Developing High Yield Hybrids, Probing The Genetic Basis Of Heterosis And Bacterial Blight Resistance In Rice

To break the yield ceiling of indica rice hybrids in China, several important issues in exploiting inter-subspecific heterosis in rice were investigated. The first one was to overcome the partial sterility of inter-subspecific rice hybrids. More than 100 wide compatibility germplasm accessions were identified from a total of 4082 rice accessions. These include three major types: primitive indica or japonica, intermediate type, and hybrid offspring between indica and japonica. A very promising restorer line, Zhong 413 was developed which has both wide compatibility and restorability to CMS. Using Zhong 413 as the male parent to cross with the common commercial CMS lines, a series of inter-subspecific hybrid cultivars (Xieyou 413, Shanyou 413, V-you 413, D-you 413 and Maxieyou 413) were developed. These hybrid cultivaxs showed superior heterosis over the check variety for a wide range of agronomic traits including grain leaf quality, grain components and yield in different locations and years. These hybrids are characterized with rapid growth rate, large leaf area index, heavy panicles, great dry matter production, resistance to lodging, and thus high yield potential. Xieyou 413 and Shanyou 413 were registered in Zhejing (1995) and Sichuan(1996) provinces, respectively. Up to 1997, acreage of Zhong 413 hybrid series reached about 60,000 ha in China.To understand the genetic basis of heterosis, a set of 200 test Fl hybrids were developed using Zhong 413 as the male parent and 200 F10 recombination inbred lines (RILs) as female parents. A complete linkage map with 178 makers was constructed, which covered all 12-rice chromosomes with an average interval of 13.1 cM between markers. Using Zhong413 as the male parent, the level of heterosis for grain yield in the test Fl hybrids was possitively associated with the japonica genome content of the female RILs with the strongest heterosis associated with about 75% japonica genome in the female RILs. Using an improved mixed model approach with adequate background genetic variation control, a total of 17 quantitative trait loci (main-effect QTLs) and 56 pairs of epistatic loci associated with 8 yield related traits were identified and mapped to all 12 rice chromosomes. On average, the main-effect QTLs explained 19.5 10.3% of ths total phenotjpic variatioi in the test Fl population while interactions explained 50.6 15. 9 % cf the total phenotypic variation in the test Fl population. Most of QTLs and epstatic loci were overdominant. The results indicated that the overdominant epistasis loci were largely responsible for high levels ofheterosis observed in the Zhong413 test Fl population.A major gene (Xa4), 10 QTLs and 9 pairs of epistasis loci conferring horizontal resistance to three strains of bacterial blight (Xanthomonas oryza pv oryza) were mapped to 8 of 12 rice chromosomes by using the same set of the RILs and the complete linkage map generated The Teqing allete at Xa4 on chromosome 11 acts as a dominant resistant gene against pathogen race CR4 and CX08, but as an additive QTL with a significantly (47%) reduced effect against the virulent strain, CR6. While Xa4 exhibited a strong degree of race specificity, most QTLs showed consistent levels of resistance against all three strains. The results suggest that a high level durable resistance to bacterial blight may be achieved by cumulative effects of multiple QTLs.