Simultaneous Improvement And Genetic Dissection For Grain Yield,drought Tolerance And Rice Yellow Mottle Virus Resistance In Rainfed Lowland Rice(Oryza Sativa L.)

Breeding for improvement of multiple traits is the most feasible option for sustainable increase in rice production in rainfed lowland ecosystem.Five BC₁F₂ Weed Tolerance Rice No.1（WTR1）populations were adopted for evaluation for adaptation in the rainfed lowland ecosystem.The evaluation focused on improvement of three target traits:high grain yield（HY）,drought tolerance（DT）and resistance to rice yellow mottle virus（RYMV）.High grain yield was adopted as the basic criterion for selection of promising lines in addition to disease free phenotype.The BC₁F₃ populations were concurrently screened for RYMV resistance by field evaluation under both severe drought and irrigation（normal）conditions at Bagamoyo（Tanzania）.A total of 65 selective introgression lines（ILs）with RYMV tolerances and better grain yield（GY）than RP and local check;including 31 from irrigation and34 from drought stress were advanced for the progeny test.The promising ILs were progeny tested for the yield component traits under both irrigation and drought stress at three different locations in:Bagamoyo（BM,Tanzania）,Beijing（BJ,Northern China）,and Hainan（HN,Southern China）.Resistance to RYMV was tested by artificial inoculation of the most virulent RYMV strain S4 at the seedling stage in screen house in Tanzania.Considering the different environmental factors that could have affected the behaviors of the ILs,we adopted a relatively stringent threshold of 3%higher GY than the RP which was higher than the local control.Also we studied the genetic characteristics of the promising WTR1 ILs for the gene/QTL governing the RYMV resistance,drought tolerance and high grain yield under multiple stresses.Among the selected 65 ILs,a total of 35（53.8%）were approved to be resistant（12）or moderately resistant（23）to RYMV by artificial inoculation.Only 20%and 12.5%of the resistance and moderate resistance phenotype of the ILs was associated with rymv1-2 allele found in resistant cultivar Gigante.A main effect QTL qDfs1-5 was identified in the distal end of long arm of chromosome 1 collocated with RYMV–QTL and near RYMV 2 gene;a gene reported only in O.glaberrima.The QTL spans the region between 36380096-40051014 bp and explained more than 80%of the phenotypic variances of the ILs.A total of 29 ILs（44.6%）were found to be better than RP in GY under irrigation and/or drought stress conditions.Of these 29 ILs,7 lines showed improved performance in all three target traits（HY,DT&RYMV）,14 ILs showed improved performance of 2 target traits:5（DT&HY）,6（HY&RYMV）and 3（DT&RYMV）the rest 4 ILs improved only the HY and 4 DT only.The variation of the yield component traits of the ILs was significantly high across treatment conditions except for PN under drought stress.The variations were mainly accounted by environmental factors,genotype differences,and the interactions between the environment and genotype.Genome wide association study revealed several lead SNPs significantly associated with yield component traits across treatment,location and or season.Interestingly 24 lead SNPs were significantly associated with two or more yield component traits across treatment,location and or season.Genetic overlaps（GOs）between RYMV and GY under irrigation and/or drought stress were found in this work.The main effect QTL,qDfs1-5,was found to harbor a number of minor effect QTL for high grain yield in rice in addition to other 8 GO regions combining the RYMV resistance QTL and high grain yield QTL.Among them 4 GO regions contained high grain yield QTL under drought condition.The WTR1 ILs exhibited significantly high variability and medium to high heritability of the yield component traits;thus contains great potential for further improvements.Use of multiple donor parents of diverse superior qualities and backcross breeding is an effective means of introducing new alleles into elite varieties.Selection for high grain yield in early generation backcross under severe stress and normal condition resulted into early identification of elite lines and subsequently retained the trait heritability potential.The locating of the genetic overlapping region for RYMV resistance,drought tolerance,and high grain yield traits is an important step towards improvement for multiple stress tolerances and it offers wide opportunity to explore the relationship between high grain yield and disease resistance in rice.