QTL Mapping And Candidate Gene Cloning Of Grain Weight And Its Related Traits In Common Wheat(Triticum Aestivum L.)

Wheat is the third largest crop and the enhancement of its yield potential is crucial to ensure the state food security in China.Thus,yield is an important breeding objective in wheat breeding programs.The identification of quantitative trait loci controlling grain weight,which are constitutively expressed among multiple environments or adaptively expressed in specific environment,will facilitate incorporating grain weight QTL into widely or regionally adapted cultivars in a marker assisted selection manner.In the present study,one hundred and ninety one recombinant inbred lines from a cross between ND3488(large grain)and ND459(small grain)wheat genotypes were evaluated for agronomic performance under 10 environments(8 winter-sowing conditions and 2 spring-sowing conditions).The objective of this study was to identify chromosome regions associated with QTL controlling grain weight,grain size and spike morphology under drought-heat(at the terminal growth stages)and normal field conditions.For fine mapping of one QTL cluster on chromosome 6A,QTgw-6A,F2 and F2:3 populations from a cross between ND3488 and ND459 were used for screening recombinant plants.Based on linkage analysis,gene expression,promoter activity test and single marker regression analysis,TaGW2-6A was supposed to be the candidate gene for QTgw-6A.1 region.The main results obtained are as follows:1.SSR markers and Illumina(?)Infinium 90K iSelect(?)genechip were employed to construct one genetic linkage map using the ND3488/ND459 RIL population.This map included 10,815 SNP loci and 176 SSR loci,covering 2930.13 cM and 2030 marker bins,with an average interval distance of 1.44 cM.By performing bioinformatics analysis,5527 wheat unigenes,1021 chromosome deletion bin-mapped ESTs,3675 Brachypodium genes and 3685 rice genes were incorporated onto the genetic linkage map to enhance its value in use.2.Composite interval mapping(CIM)was performed to detect QTL for 9 agronomic traits(thousand grain weights,grain area,grain circle,grain length,grain width,spike length,spikelet number per spike,grain number per spike and grain weight per spike)under 8 winter-sowing environments and 2 spring-sowing environments.The best linear unbiased predictions of 9 traits among the 2 sowing regimes were also analyzed by CIM mapping.A total of 233 minor and major QTL were detected,most of which were mapped on chromosomes 1B,2B,5A,5B,6A and 7A.One hundred and nineteen QTLs of these were constitutive QTL,explaining 3.45%-50.55%of the phenotypic variation.Eleven constitutive QTL for thousand grain weights(TGW)were specifically expressed under winter-sowing environments,among which QTgw-6A.1,QTgw-6A.2 and QTgw-6A.3 were three major QTL,explaining 6.95%-12.85%,10.15%-24.00%and 10.20%-15.87%of the phenotypic variation,respectively.Four constitutive QTL for TGW were specifically expressed under spring-sowing environments,among which QTgw-5A.5 and QTgw-5A.6 were two major QTL,explaining 20.62%-50.55%and 13.99%-37.78%of the phenotypic variation,respectively.3.By performing comparative analysis with the genomes of Aegilops tauschii,Brachypodium and rice,30 chromosome 6A specific SSR markers were explored and enriched for QTgw-6A region.By screening recombinant plants from one ND3488/ND459 F2 population,QTgw-6A region was dissected into 3 fragments,namely QTgw-6A.1,QTgw-6A.2 and QTgw-6A.3.Furthermore,fragments QTgw-6A.1 and QTgw-6A.3 were found to be associated with grain weight and grain size using 10 ND3488/ND459 F2:3 populations.4.TaGW2-6A,which was homologous to rice OsGW2,was isolated from both ND3488 and ND459.Comparing with ND459,the TaGW2-6A promoter in ND3488 had a 114-bp deletion,which decreased its promoter activity,then contributed to lower expression level in lemma.Based on the above 114-bp deletion,one co-dominant,functional marker TaGW2-6Atgw was developed,which amplified 222 or 336-bp fragments in ND3488 and ND459,respectively.Remarkably,TaGW2-6Atgw was co-localized with Xbarc118 on QTgw-6A.1 fragment in the ND3488/ND459 RIL population.Furthermore,one residual heterozygous F2 population(RHL186-F2)was used to elucidate the genetic effects of TaGW2-6A on grain weight and grain size in wheat.Results showed that TaGW2-6A was significantly correlated with TGW,grain area(GA),grain circle(GC),grain length(GL)and grain width(GW).RHL186-F2 individuals with ND3488 genotype had an increase in TGW,GA,GC,GL and GW by 11.84,6.84,2.42,1.83 and 5.19%,respectively.In addition,876 hexaploid wheat accessions and 238 tetraploid wheat accessions were used to check the genotype frequency of TaGW2-6A,among which only three hexaploid wheat accessions had the same genotype with ND3488.Collectively,it could be concluded that TaGW2-6A is the candidate gene for QTgw-6A.1 region.