Grain weight (GW) is one of the most important targets for grain yield in rice breeding. In previous studies, two quantitative trait loci (QTLs) for grain weight, tgw2 and gw8.1, were identified using progeny derived from crosses between the japonica cultivar Hwaseong and Oryza grandiglumis, and between Hwaseong and O. rufipogon (IRGC 105491), respectively. The wild alleles increased GW at two loci. An F₂ population (186 plants) was developed by crossing two near isogenic lines (NILs) harboring tgw2 and gw8.1 to test their interaction. Simple sequence repeat (SSR) markers tightly linked to the two QTLs were used to check the genotype of the F₂ population. Based on the genotype at the two loci, tgw2 and gw8.1, the F₂ plants were classified into 9 groups with a combination of three genotypes at each of the two loci. Two-way ANOVA revealed no interaction between the 2 QTLs in the F₂ population. The 1,000 grain weight (TGW) of homozygous plants with wild alleles at the two loci was significantly higher than that of plants with a single QTL in the F₂ and F₃ lines. These results indicate that two QTLs act additively, in distinct or complementary pathways, to control grain weight. Based on these results, it is desirable to pyramid the two QTLs into a single line, since the double-QTL line displayed further increase of TGW in the Hwaseong background.

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