Genetic Dissection Of Main Agronomic Traits And Combining Ability In Maize

Maize is an important feedstuffs,industrial raw materials and feed source.Maize is also one of the model crops for studying heredity,evolution and domestication.The further improvement of the B73 genone and the reduction in the cost of next genome sequencing(NGS)made high-throughput and low repetitive sequencing technology are feasible and efficient to further analyze the complex genetic basis of main agronomic traits in maize combining the recombi-ant inbred lines(RILs)which have high homozygosity,high recombination rate and relatively clear genetic backgrounde Heterosis is the basis for improving yield of maize,while general combining ability(GCA)and special combining ability(SCA)are widely applicated in hybrid breeding.In the present study,a high density genetic map was constructed with 365 RILs based on high-throughput genotyping-by-sequencing(GBS)technique.This map will facilitate identification of genes and exploration of QTL for traits per se,the hybrid performance and combining ability for plant height,flowering time,tassel and yield related traits in maize.It will also be helpful for further research into the mechanisms that control complex agronomic traits,while also providing a basis for marker-assisted selection,The main results are as follows:1.The two founder lines Ye47 8 and Qi319 were sequenced at effective sequencing depths of about 30-fold.A total of 3,549,088 homozygous polymorphic SNPs were identified among parents.The 365 RIL lines were then genotyped using GBS technology and the depth of each line is equivalent to approximately 0.17-fold coverage.Based on the parental polymorphic loci,a total of 88,268 SNPs were retained to deteramine 4,602 bin markers by using sliding window approach.A high-density genetic map was constructed by mapping these 4,602 bin markers.The total genetic distance of the linkage map was 1545,65 cM and the average distance between two adjacent markers was 0.34 cM.Collinearity and QTL analysis of cob color indicated the high power and accuracy of this genetic map in QTL mapping,which lays a foundation for further genetic analysis of complex traits.2.The RIL population was planted during 2015 to 2017 under five environments.Analyses of the QTL on each trait for single environment analysis(SA),and a joint analysis(JA)across five environments were performed based on composite interval mapping(CIM)with high genetic map.A total of 208 and 45 QTL for all traits were detected based on SA and JA,respectively,86%QTL detected by using the two QTL mapping approach were consistent.In total,15 key QTL clusters(QCs)which controlling agronomic traits development were detected,and there were nine QCs associated with flowering time.QC8,which located on chromosome 5(bin5.05)can not only regulate plant height related traits,but also had significant effects on tassel branching number and grain yield per plant.3.Phenotypic analysis of testcross populations showed that the values of hybrid performance and combining ability effects varied widely and showed a continuous and normal distribution under five environments.The analysis of variance of combining ability showed that the predominance of additive gene action was more important for main agronomic traits except grain yield per plant.Correlation analysis showed that significant positive correlation was detected between the GCA effects and the RILs per se for all the traits.A total of 714 QTL were detected for all traits datasets based on SA and 204 were stable QTL which could be detected at least two environments.A total of 231 QTL were identified by JA.Among these QTL,most QTL was significantly associated with GCA effects and 31 major QTL were identified,while only a few QTL were significantly correlated with SCA effects.4.Among the 47 QTL for GCA effects,13 and 41 QTL could be simultaneously detected for RILs per se and hybrid performance,respectively.The tPH5/tEH5-2 locus which located on chromosome 5 was significantly associated with RILs per se,hybrid performance and the GCA effects.The favorable alleles in this locus were mainly transmitted from Ye478 to its derived lines.However,the QTL hotspot on chromosome 10(bin10.04)which had the largest phenotype variation for many traits were only simultaneously detected for these traits in the testcross population and GCA effects.Notably,this locus was replaced in many Ye478 derived lines.In addition,there are many genes which located in the QTL confidence interval for RILs per se,hybrid performance and combining ability effects were the genes that have been cloned in maize or rice by homology-based cloning.Further analysis of the expression patterns of these genes in different conditions and combinations will accelerate to reveal the complex mechanism of heterosis and combining ability on the molecular level.