Genetic Analysis And Fine Mapping Of A QTL QSB-9^Tq Conferring Resistance To Sheath Blight In Rice

Sheath blight(SB) disease is one of the three most serious diseases in rice worldwide.The resistance to rice sheath blight is a typical quantitative trait.Until now,approximately 30 QTLs contributing to rice SB resistance had been detected in different populations.Each of the 12 rice chromosomes had been shown to possess one or more of the 30 QTLs.Several SB resistance QTLs(qSB) had been detected on chromosome 9.However,separate research endeavors had generated rice QTL data using different parental individuals and populations.Several researchers had detected a resistance QTL qSB-9^Tq on Teqing chromosome 9.In this study,a set of genetic analyses and fine mapping for qSB-9^Tq were conducted.The strategy of marker-assisted selection(MAS) combined with backcross identification was adopted to construct a backcross combination of Teqing/Lemont(recurrent parent) and two molecular markers RM201 and RM6971 were used for selecting qSB-9^Tq in each generation.In BC₅F₁ generation,qSB-9^Tq was validated and proved that it could be selected by two molecular markers RM201 and RM6971 efficiently.In order to accurately evaluate the resistant effect of qSB-9^Tq in BC₆ generation,seven molecular markers flanking qSB-9^Tq were used for foreground selection and other 114 molecular markers evenly distributed on other regions of rice genome were adopted for background selection.One single BC₆F₁ plant with heterozygous genotype of qSB-9^Tq and the same background as Lemont were gained and harvested.Three genotypic individuals of qSB-9^TqTq,qSB-9^LeLe and qSB-9^TqLe, distinguished by detecting marker genotypes flanking the QTL,were gained in BC₆F₂ generation and then formed the NILs in BC₆F₃ generation in two consecutive years.Two different experiment designs were adopted and both the results suggested that qSB-9^Tq located between two molecular markers RM242 and Y92.5 on chromosome 9,and it was a dominant quantitative resistance QTL(QRL,quantitative resistance locous) which could reduce disease rating about 0.7-1.0 score under Rush's '0-9' disease rating system. Two rice sheath blight(SB) resistance QTLs had been previously detected on chromosome 9 of Jasmine85 and Minghui63,respectively.According to the origin of these resistance alleles,their corresponding QTLs were named qSB-9^J85 and qSB-9^MH63,respectively.An integrated analysis between molecular markers and a rice physical map revealed that the intervals of the two QTLs and qSB-9^Tq were adjacent or partially overlapping.Marker genotype detection combined with backcross identification was applied to construct two backcross populations of Jasmine85/Lemont(recurrent parent) and Minghui63/Lemont(recurrent parent) and then 10 HCSSLs(heterozygous chromosome substitution segment lines) of each backcross combination were gained.Sequentially the two QTL intervals were validated in two different generations.The results suggested that qSB-9^J85 and qSB-9^MH63 were truly located in the interval of RM201 and RM6971 on chromosome 9 and the qSB-9^Tq on chromosome 9 of Teqing might be also in this interval.These three QRLs could reduce a disease rating score by a value of 1.0 for heterozygous genotypes and were speculated to be identical.Moreover,The QTL of resistance to rice sheath blight,mapped on chromosome 11 of Lemont had been validated and named qSB-11^Le.In this study,another QRL on chromosome 7 of Teqing was also confirmed using the same backcross combination of Teqing/Lemont(recurrent parent) and named qSB-7^Tq.Together with qSB-9^Tq,the effects and pyramiding effects of the three QRLs were studied by using a set of near-isogenic lines(NILs) under the background of Lemont.The results indicated that the three QRLs could improve the ability of resistance to rice sheath blight significantly in their single or pyramiding status.Furthermore,there were certain interactions existed among these three QRLs in this study.In order to clone and study the QRL furthermore,and conduct MAS for qSB-9^Tq more efficiently,the strategy of constructing chromosome single substitution segment lines(CSSSLs) was adopted to carried out fine mapping of the QRL.Twenty two polymorphic molecular markers were developed and screened,which could cover the interval with high density that the average physical distance was 319.753kb.These markers were used for detecting the single plant gained from the backcross combination of Teqing/Lemont in Chapter3,and the genotypes of the 22 markers were all heterozygous.Then 894 CSSSLs of 44 classes were gained according to different genotypes of the 22 markers.Two hundred and forty CSSSLs were selected for field experiment with two repeats in 2007.Disease rating and disease index were investigated 30 days after late tillering stage.Then 6 agronomic traits were also investigated and a major QTL conferring tiller angle from Teqing was discovered.In order to eliminate the disturbance of the existence of the QTL, CSSSLs without carrying the QTL were selected to conduct fine mapping for qSB-9^Tq.Two clustering approaches were used for clustering the CSSSLs by analyzing disease rating and disease index in two field repeats together. according to the clustering results the borders of the target QTLs were analyzed.The results indicated that there existed two QRLs in the interval of RM242 and Y93.5,which were L-qSB-9^Tq in the interval Z77.2-Y77.7 and R-qSB-9^Tq in the interval Y86-Y90.2 with their interval distances were 180.875kb and 207.724kb respectively.To sum up,the paper first reported and studied the effect and functional mode of the QTL conferring resistance to rice sheath blight in a relatively resistant cultivar Teqing under the condition of NILs.The QRL exhibited almost complete dominance,which would accelerate the utilization of qSB-9^Tq into the practice of hybrid rice breeding.Furthermore,there existed certain interactions between qSB-7^Tq,qSB-11^Le and qSB-9^Tq suggested the utilization of qSB-9^Tq in QRL pyramiding breeding.So,it is urgent to carry out fine mapping and cloning the QRL,which would benefit the efficiency of MAS breeding and lay a foundation for the theoretical study of the qSB-9^Tq.