| The rationale of molecular breeding approach of selected advanced backcross introgression is to use widely grown varieties as recurrent parents to develop advanced backcross populations which are imposed for screening on target traits and genotyping selected introgressed lines (ILs) to discover QTLeffectively, then develop new crop varieties with improved target traits assisted by molecular markers. The advantage of this method is to combine the QTL discovery with the practice of crop breeding, increase the power and efficiency of gene discovery of germplasm, thereby become one of the prior approach in rice gene discovery and variety improvement.In this study, Shuhui 527 and Minghui 86, widely used elite indica restorers, were applied as recurrent parents, 2004 as donors for developing two BC2F2:4 advanced backcrossing populations, which contain 233 and 226 lines respectively. The genetic composition and marker distortion of two advanced backcrossing populations were analyzed. At the same time, The QTL identification under normal irrigation and drought stress conditions were conducted with the developed populations. The main results were as follows:1. The analysis of segregation distortion of molecular markersChi-square test was conducted on gene frequency of molecular markers in the populations of Shuhui 527/2004 and Minghui 86/2004. There are 42 markers significantly distorted from the expected segregation in the population of Shuhui 527/2004 (p<0.05), accounted for 52.50% of statistics locis, and distortion of 35 markers achieved extremely significant level, accounted for 43.75% of statistics locis, among which 10 distortion segregation hotspot regions were found. There are 60 markers significantly distorted from the expected segregation in the population of Minghui 86/ 2004 (p<0.05), accounted for 75.95% of statistics locis, and the distortion of 54 markers achieved extremely significant level, accounted for 68.35% of statistics locis. A total of 11 distortion segregation hotspot regions were detected in this population. All of the hotspot loci of distortion segregation deviated toward donor in both populations.2. Phenotypic performance of of advanced backcrossing introgression lines under irrigation and drought conditionsCorrelation analysis indicated that in normal condition, yield per plant showed positive correlation with thousand grain weight at the significant level of p<0.05 in the population of Shuhui 527/2004, and positive correlation with plant height, panicle per plant and spikelet fertility at the significant level of p <0.01. In the population of Minghui 86/2004, yield per plant were significantly and positively correlated with all other traits. Similar correlations could be observed in both populations under drought stress.Comprehensive analysis of coefficient of variation and drought-resistant coefficient showed that yield per plant and spikelet fertility were most susceptible to drought,followed by plant height,and less susceptible traits were panicle per plant and thousand grain weigh. The average yield per plant of parents and populations decreased under drought stress, and there was difference of yield loss among parents and populations.3. Analysis of QTLmapping under irrigation and drought conditions using advanced backcrossing introgression linesThere are total of 30 QTLs detected in Shuhui 527/2004 population, which included 7 QTLs of plant height, 5 QTLs of fertility rate, 3 QTLs of panicle per plant, 5 QTLs of thousand grain weight and 10 QTLs of yield per plant. Among the 30 QTLs detected, 16 QTLs were mapped only in normal condition, 10 QTLs were detected only under drought stress, 4 QTLs were detected both in the two conditions. If removing 4 tightly linked or pleiotropic QTLs, only 26 QTLs were detected under two water treatments.The tightly linked or pleiotropic QTL, qph4b controlled plant height on chromosome 4 and explained 8.04% and 9.28% of phenotypic variation of plant height under normal condition and drought stress, could also be detected for thousand grain weight under normal condition and explained 4.93% of phenotypic variation.A total of 40 main effect QTLs were identified in Minghui 86/2004 population, which contain 9 QTLs of plant height, 5 QTLs of fertility rate, 4 QTLs of panicle per plant, 13 QTLs of thousand grain weight and 9 QTLs of yield per plant. Removed duplicate loci (tightly linked or pleiotropic), there are actually 31 QTLs detected at last, among which 17 QTLs were mapped only in normal condition, 11 QTLs were detected only under drought stress and 12 QTLs were detected under both conditions. There were 9 tightly linked or pleiotropic loci found in this population. qtgw1 and qtgw3b underlying thousand grain weight on chromosome 1 and chromosome 3 showed large effects, which can be detected under both conditions and explained 16.09% and 13.8% of phenotypic variation of thousand grain weight under normal condition, 11.51% and 13.69% of phenotypic variation under drought stress. They are also main QTLs controled fertility rate, explained 21.91% and 6.88% of phenotypic variation. under normal condition.
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