Importance of epistasis as the genetic basis of heterosis in an elite rice hybrid

Abstract

The genetic basis of heterosis was investigated in an elite rice hybrid by using a molecular linkage map with 150 segregating loci covering the entire rice genome. Data for yield and three traits that were components of yield were collected over 2 years from replicated field trials of 250 F(2:3) families. Genotypic variations explained from about 50% to more than 80% of the total variation. Interactions between genotypes and years were small compared with the main effects. A total of 32 quantitative trait loci (QTLs) were detected for the four traits; 12 were observed in both years and the remaining 20 were detected in only one year. Overdominance was observed for most of the QTLs for yield and also for a few QTLs for the component traits. Correlations between marker heterozygosity and trait expression were low, indicating that the overall heterozygosity made little contribution to heterosis. Digenic interactions, including additive by additive, additive by dominance, and dominance by dominance, were frequent and widespread in this population. The interactions involved large numbers of marker loci, most of which individually were not detectable on single-locus basis; many interactions among loci were detected in both years. The results provide strong evidence that epistasis plays a major role as the genetic basis of heterosis.

Figure 1

Patterns of the interactions for grains per panicle displayed by various two-locus combinations in 1994 and 1995. Paa, Pad, Pda, and Pdd are the probabilities for AA, AD, DA, and DD interactions. The vertical axis represents number of grains per panicle. The numbers 11, 12, and 22 represent the three genotypes of each locus: 11, homozygote for Minghui 63 allele; 22, homozygote for Zhenshan 97 allele; and 12, heterozygote.