Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. I. Biomass and grain yield

To understand the genetic basis of inbreeding depression and heterosis in r ice, main-effect and epistatic QTL associated with inbreeding depression an d heterosis for grain yield and biomass in five related rice mapping popula tions were investigated using a complete RFLP linkage map of 182 markers, r eplicated phenotyping experiments, and the mixed model approach. The mappin g populations included 254 F-m recombinant inbred lines derived from a cros s between Lemont (japonica) and Teqing (indica) and two BC and two testcros s hybrid populations derived from crosses between the RILs and their parent s plus two testers (Zhong 413 and IR64). For both BY and GY, there was sign ificant inbreeding depression detected in the RI population and a high leve l of heterosis in each of the BG and testcross hybrid populations. The mean performance of the BC or testcross hybrids was largely determined by their heterosis measurements. The hybrid breakdown (part of inbreeding depressio n) values of individual RILs were negatively associated with the heterosis measurements of their BC or testcross hybrids, indicating the partial genet ic overlap of genes causing hybrid breakdown and heterosis in rice. A large number of epistatic QTL pairs and a few main-effect QTL were identified, w hich were responsible for > 65% of the phenotypic variation of BY and GY in each of the populations with the former explaining a much greater portion of the variation. Two conclusions concerning the loci associated with inbre eding depression and heterosis in rice were reached from our results. First , most QTL associated with inbreeding depression and heterosis in rice appe ared to be involved in epistasis. Second, most (similar to 90%) QTL contrib uting to heterosis appeared to be overdominant. These observations tend to implicate epistasis and overdominance, rather than dominance, as the major genetic basis of heterosis in rice. The implications of our results in rice evolution and improvement are discussed.