In the previous study (Yuan et al. 2009), a quantitative trait locus (QTL) for grain weight was detected on the short arm of chromosome 5 using an advanced backcross lines (BC3F3) between Hwayeongbye (Oryza sativa) and W1944(Oryza rufipogon Griff.) .For detection of gw5 locus, a line CR6 (BC3F4) was selected and crossed to Hwayeongbyeo produce S1F2 and S1F3 population. And a plant from S1F3 population, carried W1944 homozygous segment for target region at gw5 was crossed to Hwayeong to produce S2F2 population. All these population including some S1F3 lines were grown in the field in 2007, 2008 and 2009, respectively (fig1). Frequency distribution of grain weight followed the Mendelian ratios(3:1) for single locus segregation (Χ2=1.22, 0.76, 1.34 in 2007, 2008 and 2009 respectively).In Hwayeongbye genetic background, the W1944 allele at the gw5 locusde creased grain weight, QTL analysis showed that gw5 co-segregated with RM18003 and RM194 (R2=62.7, 69.5 and 37.1% in 2007, 2008 and 2009 respectively). Addition, five QTLs plant height, culm length, secondary branch, spikelet number perplant and rationing ability were detected in the region around gw5, in 2008 and 2009. Substitution mapping with 32F3 lines, gw5 QTL was flanked by two SSRmarkers, RM18003 and RM194, in a300kb to 1.7Mb physical distance region,. QTL analysis indicated that 5 others QTLs plant height, culm length, secondary branch, spikelet number per plant and rationing ability were tightly linked.

Grain weight (GW) or grain size is the most important target not only as a major component of grain yield, but also associated with the cooking quality in rice breeding program. In a previous study, two loci for grain weight, gw8 and gw9, have been fine map to about 306.4kb and 37.4kb, respectively, in backcross populations derived from a cross between the Korean japonica cultivar Hwaseongbyeo and Oryza rufipigon (IRGC 105491). Both O.rufipogon alleles increased GW in the Hwaseongbyeo background despite the fact that O. rufipogon was the small-seeded parent. To test the effect of pyramiding the QTLs for grain weight, an F2 population consisted of 226 individuals was developed derived from the cross between the two QTLs (gw8 and gw9), and used MAS to select nearly isgenic lines for two QTLs and a pyramiding line. Molecular genotyping of F2 population revealed the existence of interaction between gw8 and gw9. Moreover, the interaction was also confirmed by phenotypic analysis of four QTL-NILs. The results suggest that epistasis interaction is important genetic basis in determining yield traits in rice.