Abstract: The traditional mapping population with complicated genetic backbround is usually derived from a cross between two parents with a lot of unfavorable agronomic traits. The identified QTLs by traditional mapping can not be applied in genetic improvement of crops due to complicated interactions between QTLs and genetic background. The advanced backcross method for QTL mapping with elite variety as recurrent parent could partially eliminate the background effects and integrate QTL mapping with genetic breeding. In present study, using Shuhui 527, an elite indica restorer in China as recurrent parent and Milagrosa from Philippines as donor, the BC₂F₂ advanced backcross population with 199 individuals was developed. A total of 85 polymorphic SSR markers evenly distributed on 12 chromosomes were applied to genotype the mapping population. The grain length, grain width, ratio of grain length to width and thousand grain weight of the parents and the BC₂F₂ population were evaluated after har-vesting. The QTL identification of the above traits was conducted by one-way ANOVA (for single QTL) and two-way ANOVA (for digenic epistatic interaction loci). Among ten QTLs for grain length, grain width, ratio of grain length to width and thousand grain weight detected by one-way ANOVA (P<0.01), there were three pleiotropic QTLs. QTLs for ratio of grain length to width could be identified in QTLs for grain length due to highly close relationship between them. The results in present study indicated that qgl3b on chromosome 3 was a major QTL controlling grain length, ratio of grain length to width and thousand grain weight, which explained 29.37%, 26.15%, and 17.15% of phenotypic variation of the three traits and showed large additive effects (posi-tive allele from Shuhui 527) and negative overdominant effects, qgw8 was also a major QTL controlling grain width with 21.47% contribution to phenotypic variation and a minor QTL controlling thousand grain weight with 5.16% contribution to phenotypic variation. It had large additive effects (positive allele from Shuhui 527) and positive partial dominant effects in both traits. Two-way ANOVA (P<0.005) showed 61 pairs of digenic epistatic interactions involved in 54 QTLs among which 23 loci had pleiotropism affecting two to four traits and eight loci were the same as those detected by one-way ANOVA. It was observed that eight of thirteen pairs of digenic epistatic interactions related to ratio of grain length to width could be found in grain length. The results above will lay a good foundation in fine mapping, cloning and designed molecular breeding of favorable genes related to rice grain size and shape.

Key words: Rice (Oryza sativa L.), Grain length, Grain width, Ratio of length and width, Thousand grain weight, QTL detection