Genetic Basis Dissection of Heterosis in Japonica Rice (<I>Oryza sativa</I> L.)

Abstract

To understand the types of gene action controlling yield-related traits and their mid-parental heterosis in japonica rice, we carried out quantitative trait locus (QTL) mapping by using 254 recombinant inbred lines (RILs) derived from a cross of japonica rice varieties Xiushui 79, C Bao and two backcross hybrid (BCF1) populations derived from these RILs. Phenotypic values and mid-parental heterosis values (HMP) were investigated for eight traits in the three populations. The eight traits were plant height, growth duration, productive panicle number per plant, panicle length, spikelet number per panicle, percentage of spikelet fertility, primary branch number per panicle, and secondary branch number per panicle. A total of 58 main-effect QTLs (M-QTL) were detected in the three populations. The percentage of phenotypic variance explained by single M-QTL ranged from 3.3% to 41.9%. Among the 58 M-QTLs detected, 45 M-QTLs (77.6%) showed additive effects, 9 M-QTLs (15.5%) showed partial-to-complete dominant effects, and 4 M-QTLs (6.9%) showed over-dominant effects. Ninety pairs of digenic epistatic QTL (E-QTL) were detected in the three populations. Among them, 44 pairs of E-QTLs were detected in RIL population, and the percentage of phenotypic variance explained by single pair of QTL ranged from 1.7% to 8.0%, with an average of 3.7%. In XSB CF1 population (Xiushui 79 as recurrent parent), 27 pairs of E-QTL were detected. Sixteen pairs of E-QTL were detected by using BCF1 phenotypic values, and the percentage of phenotypic variance explained by single E-QTL ranged from 12.7% to 78.5%, with an average of 29.2%. Eleven pairs of E-QTL were detected by using HMP values, and the percentage of phenotypic variance explained by single E-QTL ranged from 15.0% to 71.8%, with an average of 40.1%. In CBBCF1 population (C Bao as recurrent parent), 19 pairs of E-QTL were detected. Twelve pairs of E-QTL were detected by using BCF1 phenotypic value, and the percentage of phenotypic variance explained by single pair of E-QTL ranged from 2.7% to 64.4%, with an average of 30.1%. Nine pairs of E-QTL were detected by using HMP value, and the percentage of phenotypic variance explained by single pair of E-QTL ranged from 21.7% to 64.1%, with an average of 40.0%. Two pairs of E-QTL were detected by using both BCF1 phenotypic value and HMP value in CBBCF1 population. These results indicated that epistatic effects were the primary genetic basis of heterosis in this cross in japonica rice.