Identification of Quantitative Trait Loci Across Recombinant Inbred Lines and Testcross Populations for Traits of Agronomic Importance in Rice

Abstract

This study was conducted to determine whether quantitative trait loci (QTL) controlling traits of agronomic importance detected in recombinant inbred lines (RILs) are also expressed in testcross (TC) hybrids of rice. A genetic map was constructed using an RIL population derived from a cross between B5 and Minghui 63, a parent of the most widely grown hybrid rice cultivar in China. Four TC hybrid populations were produced by crossing the RILs with three maintaining lines for the widely used cytoplasmic male-sterile (CMS) lines and the genic male-sterile line Peiai64s. The mean values of the RILs for the seven traits investigated were significantly correlated to those of the F 1 hybrids in the four TC populations. Twenty-seven main-effect QTL were identified in the RILs. Of these, the QTL that had the strongest effect on each of the seven traits in the RILs was detected in two or more of the TC populations, and six other QTL were detected in one TC population. Epistatic analysis revealed that the effect of epistatic QTL was relatively weak and cross combination specific. Searching publicly available QTL data in rice revealed the positional convergence of the QTL with the strongest effect in a wide range of populations and under different environments. Since the main-effect QTL is expressed across different testers, and in different genetic backgrounds and environments, it is a valuable target for gene manipulation and for further application in rice breeding. When a restorer line that expresses main-effect QTL is bred, it could be used in a number of cross combinations. H ETEROSIS has been very successfully exploited in diverse plants and animals. In agriculture, hybrid varieties contribute strongly worldwide to the production of many crop species, including the most important food crops, such as maize and rice (Stuber 1994;Yuan 1998;Khush 2001). Hybrid rice has a yield advantage of 15-20% over the best commercial rice varieties. The area planted to hybrid rice in China accounts for .50% of the total rice area of the country at present. The cultivation of hybrid rice has started on a large scale in many Asian countries.Several hypotheses have been proposed to explain the genetic basis of heterosis. The dominance hypothesis (Bruce 1910) proposes that dominant factors from either parent mask deleterious recessive mutations from the other parent in the heterozygous F 1 population. In contrast, the overdominance hypothesis (Shull 1908) holds that heterozygosity at single loci confers properties that are superior to either homozygote. The two hypotheses have been verified with molecular biology experiments (Stuber et al. 1992;Xiao et al. 1995). A third hypothesis suggests that heterosis may arise from epistasis between alleles at different loci (Yu et al. 1997;Goodnight 1999). More recently, further results have suggested that epistasis is the primary genetic basis of heterosis. It is suggested that separate efforts should be taken for breeding high-yielding inbred and hybrid cultivars in rice Luo et al. 2001)....

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