₈. Sequence analysis of these bands showed 100% identity between lines. This result indicates that these AFLP markers could be used for the comparison of maps or assignment of linkage groups to chromosomes. Received: 12 May 1998 / Accepted: 13 November 1998     

Molecular markers in breeding for virus resistance in barley

The soil-borne barley yellow mosaic virus disease (BaMMV, BaYMV, BaYMV-2) and the aphid-transmitted barley yellow dwarf virus (BYDV) are serious threats to winter barley cultivation. Resistance to barley yellow mosaic virus disease has been identified in extensive screening programmes and several recessive resistance genes have been mapped, e.g. rym4, rym5, rym9, rym11, rym13. In contrast to barley yellow mosaic virus disease, no complete resistance to BYDV is known in the barley gene pool, but tolerant accessions have been identified and QTL for BYDV-tolerance have been detected on chromosomes 2HL and 3HL. The use of resistance and tolerance in barley breeding can be considerably improved today by molecular markers (RFLPs, RAPDs, AFLPs, SSRs, STSs, SNPs), as they facilitate (i) efficient genotyping and estimation of genetic diversity; (ii) reliable selection on a single plant level independent of symptom expression in the field (iii) acceleration of back crossing procedures; (iv) pyramiding of resistance genes; (v) detection of QTL and marker-based combination of positive alleles; and (vi) isolation of resistance genes via map-based cloning.     

Mapping of quantitative trait loci affecting Drechslera teres resistance in barley with molecular markers

 Resistance loci for seedling-stage resistance to net blotch disease (Drechslera teres) in barley were mapped with molecular markers in an F₂ population derived from a cross between the susceptible barley cultivar ‘Arena’ and the resistant Ethiopian landrace ‘Hor 9088’. Disease reactions were scored with first and second leaves of 2-week-old plants 7 and 9 days after inoculation with a single spore-derived isolate. For linkage analysis, 22 RFLP markers and 284 AFLP markers were used. The seven linkage groups covered 1153.3 cM with an average marker interval of 3.76 cM. The resistance was determined to be inherited in a quantitative manner. Altogether, 12 QTLs were mapped with positions depending on the leaf used for testing and the time period after infection. Heritability in the broad sense ranged between 0.21 and 0.37. Received: 26 May 1998 / Accepted: 9 June 1998     

Linkage mapping of two mutations that reduce phytic acid content of barley grain

 This study describes the inheritance and linkage map positions of two low phytic acid barley (Hordeum vulgare) mutations, lpa1-1 and lpa2-1, that dramatically reduce grain phytic acid content and increase inorganic seed phosphorus (P). Wide-cross, F₂ mapping populations were constructed by mating six-rowed varieties, ‘Steptoe’ and/or ‘Morex’, with two-rowed ‘Harrington’lpa donor lines homozygous for either lpa1-1 or lpa2-1. The barley lpa1-1 mutation showed normal inheritance patterns, whereas a deficiency of homozygous lpa2-1/lpa2-1 F₂ plants was observed. We identified a codominant, STS-PCR marker (aMSU21) that cosegregated with lpa1-1 in a population of 41 F₂ plants. The aMSU21 marker was then mapped to a locus on barley chromosome 2H, using a North American Barley Genome Mapping Project (NABGMP) doubled haploid population (‘Harrington’בMorex’). We determined that lpa2-1 is located within a recombination interval of approximately 30 cM between two AFLP markers that were subsequently mapped to barley chromosome 7H by integration with the same NABGMP population. Recent comparative mapping studies indicate conserved genetic map orders of several homologous molecular marker loci in maize and the Triticeae species that also show corresponding linkage to the biochemically similar lpa2 mutations of maize and barley. This observation suggests that barley and maize lpa2 mutations may affect orthologous genes. No such evidence for correspondence of the phenotypically similar lpa1 mutations of barley and maize has been revealed. Received: 22 September 1997 / Accepted: 2 December 1997     

Quantitative trait loci for resistance to spot blotch caused by Bipolarissorokiniana in wheat (T.aestivum L.) lines ‘Ning 8201’ and ‘Chirya 3’

Spot blotch caused by Bipolaris sorokiniana is a destructive disease of wheat in warm and humid wheat growing regions of the world. To identify quantitative trait loci (QTLs) for spot blotch resistance, two mapping populations were developed by making the crosses between common susceptible cultivar ‘Sonalika’ with the resistant breeding lines ‘Ning 8201’ and ‘Chirya 3’. Single seed descent derived F₆, F₇, F₈ lines of the first cross ‘Ning 8201’ × ‘Sonalika’ were evaluated for resistance to spot blotch in three blocks in each of the 3 years. After screening of 388 pairs of simple sequence repeat primers between the two parents, 119 polymorphic markers were used to genotype the mapping population. Four QTLs were identified on the chromosomes 2AS, 2BS, 5BL and 7DS and explained 62.9% of phenotypic variation in a simultaneous fit. The QTL on chromosome 2A was detected only in 1 year and explained 22.7% of phenotypic variation. In the second cross (‘Chirya 3’ × ‘Sonalika’), F₇ and F₈ population were evaluated in three blocks in each of the 2 years. In this population, five QTLs were identified on chromosomes 2BS, 2DS, 3BS, 7BS and 7DS. The QTLs identified in the ‘Chirya 3’ × ‘Sonalika’ population explained 43.4% of phenotypic variation in a simultaneous fit. The alleles for reduced disease severity in both the populations were derived from the respective resistant parent. The QTLs QSb.bhu-2B and QSb.bhu-7D from both populations were placed in the same deletion bins, 2BS1-0.53-0.75 and 7DS5-0.36-0.61, respectively. The closely linked markers Xgwm148 to the QTL on chromosome 2B and Xgwm111 to the QTL on chromosome 7D are potentially diagnostic markers for spot blotch resistance.     

Molecular mapping of leaf rust resistance gene <Emphasis Type="Italic">Rph14</Emphasis> in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

An incompletely dominant gene conferring resistance to Puccinia hordei, Rph14, identified previously in an accession of Hordeum vulgare, confers resistance to all known pathotypes of P. hordei in Australia. Knowledge of the chromosomal location of Rph14 and the identification of DNA markers closely linked to it will facilitate combining it with other important leaf rust resistance genes to achieve long lasting resistance. The inheritance of Rph14 was confirmed using 146 and 106 F₃ lines derived from the crosses ‘Baudin’/‘PI 584760’ (Rph14) and ‘Ricardo’/‘PI 584760’ (Rph14), respectively. Bulk segregant analysis on DNA from the parental genotypes and resistant and susceptible DNA bulks using DArT markers located Rph14 to the short arm of chromosome 2H. DArT marker bPb-1664 was identified as having the closest genetic association with Rph14. PCR based marker analysis identified a single SSR marker, Bmag692, linked closely to Rph14 at a map distance of 2.1 and 3.8 cm in the ‘Baudin’/‘PI 584760’and ‘Ricardo’/‘PI 584760’ populations, respectively.     

QTL analysis of tolerance to a German strain of BYDV-PAV in barley (Hordeum vulgare L.)

One hundred and forty six barley doubled-haploid lines (DH lines) were tested for variation in grain yield, yield components, plant height, and heading date after artificial infection with a German isolate of barley yellow dwarf virus (BYDV-PAV-Braunschweig). Of these 146 lines 76 were derived from the cross of the barley yellow dwarf virus (BYDV) tolerant cultivar ’Post’ to cv ’Vixen’ (Ryd2) and 70 from the cross of Post to cv ’Nixe’. Phenotypic measurements were gathered on both non-infected plants and plants artificially inoculated with BYDV-PAV by viruliferous aphids in pot and field experiments for three years at two locations. For all traits a continuous variation was observed suggesting a quantitative mode of inheritance for tolerance against BYDV-PAV. Using skeleton maps constructed using SSRs, AFLPs and RAPDs, two QTLs for relative grain yield per plant after BYDV infection, explaining about 47% of the phenotypic variance, were identified in Post × Vixen at the telomeric region of chromosome 2HL and at a region containing the Ryd2 gene on chromosome 3HL. In Post × Nixe, a QTL was found in exactly the same chromosome 2HL marker interval. In this cross, additional QTL were mapped on chromosomes 7H and 4H and together these explained about 40% of the phenotypic variance. QTL for effects of BYDV infection on yield components, plant height, and heading date generally mapped to the same marker intervals, or in the vicinity of the QTL for relative grain yield, on chromosomes 2HL and 3HL, suggesting that these regions are of special importance for tolerance to the Braunschweig isolate of BYDV-PAV. Possible applications of marker-assisted selection for BYDV tolerance based on these results are discussed. Received: 1 December 2000 / Accepted: 9 March 2001