Major haplotype divergence including multiple germin-like protein genes, at the wheat Sr2 adult plant stem rust resistance locus

Intelligent reprogramming of wheat for enhancement of fungal and nematode disease resistance using advanced molecular techniques

Wheat (Triticum aestivum L.) diseases are major factors responsible for substantial yield losses worldwide, which affect global food security. For a long time, plant breeders have been struggling to improve wheat resistance against major diseases by selection and conventional breeding techniques. Therefore, this review was conducted to shed light on various gaps in the available literature and to reveal the most promising criteria for disease resistance in wheat. However, novel techniques for molecular breeding in the past few decades have been very fruitful for developing broad-spectrum disease resistance and other important traits in wheat. Many types of molecular markers such as SCAR, RAPD, SSR, SSLP, RFLP, SNP, and DArT, etc., have been reported for resistance against wheat pathogens. This article summarizes various insightful molecular markers involved in wheat improvement for resistance to major diseases through diverse breeding programs. Moreover, this review highlights the applications of marker assisted selection (MAS), quantitative trait loci (QTL), genome wide association studies (GWAS) and the CRISPR/Cas-9 system for developing disease resistance against most important wheat diseases. We also reviewed all reported mapped QTLs for bunts, rusts, smuts, and nematode diseases of wheat. Furthermore, we have also proposed how the CRISPR/Cas-9 system and GWAS can assist breeders in the future for the genetic improvement of wheat. If these molecular approaches are used successfully in the future, they can be a significant step toward expanding food production in wheat crops.

Wheat Genes Associated with Different Types of Resistance against Stem Rust (Puccinia graminis Pers.)

Stem rust is one wheat's most dangerous fungal diseases. Yield losses caused by stem rust have been significant enough to cause famine in the past. Some races of stem rust are considered to be a threat to food security even nowadays. Resistance genes are considered to be the most rational environment-friendly and widely used way to control the spread of stem rust and prevent yield losses. More than 60 genes conferring resistance against stem rust have been discovered so far (so-called Sr genes). The majority of the Sr genes discovered have lost their effectiveness due to the emergence of new races of stem rust. There are some known resistance genes that have been used for over 50 years and are still effective against most known races of stem rust. The goal of this article is to outline the different types of resistance against stem rust as well as the effective and noneffective genes, conferring each type of resistance with a brief overview of their origin and usage.

The Use and Limitations of Exome Capture to Detect Novel Variation in the Hexaploid Wheat Genome

The bread wheat (Triticum aestivum) pangenome is a patchwork of variable regions, including translocations and introgressions from progenitors and wild relatives. Although a large number of these have been documented, it is likely that many more remain unknown. To map these variable regions and make them more traceable in breeding programs, wheat accessions need to be genotyped or sequenced. The wheat genome is large and complex and consequently, sequencing efforts are often targeted through exome capture. In this study, we employed exome capture prior to sequencing 12 wheat varieties; 10 elite T. aestivum cultivars and two T. aestivum landrace accessions. Sequence coverage across chromosomes was greater toward distal regions of chromosome arms and lower in centromeric regions, reflecting the capture probe distribution which itself is determined by the known telomere to centromere gene gradient. Superimposed on this general pattern, numerous drops in sequence coverage were observed. Several of these corresponded with reported introgressions. Other drops in coverage could not be readily explained and may point to introgressions that have not, to date, been documented.

Characterization and Use in Wheat Breeding of Leaf Rust Resistance Genes from Durable Varieties

Simple Summary

Wheat leaf rust is one of the most significant diseases worldwide, incited by a parasitic fungus which infects leaves, affecting grain yield. This pathogen is spread by the wind over large areas through microscopic spores. This huge number of spores favors the selection of virulent forms; therefore, there is a continuous need for new resistance genes to control this disease without fungicides. These resistant genes are naturally found in resistant wheat varieties and can be introduced by standard crosses. In this work, seven resistant genes were introduced into several commercial susceptible varieties. The selection of resistance genes was assisted by DNA markers that are close to these genes on the chromosome. Additionally, the selection of desirable traits from the commercial variety was also assisted by DNA markers to accelerate the process. In field testing, the varieties developed here were resistant to leaf rust, and suitable for commercial use.

Abstract

Leaf rust is one of the most significant diseases of wheat worldwide. In Argentina, it is one of the main reasons for variety replacement that becomes susceptible after large-scale use. Some varieties showed durable resistance to this disease, including Buck Manantial and Sinvalocho MA. RILs (Recombinant Inbred Lines) were developed for each of these varieties and used in genetics studies to identify components of resistance, both in greenhouse inoculations using leaf rust races, and in field evaluations under natural population infections. In Buck Manantial, the APR gene LrBMP1 was associated with resistance in field tests. In crosses involving Sinvalocho MA, four genes were previously identified and associated with resistance in field testing: APR (Adult Plant Resistance) gene LrSV1, the APR genetic system LrSV2 + LrcSV2 and the ASR (All Stage Resistance) gene LrG6. Using backcrosses, LrBMP1 was introgressed in four commercial susceptible varieties and LrSV1, LrSV2 + LrcSV2 and LrG6 were simultaneously introgressed in three susceptible commercial varieties. The use of molecular markers for recurrent parent background selection allowed us to select resistant lines with more than 80% similarity to commercial varieties. Additionally, progress towards positional cloning of the genetic system LrSV2 + LrcSV2 for leaf rust APR is reported.

A wheat cysteine-rich receptor-like kinase confers broad-spectrum resistance against Septoria tritici blotch

The poverty of disease resistance gene reservoirs limits the breeding of crops for durable resistance against evolutionary dynamic pathogens. Zymoseptoria tritici which causes Septoria tritici blotch (STB), represents one of the most genetically diverse and devastating wheat pathogens worldwide. No fully virulent Z. tritici isolates against synthetic wheats carrying the major resistant gene Stb16q have been identified. Here, we use comparative genomics, mutagenesis and complementation to identify Stb16q, which confers broad-spectrum resistance against Z. tritici. The Stb16q gene encodes a plasma membrane cysteine-rich receptor-like kinase that was recently introduced into cultivated wheat and which considerably slows penetration and intercellular growth of the pathogen.

Fine mapping QSc.VR4, an effective and stable scald resistance locus in barley (Hordeum vulgare L.), to a 0.38-Mb region enriched with LRR-RLK and GLP genes

An effective and stable quantitative resistance locus, QSc.VR4, was fine mapped, characterized and physically anchored to the short arm of 4H, conferring adult plant resistance to the fungus Rhynchosporium commune in barley. Scald caused by Rhynchosporium commune is one of the most destructive barley diseases worldwide. Accumulation of adult plant resistance (APR) governed by multiple resistance alleles is predicted to be effective and long-lasting against a broad spectrum of pathotypes. However, the molecular mechanisms that control APR remain poorly understood. Here, quantitative trait loci (QTL) analysis of APR and fine mapping were performed on five barley populations derived from a common parent Vlamingh, which expresses APR to scald. Two QTLs, designated QSc.VR4 and QSc.BR7, were detected from a cross between Vlamingh and Buloke. Our data confirmed that QSc.VR4 is an effective and stable APR locus, residing on the short arm of chromosome 4H, and QSc.BR7 derived from Buloke may be an allele of reported Rrs2. High-resolution fine mapping revealed that QSc.VR4 is located in a 0.38 Mb genomic region between InDel markers 4H2282169 and 4H2665106. The gene annotation analysis and sequence comparison suggested that a gene cluster containing two adjacent multigene families encoding leucine-rich repeat receptor kinase-like proteins (LRR-RLKs) and germin-like proteins (GLPs), respectively, is likely contributing to scald resistance. Adult plant resistance (APR) governed by QSc.VR4 may confer partial levels of resistance to the fungus Rhynchosporium commune and, furthermore, be an important resource for gene pyramiding that may contribute broad-based and more durable resistance.

Phenotypes Conferred by Wheat Multiple Pathogen Resistance Locus, Sr2, Include Cell Death in Response to Biotic and Abiotic Stresses

The wheat Sr2 locus confers partial resistance to four biotrophic pathogens: wheat stem rust (Puccinia graminis f. sp. tritici), leaf rust (P. triticina), stripe rust (P. striiformis f. sp. tritici), and powdery mildew (Blumeria graminis f. sp. tritici). In addition, Sr2 is linked with a brown coloration of ears and stems, termed pseudo-black chaff (PBC). PBC, initially believed to be elicited by stem rust infection, was subsequently recognized to occur in the absence of pathogen infection. The current study demonstrates that the resistance response to stem rust is associated with the death of photosynthetic cells around rust infection sites in the inoculated leaf sheath. Similarly, Sr2-dependent resistance to powdery mildew was associated with the death of leaf mesophyll cells around mildew infection sites. We demonstrate that PBC occurring in the absence of pathogen inoculation also corresponds with death and the collapse of photosynthetic cells in the affected parts of stems and ears. In addition, Sr2-dependent necrosis was inducible in leaves by application of petroleum jelly or by heat treatments. Thus, Sr2 was found to be associated with cell death, which could be triggered by either biotic or abiotic stresses. Our results suggest a role for the Sr2 locus in controlling cell death in response to stress.

Divergence between bread wheat and Triticum militinae in the powdery mildew resistance QPm.tut-4A locus and its implications for cloning of the resistance gene

A segment of Triticum militinae chromosome 7G harbors a gene(s) conferring powdery mildew resistance which is effective at both the seedling and the adult plant stages when transferred into bread wheat (T. aestivum). The introgressed segment replaces a piece of wheat chromosome arm 4AL. An analysis of segregating materials generated to positionally clone the gene highlighted that in a plant heterozygous for the introgression segment, only limited recombination occurs between the introgressed region and bread wheat 4A. Nevertheless, 75 genetic markers were successfully placed within the region, thereby confining the gene to a 0.012 cM window along the 4AL arm. In a background lacking the Ph1 locus, the localized rate of recombination was raised 33-fold, enabling the reduction in the length of the region containing the resistance gene to a 480 kbp stretch harboring 12 predicted genes. The substituted segment in the reference sequence of bread wheat cv. Chinese Spring is longer (640 kbp) and harbors 16 genes. A comparison of the segments' sequences revealed a high degree of divergence with respect to both their gene content and nucleotide sequence. Of the 12 T. militinae genes, only four have a homolog in cv. Chinese Spring. Possible candidate genes for the resistance have been identified based on function predicted from their sequence.

Rapid gene cloning in cereals

The large and complex genomes of many cereals hindered cloning efforts in the past. Advances in genomics now allow the rapid cloning of genes from humanity's most valuable crops. The past two decades were characterized by a genomics revolution that entailed profound changes to crop research, plant breeding, and agriculture. Today, high-quality reference sequences are available for all major cereal crop species. Large resequencing and pan-genome projects start to reveal a more comprehensive picture of the genetic makeup and the diversity among domesticated cereals and their wild relatives. These technological advancements will have a dramatic effect on dissecting genotype-phenotype associations and on gene cloning. In this review, we will highlight the status of the genomic resources available for various cereal crops and we will discuss their implications for gene cloning. A particular focus will be given to the cereal species barley and wheat, which are characterized by very large and complex genomes that have been inaccessible to rapid gene cloning until recently. With the advancements in genomics and the development of several rapid gene-cloning methods, it has now become feasible to tackle the cloning of most agriculturally important genes, even in wheat and barley.

Advances in Wheat and Pathogen Genomics: Implications for Disease Control

The gene pool of wheat and its wild and domesticated relatives contains a plethora of resistance genes that can be exploited to make wheat more resilient to pathogens. Only a few of these genes have been isolated and studied at the molecular level. In recent years, we have seen a shift from classical breeding to genomics-assisted breeding, which makes use of the enormous advancements in DNA sequencing and high-throughput molecular marker technologies for wheat improvement. These genomic advancements have the potential to transform wheat breeding in the near future and to significantly increase the speed and precision at which new cultivars can be bred. This review highlights the genomic improvements that have been made in wheat and its pathogens over the past years and discusses their implications for disease-resistance breeding.

Chromosome-scale comparative sequence analysis unravels molecular mechanisms of genome dynamics between two wheat cultivars

BACKGROUND

Recent improvements in DNA sequencing and genome scaffolding have paved the way to generate high-quality de novo assemblies of pseudomolecules representing complete chromosomes of wheat and its wild relatives. These assemblies form the basis to compare the dynamics of wheat genomes on a megabase scale.

RESULTS

Here, we provide a comparative sequence analysis of the 700-megabase chromosome 2D between two bread wheat genotypes-the old landrace Chinese Spring and the elite Swiss spring wheat line 'CH Campala Lr22a'. Both chromosomes were assembled into megabase-sized scaffolds. There is a high degree of sequence conservation between the two chromosomes. Analysis of large structural variations reveals four large indels of more than 100 kb. Based on the molecular signatures at the breakpoints, unequal crossing over and double-strand break repair were identified as the molecular mechanisms that caused these indels. Three of the large indels affect copy number of NLRs, a gene family involved in plant immunity. Analysis of SNP density reveals four haploblocks of 4, 8, 9 and 48 Mb with a 35-fold increased SNP density compared to the rest of the chromosome. Gene content across the two chromosomes was highly conserved. Ninety-nine percent of the genic sequences were present in both genotypes and the fraction of unique genes ranged from 0.4 to 0.7%.

CONCLUSIONS

This comparative analysis of two high-quality chromosome assemblies enabled a comprehensive assessment of large structural variations and gene content. The insight obtained from this analysis will form the basis of future wheat pan-genome studies.

Inferring defense-related gene families in Arabidopsis and wheat

BACKGROUND

A large number of disease resistance genes or QTLs in crop plants are identified through conventional genetics and genomic tools, but their functional or molecular characterization remains costly, labor-intensive and inaccurate largely due to the lack of deep sequencing of large and complex genomes of many important crops such as allohexaploid wheat (Triticum aestivum L.). On the other hand, gene annotation and relevant genomic resources for disease resistance and other defense-related traits are more abundant in model plant Arabidopsis (Arabidopsis thaliana). The objectives of this study are (i) to infer homology of defense-related genes in Arabidopsis and wheat and (ii) to classify these homologous genes into different gene families.

RESULTS

We employed three bioinformatics and genomics approaches to identifying candidate genes known to affect plant defense and to classifying these protein-coding genes into different gene families in Arabidopsis. These approaches predicted up to 1790 candidate genes in 11 gene families for Arabidopsis defense to biotic stresses. The 11 gene families included ABC, NLR and START, the three families that are already known to confer rust resistance in wheat, and eight new families. The distributions of predicted SNPs for individual rust resistance genes were highly skewed towards specific gene families, including eight one-to-one uniquely matched pairs: Lr21-NLR, Lr34-ABC, Lr37-START, Sr2-Cupin, Yr24-Transcription factor, Yr26-Transporter, Yr36-Kinase and Yr53-Kinase. Two of these pairs, Lr21-NLR and Lr34-ABC, are expected because Lr21 and Lr34 are well known to confer race-specific and race-nonspecific resistance to leaf rust (Puccinia triticina) and they encode NLR and ABC proteins.

CONCLUSIONS

Our inference of 11 known and new gene families enhances current understanding of functional diversity with defense-related genes in genomes of model plant Arabidopsis and cereal crop wheat. Our comparative genomic analysis of Arabidopsis and wheat genomes is complementary to the conventional map-based or marker-based approaches for identification of genes or QTLs for rust resistance genes in wheat and other cereals. Race-specific and race-nonspecific candidate genes predicted by our study may be further tested and combined in breeding for durable resistance to wheat rusts and other pathogens.

Extensive gene content variation in the Brachypodium distachyon pan-genome correlates with population structure

While prokaryotic pan-genomes have been shown to contain many more genes than any individual organism, the prevalence and functional significance of differentially present genes in eukaryotes remains poorly understood. Whole-genome de novo assembly and annotation of 54 lines of the grass Brachypodium distachyon yield a pan-genome containing nearly twice the number of genes found in any individual genome. Genes present in all lines are enriched for essential biological functions, while genes present in only some lines are enriched for conditionally beneficial functions (e.g., defense and development), display faster evolutionary rates, lie closer to transposable elements and are less likely to be syntenic with orthologous genes in other grasses. Our data suggest that differentially present genes contribute substantially to phenotypic variation within a eukaryote species, these genes have a major influence in population genetics, and transposable elements play a key role in pan-genome evolution.

The role of differential gene content in the evolution and function of eukaryotic genomes remains poorly explored. Here the authors assemble and annotate the Brachypodium distachyon pan-genome consisting of 54 diverse lines and reveal the differential present genes as a major driver of phenotypic variation.

Rapid cloning of genes in hexaploid wheat using cultivar-specific long-range chromosome assembly

Cereal crops such as wheat and maize have large repeat-rich genomes that make cloning of individual genes challenging. Moreover, gene order and gene sequences often differ substantially between cultivars of the same crop species. A major bottleneck for gene cloning in cereals is the generation of high-quality sequence information from a cultivar of interest. In order to accelerate gene cloning from any cropping line, we report 'targeted chromosome-based cloning via long-range assembly' (TACCA). TACCA combines lossless genome-complexity reduction via chromosome flow sorting with Chicago long-range linkage to assemble complex genomes. We applied TACCA to produce a high-quality (N50 of 9.76 Mb) de novo chromosome assembly of the wheat line CH Campala Lr22a in only 4 months. Using this assembly we cloned the broad-spectrum Lr22a leaf-rust resistance gene, using molecular marker information and ethyl methanesulfonate (EMS) mutants, and found that Lr22a encodes an intracellular immune receptor homologous to the Arabidopsis thaliana RPM1 protein.

Repeat-length variation in a wheat cellulose synthase-like gene is associated with altered tiller number and stem cell wall composition

The tiller inhibition gene (tin) that reduces tillering in wheat (Triticum aestivum) is also associated with large spikes, increased grain weight, and thick leaves and stems. In this study, comparison of near-isogenic lines (NILs) revealed changes in stem morphology, cell wall composition, and stem strength. Microscopic analysis of stem cross-sections and chemical analysis of stem tissue indicated that cell walls in tin lines were thicker and more lignified than in free-tillering NILs. Increased lignification was associated with stronger stems in tin plants. A candidate gene for tin was identified through map-based cloning and was predicted to encode a cellulose synthase-like (Csl) protein with homology to members of the CslA clade. Dinucleotide repeat-length polymorphism in the 5'UTR region of the Csl gene was associated with tiller number in diverse wheat germplasm and linked to expression differences of Csl transcripts between NILs. We propose that regulation of Csl transcript and/or protein levels affects carbon partitioning throughout the plant, which plays a key role in the tin phenotype.

Single Marker and Haplotype-Based Association Analysis of Semolina and Pasta Colour in Elite Durum Wheat Breeding Lines Using a High-Density Consensus Map

Association mapping is usually performed by testing the correlation between a single marker and phenotypes. However, because patterns of variation within genomes are inherited as blocks, clustering markers into haplotypes for genome-wide scans could be a worthwhile approach to improve statistical power to detect associations. The availability of high-density molecular data allows the possibility to assess the potential of both approaches to identify marker-trait associations in durum wheat. In the present study, we used single marker- and haplotype-based approaches to identify loci associated with semolina and pasta colour in durum wheat, the main objective being to evaluate the potential benefits of haplotype-based analysis for identifying quantitative trait loci. One hundred sixty-nine durum lines were genotyped using the Illumina 90K Infinium iSelect assay, and 12,234 polymorphic single nucleotide polymorphism (SNP) markers were generated and used to assess the population structure and the linkage disequilibrium (LD) patterns. A total of 8,581 SNPs previously localized to a high-density consensus map were clustered into 406 haplotype blocks based on the average LD distance of 5.3 cM. Combining multiple SNPs into haplotype blocks increased the average polymorphism information content (PIC) from 0.27 per SNP to 0.50 per haplotype. The haplotype-based analysis identified 12 loci associated with grain pigment colour traits, including the five loci identified by the single marker-based analysis. Furthermore, the haplotype-based analysis resulted in an increase of the phenotypic variance explained (50.4% on average) and the allelic effect (33.7% on average) when compared to single marker analysis. The presence of multiple allelic combinations within each haplotype locus offers potential for screening the most favorable haplotype series and may facilitate marker-assisted selection of grain pigment colour in durum wheat. These results suggest a benefit of haplotype-based analysis over single marker analysis to detect loci associated with colour traits in durum wheat.

Construction of BAC Libraries from Flow-Sorted Chromosomes

Cloned DNA libraries in bacterial artificial chromosome (BAC) are the most widely used form of large-insert DNA libraries. BAC libraries are typically represented by ordered clones derived from genomic DNA of a particular organism. In the case of large eukaryotic genomes, whole-genome libraries consist of a hundred thousand to a million clones, which make their handling and screening a daunting task. The labor and cost of working with whole-genome libraries can be greatly reduced by constructing a library derived from a smaller part of the genome. Here we describe construction of BAC libraries from mitotic chromosomes purified by flow cytometric sorting. Chromosome-specific BAC libraries facilitate positional gene cloning, physical mapping, and sequencing in complex plant genomes.

Suppressed recombination and unique candidate genes in the divergent haplotype encoding Fhb1, a major Fusarium head blight resistance locus in wheat

Fine mapping and sequencing revealed 28 genes in the non-recombining haplotype containing Fhb1 . Of these, only a GDSL lipase gene shows a pathogen-dependent expression pattern. Fhb1 is a prominent Fusarium head blight resistance locus of wheat, which has been successfully introgressed in adapted breeding material, where it confers a significant increase in overall resistance to the causal pathogen Fusarium graminearum and the fungal virulence factor and mycotoxin deoxynivalenol. The Fhb1 region has been resolved for the susceptible wheat reference genotype Chinese Spring, yet the causal gene itself has not been identified in resistant cultivars. Here, we report the establishment of a 1 Mb contig embracing Fhb1 in the donor line CM-82036. Sequencing revealed that the region of Fhb1 deviates from the Chinese Spring reference in DNA size and gene content, which explains the repressed recombination at the locus in the performed fine mapping. Differences in genes expression between near-isogenic lines segregating for Fhb1 challenged with F. graminearum or treated with mock were investigated in a time-course experiment by RNA sequencing. Several candidate genes were identified, including a pathogen-responsive GDSL lipase absent in susceptible lines. The sequence of the Fhb1 region, the resulting list of candidate genes, and near-diagnostic KASP markers for Fhb1 constitute a valuable resource for breeding and further studies aiming to identify the gene(s) responsible for F. graminearum and deoxynivalenol resistance.

Haplotype divergence and multiple candidate genes at Rphq2, a partial resistance QTL of barley to Puccinia hordei

KEY MESSAGE

Rphq2, a minor gene for partial resistance to Puccinia hordei , was physically mapped in a 188 kbp introgression with suppressed recombination between haplotypes of rphq2 and Rphq2 barley cultivars.

ABSTRACT

Partial and non-host resistances to rust fungi in barley (Hordeum vulgare) may be based on pathogen-associated molecular pattern (PAMP)-triggered immunity. Understanding partial resistance may help to understand non-host resistance, and vice versa. We constructed two non-gridded BAC libraries from cultivar Vada and line SusPtrit. Vada is immune to non-adapted Puccinia rust fungi, and partially resistant to P. hordei. SusPtrit is susceptible to several non-adapted rust fungi, and has been used for mapping QTLs for non-host and partial resistance. The BAC libraries help to identify genes determining the natural variation for partial and non-host resistances of barley to rust fungi. A major-effect QTL, Rphq2, for partial resistance to P. hordei was mapped in a complete Vada and an incomplete SusPtrit contig. The physical distance between the markers flanking Rphq2 was 195 Kbp in Vada and at least 226 Kbp in SusPtrit. This marker interval was predicted to contain 12 genes in either accession, of which only five genes were in common. The haplotypes represented by Vada and SusPtrit were found in 57 and 43%, respectively, of a 194 barley accessions panel. The lack of homology between the two haplotypes probably explains the suppression of recombination in the Rphq2 area and limit further genetic resolution in fine mapping. The possible candidate genes for Rphq2 encode peroxidases, kinases and a member of seven-in-absentia protein family. This result suggests that Rphq2 does not belong to the NB-LRR gene family and does not resemble any of the partial resistance genes cloned previously.

Dynamic evolution of resistance gene analogs in the orthologous genomic regions of powdery mildew resistance gene MlIW170 in Triticum dicoccoides and Aegilops tauschii

Rapid evolution of powdery mildew resistance gene MlIW170 orthologous genomic regions in wheat subgenomes. Wheat is one of the most important staple grain crops in the world and also an excellent model for plant ploidy evolution research with different ploidy levels from diploid to hexaploid. Powdery mildew disease caused by Blumeria graminis f.sp. tritici can result in significant loss in both grain yield and quality in wheat. In this study, the wheat powdery mildew resistance gene MlIW170 locus located at the Triticum dicoccoides chromosome 2B short arm was further characterized by constructing and sequencing a BAC-based physical map contig covering a 0.3 cM genetic distance region (880 kb) and developing additional markers to delineate the resistance gene within a 0.16 cM genetic interval (372 kb). Comparative analyses of the T. dicoccoides 2BS region with the orthologous Aegilops tauschii 2DS region showed great gene colinearity, including the structure organization of both types of RGA1/2-like and RPS2-like resistance genes. Comparative analyses with the orthologous regions from Brachypodium and rice genomes revealed considerable dynamic evolutionary changes that have re-shaped this MlIW170 region in the wheat genome, resulting in a high number of non-syntenic genes including resistance-related genes. This result might reflect the rapid evolution in R-gene regions. Phylogenetic analysis on these resistance-related gene sequences indicated the duplication of these genes in the MlIW170 region, occurred before the separation of the wheat B and D genomes.