Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. II. Grain yield components

The genetic basis underlying inbreeding depression and heterosis for three grain yield components of rice was investigated in five interrelated mappin g populations using a complete RFLP linkage map, replicated phenotyping, an d the mixed model approach. The populations included 254 F-10 recombinant i nbred lines (RILs) derived from a cross between Lemont (japonica) and Teqin g (indica), two backcross (BC) and two testcross populations derived from c rosses between the RILs and the parents plus two testers (Zhong413 and IR64 ). For the yield components, the RILs showed significant inbreeding depress ion and hybrid breakdown, and the BC and testcross populations showed high levels of heterosis. The average performance of the BC or testcross hybrids was largely determined by heterosis. The inbreeding depression values of i ndividual RILs were negatively associated with the heterosis measurements o f the BC or testcross hybrids. We identified man), epistatic QTL pairs and a few main-effect QTL responsible for > 65% of the phenotypic variation of (lie yield components in each of the populations. Most epistasis occurred b etween complementary loci, suggesting that grain yield components were asso ciated more with multilocus genotypes than with specific alleles at individ ual loci. Overdominance was also an important property of most loci associa ted with heterosis, particularly for panicles per plant and grains per pani cle. Two independent groups of genes appeared to affect grain weight: one s howing primarily nonadditive gene action explained 62.1 % of the heterotic variation of the trait, and the other exhibiting only additive gene action accounted for 28.1 % or the total trait variation of the F-1 mean values. W e found no evidence suggesting that pseudo-overdominance from the repulsive linkage of completely or partially dominant QTL for yield components resul ted in the overdominant QTL for grain yield. Pronounced overdominance resul ting from epistasis expressed by multilocus genotypes appeared to explain t he long-standing dilemma of how inbreeding depression Could arise from over dominant genes.