Mapping And Pyramiding Of Drought Tolerant QTLs By Using Selected Backcross Introgression Lines In Rice

Drought is one of the most important factors limiting the production in rice (Oryza sativa L.). It is the most simple and effective methods to improve drought tolerance (DT) of rice varities by discovering and applying DT QTLs from germplasm. The new strategy of molecular breeding to combine QTL discovery with crop breeding can be conducted by screening the target trait of introgressed lines (ILs) derived from large scale backcross and genotyping ILs. Furtherly, DT QTLs can be pyramided by intercross DT ILs with favorable DT QTLs from different donors assisted by marker selection to develop DT rice varieties.In this study, C418, an elite japonica restorer of Northern China, was used as recurrent parent to cross and backcross with C71 and Zaoxian 14 (donors) respectively, from which the random BC₂F₂ populations were developed. The bulks of 2 BC2F2 populations were imposed critical selections for drought tolerance (DT) under severe drought stress, from which a total of 16 introgression lines (ILs) survived the stress. The progeny of the DT BC₂F₅ ILs were repeatedly evaluated for DT and genotyped with molecular markers for DT QTL detection. Pyramiding populations were developed by making crosses between 4 of DT ILs and imposed DT selections in F₂ under the severe drought condition, from which 2 sets of selected DT pyramiding lines (PLs) were repeatedly phenotyped for DT and genotyped with molecular markers for QTL analysis. The main results summarized as follows:1. The progeny test of 2 sets of DT ILs were conducted for DT under drought and irrigation conditions in Hainan and Beijing. There existed some variations between populations, experiment locations and water treatments. In summary, no significant differences were observed between recurrent parent and the ILs in DT with the grain yield as the DT index. However, some ILs in the progeny performed much better DT and yield than the recurrent parent. Among the grain yield components, panicles per plant and grains per panicle were closely correlated with grain yield and were much more susceptible to drought stress. The ILs under drought condition with more panicles per plant and more grains per panicle showed higher yield.The DT ILs were genotyped with 106 well distributed polymorphic SSR markers. A total of 16 DT QTLs were identified through X² test on introgression frequency (P＜0.0005), distributed on the whole genome except for chromosome 2 and 7. The additive effects of donor alleles of most DT QTLs were positive. The results in this study suggested it was practical to improve rice DT and identify favorable DT genes/QTLs by backcrossing and imposing critical selection on the target trait.2. The progeny test of 2 sets of DT pyramiding lines under the drought and irrigation conditions in Hainan were imposed for DT. The results indicated that the PLs had better DT and higher yield in normal irrigation than drought stress. However, the average DT and yield of most PLs were less than recurrent parent and/or ILs parents up to the performance of parental ILs. Few positive overparent PLs could be found if the parental ILs were excellent in DT and yield. The analysis of correlation and path suggested that the PLs under drought condition with more panicles per plant and more grains per panicle had better DT and higher yield.X² tests and One-way ANOVA were used to identify DT QTLs. A total of 26 and 23 QTLs (P＜0.001) were identified in 2 pyramiding populations respectively by X2 test, among which 9 QTLs could be detected in both of the populations. ANOVA detected 28 and 5 DT QTLs (yield and fertility rate as DT index) in two populations, 16 of which were overlapped to ones mapped by X² test. Most DT QTLs declared in this study were consistent with those documented before.Gametic linkage disequilibrium (LD) analyses discovered 2 association loops (ALs) or QTL groups among 20 and 8 DT QTLs in 2 populations respectively, which suggested that there might exist the putative genetic network related to DT. All PLs could be classified into 3 genotypic groups (GG) in both populations based on genotype of QTL groups. The more the ALs with higher introgressed alleles, the better the DT and yield.