Plant & Animal Genome IX Conference

Workshop: Molecular Markers
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GENETIC BASIS OF INBREEDING DEPRESSION AND HETERIOSIS IN RICE (Oryza sativa L.)

¹ Plant Breeding, Genetics, and Biochemistry Division, International Rice Research Institute, P.O. Box 3127, 1271 Makati City, Philippines

² China National Rice Research Institute, 359 Ti-Yu-Chang Rd, Hangzhou, China

³ Dept. of Agronomy, Zhejiang Agricultural University, Hangzhou, China

⁴ Dept. of Crop and Soil Sciences, Texas A&M University, College Station, Texas 77843, USA

The genetic basis underlying inbreeding depression and heterosis for grain yield and its components (biomass, grains per panicle, panicles per plant and grain weight) was investigated in five inter-related rice mapping populations using a complete RFLP linkage map, replicated phynotyping, and the mixed model approach. The populations included 254 F10 recombinant inbred lines (RILs) derived from a cross between Lemont (japonica) and Teqing (indica), two BC and two testcross populations derived from crosses between the RILs and their parents plus two testers (Zhong413 and IR64). For yield and its components, the RILs showed significant inbreeding depression and hybrid breakdown, while the BC and testcross populations showed high levels of heterosis. The average performance of BC or testcross hybrids was largely determined by heterosis. The inbreeding depression values of individual RILs were negatively associated with the heterosis measurements of the BC or testcross hybrids. We identified many epistatic QTL pairs and a few main-effect QTLs, which were responsible for over 65% of the phenotypic variation of the traits in each of the populations. Two conclusions concerning the loci associated with inbreeding depression and heterosis in rice have been reached. First, most QTLs associated with inbreeding depression and heterosis in rice appeared to be involved in epistasis. Most epistasis occurred between complementary loci, suggesting that grain yield components are associated more with multilocus genotypes than with specific alleles at individual loci. Second, most (~90%) QTLs contributing to heterosis appeared to be overdominant, particularly for grain yield, biomass, panicles per plant, and grains per panicle. There appeared two independent groups of genes affecting grain weight. The first group of primarily non-additive gene action explained 62.1% of the trait variation, and the other exhibiting only additive gene action accounted for 28.1% of the total trait variation of the F1 mean values. We found no evidence suggesting that pseudo-overdominance from the repulsive linkage of completely or partially dominant QTLs for yield components resulted in the overdominant QTLs for grain yield. These observations tend to implicate epistasis and overdominance, rather than dominance as the major genetic basis of heterosis in rice. Pronounced overdominance resulted from epistasis expressed by multilocus genotypes appeared to explain the long-standing dilemma of how inbreeding depression could arise from overdominant genes. The implications of our results in rice evolution and improvement were discussed.