| High-yield is always the important goal of wheat breeding. And in the future, the global demand for wheat will present a significant growth trend. Therefore, the emphasis of study is continuously focused on increase of yield per unit area. Yield is a complex quantitative trait, which composed of the effective number of panicle, grain number per spike (GN) and1000-kernals weight (TKW), and controlled by multiple genes. Research showed that increase of yield after the1960was mainly depended on GN and TKW. Therefore, we used the Chinese wheat mini core collection (MCC) to undertake a genome-wide association study (GWAS) of GN and TKW using531SSR markers randomly located on all21chromosomes, and the associated loci were further validated in selected introgression lines (ILs). In addition, we have a detailed analysis on canopy temperature (CT) and chlorophyll content (CC) in ILs, and the relationship between CT/CC and yield/component factors. Then, we also identified loci influencing CT and CC using association mapping. The results and main conclusions were followed.(1) Genome-wide association study indicated that important QTLs influencing GN and TKW were explored. Some associated loci included numerous breeder favorable alleles with strong positive effects. Significant or extremely significant differences were detected on phenotypic values between varieties conveying favored allele and varieties with other alleles. Due to positive effects, these favorable alleles have important value for molecular marker assisted selection and molecular design breeding. Moreover, statistical simulation showed that the favored alleles have additive genetic effects. The nature of breeding is trying to polymerize more and more favored alleles. There was a significantly positive correlation between GN and TKW in modern varieties and ILs. We concluded that artificial selection played a major role. And in these two populations, the numbers of favorable alleles both GN and TKW are increased with breeding process. Moreover, we found that those pleiotropic favored alleles increased the phenotypic value of one trait, without decreased the phenotypic value of other, and some could significantly increase GN and TKW. Similarly, the interactions between loci associated GN and loci associated TKW were not entirely negative, some were even positive.(2) Yield increase depends on harmonious development of three factors. Analysis of genotype indicated that high-yield varieties (such as Z18and HD6172et al) had already included much favorable alleles at associated loci. By hybrid, backcross methods and selection at phenotypic level, the ILs further polymerized a few of favorable alleles. However, the yields of ILs were not significant difference with their receptor. It indicated that it was very difficult to increase yield by change of few loci. And it was also difficult to achieve polymerization of more favorable alleles by phenotypic selection. Therefore, in the future, the combination between conventional breeding and molecular marker-assisted selection is an irresistible trend.(3) The relationships were significantly negative between CT and TKW/yield, while the relationships were significantly positive between CC and TKW/yield. And as time goes on, the degree of relation was gradually increased. Therefore, we concluded that good root system should be the benefit of low CT, which guaranteed the supply of water and nutrition. And low CT should protect chlorophyll and photosynthesis, which made TKW and yield increased.(4) Among loci associated CT and CC, many loci showed simultaneously associated with GN and TKW. Pleiotropy or linkage disequilibrium among genes could be important reason causing the correlation between traits. Similarly with GN and TKW, statistical simulation showed that the favored alleles have additive genetic effects. Pleiotropic favorable alleles showed decreasing CT, increasing CC and improving TKW. Therefore, these pleiotropic favorable alleles should be significant for future high-yield breeding of wheat. |
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