Physical Mapping Powdery Mildew Resistance Gene MIIW170 Derived From Wild Emmer And Comparative Analyses With 2DS Orthologous Region Of Aeilops Tauschii

Wheat is one of the most important staple grain crops in the world and has a special significance to human being.China has been the world's largest wheat producer,importer and consumer.However,wheat disease causes tremendous yield loss every year and is one of the major threads in world hunger.Among the wheat diseases,powdery mildew caused by Blumeria graminis f.sp.tritici(Bgt)is one of the most devastating diseases that can severely reduce wheat production.The discovery of resistance genes and their use in breeding program are considered an economical and effective way to control wheat diseases.Bread wheat(Triticum aestivum)is hexaploid with three closely related subgenomes designated A,B and D.Wild emmer(T.dicoccoides)(AABB,2n = 4X = 28),the immediate progenitor of both cultivated tetraploid and hexaploid wheats,is the great source for high resistance to powdery mildew.In this study,a high-density genetic linkage map of powdery mildew resistance gene MlIW170,originated from wild emmer accession IW170 and located on the short arm of chromosome 2B,was constructed.Also,a physical map ofMlIW17 was constructed through chromosome landing and chromosome walking by screening a BAC library of wild emmer accession TZ-2.The MlIW170 orthologous sub-genome region on Aegilops tauschii,the D sub-genome donor of common wheat,was identified and sequenced for comparative genemics and phylogenetic analyses.The main results are described as follows:1.Three F2 and F2:3 segregating populations were constructed by crossing T.durum line 81086A and wild emmer accession IW170,common wheat cultivar Chinese Spring and wheat powdery mildew resistance gene Pm26 substitution line CS-Pm26,and common wheat line Xuezao and wild emmer introgression line WE74.Comparative genetic mapping results revealed that the powdery mildew resistance genes MlIWI70,Pm26 and MlWE74 in lines IW170,CS-Pm26 and WE74 are located in one resistance cluster and may be same gene or multiple alleles.2.Molecular markers tightly linked to MlIW170 were used as probes to screen a Hind? BAC library of wild emmer accession TZ-2.Chromosome landing and chromosome walking were applied to identify a series of overlapping BAC clones to construct a physical map covering the genomic region of powdery mildew resistance gene MlIW170.Two Contigs of 590,960 bp(GeneBank accession No.KR021047)and 289,613 bp(GeneBank accession No.KR021048)were assembled after sequencing 12 and 5 overlapping BAC clones distal and proximal to the MlIW 70 locus,respectively,using Illumina MiSeq platform.A total of 2-1 putative genes including7 resistance geneanalogs(RGA)were predicated in this 880 kb scaffold.Additional markers WGGC1115,WGGC1323,WGGC8911,WGGC8969,WGGC8977 and WGGC9140 were developed and MlIW170 is delineated within a 372 kb interval containing at least 5 putative genes.3.The BAC Contig 1774 on Ae.tauschii chromosome 2D physical map was identified as orthologous region of MlIW170 in T.dicoccoides chromosome 2B.The 12 MTP BACs from SNP marker AT2D1036 to AT2D1040 on Contig 1774 were sequenced to generate a 852,217 bp single scaffold(GeneBank accession No.KR021046)using Roche 454 platform.A total of 26 putative genes were predicated including 12 resistance gene analogs.4.Comparative genomics analyses of the T.dicoccoides 2BS region with the orthologous Ae.tauschii 2DS region showed great gene colinearity.Three types of resistance gene analogs were identified as RGA1/2,RPM and RPS2 in the orthologous 2B and 2D genomic regions.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 nonsyntenic genes including resistance related genes.This result might reflect the rapid evolution in R gene regions in wheat genome.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.The comparative analysis of puritive genes from MIIW170 region between wild emmer,Ae.Tauschii and hexaploid common wheat Chinese Spring indicated that some genes kept conservative and others changed during the wheat polyploidization evalution process.