Russian Wheat Aphid Resistance Genes Research Articles

Fine Mapping of Russian Wheat Aphid Resistance Gene in Bread Wheat (Triticum aestivum L.)

Amaç: Bu çalışmanın amacı Rus buğday afidine dayanıklılık sağlayan Dn100695 geninin bulunduğu 7D kromozomunu SSR markörleriyle ayrıntılı bir şekilde taramak ve dayanıklılık genini çevreleyen yeni SSR markörlerini belirlemektir. 
 Materyal ve Yöntem: Basribey x IG 100695 melezinin 190 bireyinden oluşan F2 populasyonu haritalama amacıyla kullanılmıştır. Haritalama populasyonu daha önceki markörlere ilave olarak 7D kromozomunda bulunan 20 yeni SSR markörleri ile taranmıştır. Zararlının populasyondaki kloroz, kıvrılma ve afit yoğunluğu özellikleri fenotipik veriler olarak ele alınmıştır. 
 Araştırma Bulguları ve Sonuç: Çalışma sonucunda 85 polimorfik markör içerisinden 46’sı toplamda 1226.9 cM’lık bir genom bölgesini haritalandırmış ve 14 farklı bağlantı grubu oluşturmuştur. 7D kromozomu üzerinde dayanıklılık genini çevreleyen özgün bir SSR markörü (GWM121) belirlenmiş ve Dn100695 genini her iki yönden çevreleyen markörler haritalanmıştır. Ayrıca markör regresyon analizi sonucunda Rus buğday afidi dayanıklılığına katkı sağlama potansiyeli bulunan SSR markörleri de saptanmıştır.

Accessing a Russian Wheat Aphid Resistance Gene in Bread Wheat by Long-Read Technologies.

Russian wheat aphid (RWA) ( Kurdjumov) is a serious invasive pest of small-grain cereals and many grass species. An efficient strategy to defy aphid attacks is to identify sources of natural resistance and transfer resistance genes into susceptible crop cultivars. Revealing the genes helps understand plant defense mechanisms and engineer plants with durable resistance to the pest. To date, more than 15 RWA resistance genes have been identified in wheat ( L.) but none of them has been cloned. Previously, we genetically mapped the RWA resistance gene into an interval of 0.83 cM on the short arm of chromosome 7D and spanned it with five bacterial artificial chromosome (BAC) clones. Here, we used a targeted strategy combining traditional approaches toward gene cloning (genetic mapping and sequencing of BAC clones) with novel technologies, including optical mapping and long-read nanopore sequencing. The latter, with reads spanning the entire length of a BAC insert, enabled us to assemble the whole region, a task that was not achievable with short reads. Long-read optical mapping validated the DNA sequence in the interval and revealed a difference in the locus organization between resistant and susceptible genotypes. The complete and accurate sequence of the region facilitated the identification of new markers and precise annotation of the interval, revealing six high-confidence genes. Identification of as the most likely candidate opens an avenue for its validation through functional genomics approaches.

Chromosomal genomics facilitates fine mapping of a Russian wheat aphid resistance gene.

Making use of wheat chromosomal resources, we developed 11 gene-associated markers for the region of interest, which allowed reducing gene interval and spanning it by four BAC clones. Positional gene cloning and targeted marker development in bread wheat are hampered by high complexity and polyploidy of its nuclear genome. Aiming to clone a Russian wheat aphid resistance gene Dn2401 located on wheat chromosome arm 7DS, we have developed a strategy overcoming problems due to polyploidy and enabling efficient development of gene-associated markers from the region of interest. We employed information gathered by GenomeZipper, a synteny-based tool combining sequence data of rice, Brachypodium, sorghum and barley, and took advantage of a high-density linkage map of Aegilops tauschii. To ensure genome- and locus-specificity of markers, we made use of survey sequence assemblies of isolated wheat chromosomes 7A, 7B and 7D. Despite the low level of polymorphism of the wheat D subgenome, our approach allowed us to add in an efficient and cost-effective manner 11 new gene-associated markers in the Dn2401 region and narrow down the target interval to 0.83cM. Screening 7DS-specific BAC library with the flanking markers revealed a contig of four BAC clones that span the Dn2401 region in wheat cultivar 'Chinese Spring'. With the availability of sequence assemblies and GenomeZippers for each of the wheat chromosome arms, the proposed strategy can be applied for focused marker development in any region of the wheat genome.

Characterization of eight Russian wheat aphid (Hemiptera: Aphididae) biotypes using two-category resistant-susceptible plant responses.

Eight biotypes of the Russian wheat aphid, Diuraphis noxia (Kurdjumov), have been discovered in the United States since 2003. Biotypes are identified by the distinct feeding damage responses they produce on wheat carrying different Russian wheat aphid resistance genes, namely, from Dn1 to Dn9. Each Russian wheat aphid biotype has been named using plant damage criteria and virulence categories that have varied between studies. The study was initiated to compare the plant damage caused by all the eight known Russian wheat aphid biotypes, and analyze the results to determine how Russian wheat aphid virulence should be classified. Each Russian wheat aphid biotype was evaluated on 16 resistant or susceptible cereal genotypes. Plant damage criteria included leaf roll, leaf chlorosis, and plant height. The distribution of chlorosis ratings followed a bimodal pattern indicating two categories of plant responses, resistant or susceptible. Correlations were significant between chlorosis ratings and leaf roll (r(2) = 0.72) and between chlorosis ratings and plant height (r(2) = 0.48). The response of 16 cereal genotypes to feeding by eight Russian wheat aphid biotypes found RWA1, RWA2, RWA6, and RWA8 to differ in virulence, while Russian wheat aphid biotypes RWA3, RWA4, RWA5, and RWA7 produced similar virulence profiles. These biotypes have accordingly been consolidated to what is hereafter referred to as RWA3/7. Our results indicated that the five main biotypes RWA1, RWA2, RWA3/7, RWA6, and RWA8 can be identified using only four wheat genotypes containing Dn3, Dn4, Dn6, and Dn9.

Selection of reference genes for expression analysis in Diuraphis noxia (Hemiptera: Aphididae) fed on resistant and susceptible wheat plants.

The Russian wheat aphid (RWA), Diuraphis noxia Kurdjumov, is a major global pest of wheat and barley production that causes enormous economic damage. Few studies have been conducted to explore and decipher the molecular basis of RWA strategies to evade plant defense mechanisms. Gene expression studies of RWA in response to wheat genotypes carrying different RWA resistance genes have been initiated in our group; however, a secure and accurate understanding of RWA gene expression is dependent on identification of suitable reference genes. This study analyzed expression profiles of five potential reference genes selected and sequenced during RNA sequencing experiments. The expression of genes coding for actin and ribosomal protein L27 was comparatively less variable in RWA fed on different wheat hosts. Results of geNorm, NormFinder, and BestKeeper expression analyses support the use of actin and ribosomal protein L27 in RT-qPCR studies of RWA gene expression in studies involving RWA-wheat interactions.

Mapping resistance genes conferring tolerance to RWA (Diuraphis noxia) in barley (Hordeum vulgare)

The Russian wheat aphid (RWA) is one of the most aggressive pests of barley and wheat. The outbreak of RWA occurred in Argentina in 2008 caused serious damage to barley cultivars. The most effective and sustainable method of RWA control is to identify new resistance genes. The purpose of the current research was to map RWA resistance genes in a set of double haploid (DH) lines of the Oregon-Wolfe Barley (OWB) mapping population derived from the cross between OWBDOM and OWBREC. The DH and both parental lines were screened for antixenosis, tolerance and antibiosis to RWA. There was significant variation among the DH lines in most of the traits studied. However, only tolerance resulted in significant quantitative trait loci (QTLs) associated with the molecular markers. Two main QTLs were identified. These explained 90 and 79 % of the variability of foliar area and chlorophyll content, respectively, of infested and control plants. The initial and final foliar area and the variation in foliar area were associated with the same molecular markers on chromosome 2H (BmAc0125, Vrs1, BmAc0144f and BmAg0113e). The positive alleles were provided by OWBDOM. The content of chlorophyll was associated with the marker loci WMC1E8, MWG912, ABC261, MWG2028 and Blp on chromosome 1H, with the positive alleles provided by OWBREC. Both parents contributed to different tolerance traits, with foliar area and chlorophyll content remaining as the plant traits most affected by aphid feeding. The QTLs found in this population are new RWA resistance loci. A sequence homology search was performed to derive the putative function of the genes linked to the QTLs.

Polymorphism and Methylation of Four Genes Expressed in Salivary Glands of Russian Wheat Aphid (Homoptera: Aphididae)

DNA methylation is a general epigenetic mechanism for plants, animals, and fungi to adapt to environmental variation. Two biotypes of the Russian wheat aphid (Diuraphis noxia), Biotype 1 and Biotype 2, have different virulence to host plants. In this study, in addition to a high polymorphism, DNA methylation at cytosines were observed in genomic fragments of four genes in Biotype 1 and Biotype 2, after the genomic DNA was treated with sodium bisulfite. These genes presumably encode proteins and enzymes in salivary glands of aphids. The two Biotype 1 showed different methylation levels, that is, Biotype 1 showed a higher methylation on the four genes. Two thirds of methyl cytosines were in a sequence context of CHH (H = A, C, or T). Some polymorphism and methylation sites were located at important positions in terms of enzyme function, such as close to catalytic residues or inhibitor binding residues. These findings may provide clues to explore the evolutionary mode between Russian wheat aphid virulence and resistance genes of host plants.

Aphid feeding activates expression of a transcriptome of oxylipin-based defense signals in wheat involved in resistance to herbivory.

Damage by the Russian wheat aphid (RWA), Diuraphis noxia, significantly reduces wheat and barley yields worldwide. In compatible interactions, virulent RWA populations flourish and susceptible plants suffer extensive leaf chlorophyll loss. In incompatible interactions, RWA reproduction and population growth are significantly reduced and RWA-related chlorophyll loss in resistant plants is minor. The objectives of this study were to develop an understanding of the molecular and phytochemical bases of RWA resistance in plants containing the Dnx resistance gene. Microarray, real-time polymerase chain reaction, and phytohormone assays were conducted to identify transcriptome components unique to RWA-infested Dnx plants and susceptible (Dn0) plants, and to identify and characterize putative genes involved in Dnx plant defense responses. We found that RWA-infested Dnx plants upregulated >180 genes related to reactive oxygen species, signaling, pathogen defense, and arthropod allelochemical and physical defense. The expression of several of these genes in RWA-infested Dnx plants increased significantly from 6- to 24-h post infestation (hpi), but their expression in Dn0 plants, when present, was delayed until 48- to 96 hpi. Concentrations of 16- and 18-carbon fatty acids, trans-methyl-12-oxophytodienoic acid, and abscisic acid were significantly greater in Dnx foliage than in Dn0 foliage after RWA infestation, suggesting that Dnx RWA defense and resistance genes may be regulated via the oxylipin pathway. These findings provide a foundation for the elucidation of the molecular basis for compatible- and incompatible plant-aphid interactions.

Effect of segregation distortion on genetic mapping of a PI 294994-derived Russian Wheat Aphid resistance gene

An earlier genetic mapping study with an unidentified Russian Wheat Aphid resistance gene, referred to as Dn? and derived from PI 294994, suggested that the gene occurs close to the centromere on 7DS. Closer inspection of the map data showed that mild segregation distortion occurred at some of the loci (including Dn? which could have affected conclusions regarding the gene order. As a result, and depending on the gene order inferred in the analysis, Dn? could be mapped on either side of the 7D centromere. A telosomic analysis was therefore undertaken in order to confirm the map position or to determine the true location of Dn? However, physical mapping showed that Dn? was not even located on chromosome 7D and it was concluded that it may be the same gene as Dn4 on 1DL. The result cautions that even mildly distorted segregation data could be very misleading and that genetic mapping results based on such data need to be confirmed independently, preferably by physical mapping.

Chromosomal Location of the Russian Wheat Aphid Resistance Gene, Dn5

Recent reports suggested that the Russian wheat aphid (Diuraphis noxia Mordvilko, RWA) resistance gene Dn5 is located on wheat (Triticum aestivum L.) chromosome arm 7DS rather than on 7DL. A further attempt was therefore made to physically map the gene employing the same source material previously used to describe, name, and map it. It was possible to derive monotelosomic 7DL plants carrying Dn5 on the telosome. Such plants could not be developed for the short arm. The identities of the 7DS and 7DL telosomes were confirmed using mapped microsatellite a‐nd endopeptidase markers and showed unequivocally that Dn5 occurs on 7DL. It appears that use of the wrong Dn5 source material could explain some of the inconsistencies in earlier genetic studies involving this gene. In an attempt to also genetically map Dn5, a doubled haploid (DH) mapping population was derived from the cross PI 294994 × ‘Chinese Spring’ (CS). A single RWA resistance gene, believed to be Dn5, segregated in the population and was mapped together with 12 microsatellite loci and the Ep‐D1 locus. The Dn gene mapped near the centromere on 7DS. However, closer inspection of the data suggested that segregation distortion with opposite effect occurred within two areas of the region that was being mapped. The combined distortion effect rendered the data unreliable and it was not possible to unambiguously determine the chromosome arm on which the Dn gene resides. It appeared that the chromosome 7D proximal regions may be problematic in terms of genetic analysis based on segregation data.

Molecular Mapping and Allelic Relationships of Russian Wheat Aphid–Resistance Genes

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious economic pest of wheat (Triticum aestivum L.) worldwide. Although resistant wheat cultivars provide good control of RWA, genetic characterization is critical to the continual use of aphid‐resistance genes. The objective of this study was to determine the chromosome location and the genetic relationships of the RWA‐resistance genes Dn1, Dn2, Dn4, Dn5, Dn6, Dnx, and several uncharacterized Dn genes. Molecular mapping demonstrated that Dn genes in four uncharacterized wheat lines (PI 47545, PI 222666, PI 222668, and PI 225245) as well as Dn1, Dn2, Dn5, Dn6, and Dnx are tightly linked to microsatellite markers Xgwm44 and Xgwm111 Deletion bin mapping assigned Xgwm44 physically in the region distal to the breakpoints 7DS‐4 (FL 0.61) and Xgwm111 distal to 7DS‐5 (FL 0.36) of wheat chromosome arm 7DS. Allelism tests detected no segregation for susceptibility among TC1F1 plants from the test crosses or among F2 plants derived from intercrosses between wheat lines containing each of these known Dn genes. Both marker‐linkage analyses and allelism tests revealed that all these Dn genes are clustered in a region linked to Xgwm111 and are either alleles of a single locus or are closely related members of a Dn gene family. Dn4 and the uncharacterized Dn gene in PI 151918 are either allelic or linked on wheat chromosome 1DS. The linked markers and genetic relationships of these Dn genes will greatly facilitate their use in wheat breeding and the deployment of aphid‐resistant cultivars.

Comparison of Dn4- and Dn7-Carrying Spring Wheat Genotypes Artificially Infested with Russian Wheat Aphid (Homoptera: Aphididae) Biotype 1

Genetic resistance is a useful control strategy for managing Russian wheat aphid, Diuraphis noxia (Mordvilko), in wheat, Triticum aestivum L. In 2003, a Russian wheat aphid population (denoted as biotype 2) identified in Colorado was virulent to genotypes carrying the Dn4 Russian wheat aphid resistance gene, necessitating the rapid identification and deployment of new sources of resistance. Although the Dn7 gene had shown excellent resistance to Russian wheat aphid biotypes 1 and 2 in evaluations in the greenhouse, no information is available on the amount of protection provided by Dn7 under field conditions. The objective of this study was to compare the reaction of Dn4- and Dn7-carrying spring wheat genotypes under artificial infestation by Russian wheat aphid biotype 1 in the field. Irrigated field experiments were conducted in 2003 and 2004 in a split-split plot arrangement with six replications. The whole plot treatment was infestation level (control, 1x, and 10x Russian wheat aphids), and the subplot treatment was resistance source (Dn4- and Dn7-carrying genotypes). The sub-subplot treatment consisted of side-by-side planting of resistant and susceptible genotypes. The Dn4 subplot was significantly more damaged than the Dn7 subplot in 2003, but not in 2004. Interaction effects observed in 2004 suggested an advantage of Dn7 relative to Dn4 in terms of reduced Russian wheat aphid abundance and plant damage. Deployment of the Dn7 Russian wheat aphid resistance gene should provide protection in the field comparable with that provided by the Dn4 resistance gene for management of Russian wheat aphid biotype 1.

Mapping antixenosis genes on chromosome 6A of wheat to greenbug and to a new biotype of Russian wheat aphid

AbstractGreenbug and Russian wheat aphid (RWA) are two devastating pests of wheat. The first has a long history of new biotype emergence and recently. RWA resistance has just started to break down. Thus, it is necessary to find new sources of resistance that will broaden the genetic base against these pests in wheat. Seventy‐five doubled haploid recombinant (DHR) lines for chromosome 6A from the F1 of the cross between “Chinese Spring’ and the “Chinese Spring (Synthetic 6A) (Triticum dicoccoides × Aegilops tauschii)” substitution line were used as a mapping population for testing resistance to greenbug biotype C and to a new strain of RWA that appeared in Argentina in 2003. A quantitative trait locus (QTL) (br antixenosis to greenbug was significantly associated with the marker loci Xgwm1009 and Xgwm1185 located in the centromere region of chromosome 6A. Another QTL which accounted for most of the antixenosis against RWA was associated with the marker loci Xgwm1291 and Xiinni1150. both located on the long arm of chromosome 6A. This is the first report of greenbug and RWA resistance genes located on chromosome 6A. It is also the first report of antixenosis against the new strain of RWA. As most of the RWA resistance genes present in released cultivars have been located in [he D‐genome, it is highly desirable to find new sources in other genomes to combine the existing resistance genes with new sources.

DNA and morphological markers for a Russian wheat aphid resistance gene

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a significant insect pest of wheat worldwide. Morphological and molecular markers associated with RWA resistance could be used to increase the accuracy and efficiency of selection of resistant germplasm and facilitate transfer to desirable wheat genotypes. The objective of this work was to identify microsatellite (SSR) markers linked to the RWA resistance gene (Dn4) and glume-colour gene (Rg2) using a population of F2-derived F3 families originating from a cross between a susceptible line (synthetic hexaploid-11) and a resistance cultivar (Halt). Two microsatellite markers Xgwm106 and Xgwm337 flanked Dn4 on the short arm of chromosome 1D at 5.9 and 9.2 cM, respectively. Two other microsatellite markers, Xpsp2999 and Xpsp3000, at the distal part of this chromosome arm are linked to Dn4 and to Rg2. The accuracy and efficiency of marker-assisted selection were calculated for homozygous Dn4Dn4 genotypes in the F2 generation. The gene Rg2 for red glume colour can also be used for marker-assisted selection of Dn4 gene individually and in combination with microsatellite markers. When used together, the closest markers Xgwm106 and Xgwm337, provide 100% accuracy and 75% efficiency. One hundred percent accuracy is also achieved when the morphological marker red glume is used in combination with either Xgwm106 or Xgwm337. Using these flanking markers, it may be possible to fix resistance to RWA in the first segregating generation of an F2 population without infestation with aphids.

Microsatellite mapping of a Russian wheat aphid resistance gene on chromosome 7B of an Iranian tetraploid wheat line: preliminary results

Russian wheat aphid (Diuraphis noxia Mordvilko) is a cosmopolitan pest of wheat. Development of resistant genetic sources is economically and environmentally effective. The objective of this study was to localize and tag a Russian wheat aphid resistance gene in Iranian tetraploid wheat line ‘1881’ using microsatellite markers. Thirty pairs of microsatellite primers of group 7 chromosomes of wheat were screened, using bulked-segregant analysis. Four microsatellite primer pairs of chromosome 7B revealed polymorphism between susceptible and resistant parents and bulk DNA samples of resistant and susceptible individuals, among which the linkage of 2 markers was confirmed. The locus Xgwm46-7B was the closest to the RWA resistance locus with a distance of 10 cM. The identified locus on chromosome 7B of the tetraploid wheat line ’1881’ may be orthologous to one of the previously identified RWA resistance loci on chromosome 7D.

Field evaluation of emmer wheat-derived synthetic hexaploid wheat for resistance to Russian wheat aphid (Homoptera: Aphididae).

Broadening the genetic base for resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), in bread wheat, Triticum aestivum L., is desirable. To date, identified Russian wheat aphid resistance genes are either located to the D chromosomes or to rye translocation of wheat, and resistance derived from the A or B genomes of tetraploid Triticum spp. would therefore be highly beneficial. Fifty-eight synthetic hexaploid wheat, derived from interspecific crosses of Triticum dicoccum Schrank. and Aegilops tauschii (Coss.) Schmal. and their parents were evaluated for resistance to Russian wheat aphid under field conditions. Plots infested with aphids were compared with plots protected with insecticides. The T. dicoccum parents were highly resistant to Russian wheat aphids, whereas the Ae. tauschii parents were susceptible. Resistance levels observed in the synthetic hexaploids were slightly below the levels of their T. dicoccum parents when a visual damage scale was used. but no major resistance suppression was observed among the synthetics. Russian wheat aphid infestation on average reduced plant height and kernel weight at harvest in the synthetic hexaploids and the T. dicoccum parents by 3-4%, whereas the susceptible control 'Seri M82' suffered losses of above 20%. Because resistance in the synthetic hexaploid wheat is derived from their T. dicoccum parent, resistance gene(s) must be located on the A and/or B genomes. They must therefore be different from previously identified Russian wheat aphid resistance genes, which have all been located on the D genome of wheat or on translocated segments.

Field Evaluation of Emmer Wheat-Derived Synthetic Hexaploid Wheat for Resistance to Russian Wheat Aphid (Homoptera: Aphididae)

Broadening the genetic base for resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), in bread wheat, Triticum aestivum L., is desirable. To date, identified Russian wheat aphid resistance genes are either located to the D chromosomes or to rye translocation of wheat, and resistance derived from the A or B genomes of tetraploid Triticum spp. would therefore be highly beneficial. Fifty-eight synthetic hexaploid wheat, derived from interspecific crosses of Triticum dicoccum Schrank. and Aegilops tauschii (Coss.) Schmal. and their parents were evaluated for resistance to Russian wheat aphid under field conditions. Plots infested with aphids were compared with plots protected with insecticides. The T. dicoccum parents were highly resistant to Russian wheat aphids, whereas the Ae. tauschii parents were susceptible. Resistance levels observed in the synthetic hexaploids were slightly below the levels of their T. dicoccum parents when a visual damage scale was used, but no major resistance suppression was observed among the synthetics. Russian wheat aphid infestation on average reduced plant height and kernel weight at harvest in the synthetic hexaploids and the T. dicoccum parents by 3–4%, whereas the susceptible control ‘Seri M82′ suffered losses of above 20%. Because resistance in the synthetic hexaploid wheat is derived from their T. dicoccum parent, resistance gene(s) must be located on the A and/or B genomes. They must therefore be different from previously identified Russian wheat aphid resistance genes, which have all been located on the D genome of wheat or on translocated segments.

DNA and morphological markers for a Russian wheat aphid resistance gene

DNA and morphological markers for a Russian wheat aphid resistance gene

The allelic relationships among Russian wheat aphid resistance genes in two Iranian wheat lines and known genes

The availability of more resistance genes to Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) may provide additional protection from new virulent strains or biotypes of the insect. This genetic study was conducted to determine the allelic relationships of resistance genes in two Iranian wheat (Triticum aestivum L.) lines, SHZ.W.102 and SHZ.W.104 and lines PI 137739, PI 262660, PI 372129, PI 294994 and PI 243781, carrying resistance genes Dn1, Dn2, Dn4, Dn5 and Dn6 respectively. The two Iranian lines were crossed to each of the other resistant lines, and F1 and F2 seedlings were screened for RWA reaction. The resistance gene in 102 was allelic to Dn1, however, the resistance gene in 104 was different from other known genes. The resistant line 104 is an effective source of genetic resistance to RWA and the gene symbol Dn7 is proposed for its resistance gene.

Identification of microsatellite markers linked to Russian wheat aphid resistance genes Dn4 and Dn6.

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious economic pest of wheat worldwide. Host plant resistance is the preferred method to control RWA infestations. The identification and mapping of RWA-resistant genes and the development of resistant wheat cultivars can be facilitated through the use of molecular markers. In the present study, microsatellite (SSR) markers linked to the RWA-resistant genes Dn4 and Dn6 were identified using several F(2) mapping populations derived from crosses of susceptible wheat cultivars and resistant sources. Two flanking microsatellite markers Xgwm106 and Xgwm337 are linked in coupling phase with Dn4 on the short arm of wheat chromosome 1D at 7.4 cM and 12.9 cM, respectively. Two other microsatellite markers Xgwm44 and Xgwm111 are linked to Dn6 in coupling phase near the centromere on the short arm of chromosome 7D at 14.6 cM and 3.0 cM, respectively. This is the first report on the chromosomal location of Dn6, which proved to be either allelic or tightly linked to Dn1, Dn2 and Dn5. This result of Dn6 location contradicts previous reports that Dn6 was independent of Dn1, Dn2 and Dn5. The linked markers can be conveniently used for marker-assisted selection in wheat breeding programs for the identification and/or pyramiding of Dn4 and Dn6 genes.