Genetic mechanism of heterosis for rice milling and appearance quality in an elite rice hybrid

Development of hybrid rice with high yield and grain quality is a goal of rice breeding. To investigate the genetic mechanism of heterosis for rice milling and appearance quality in indica/xian rice, QTL mapping was conducted using 1061 recombinant inbred lines (RILs) derived from a cross of the xian rice cultivars Quan9311B (Q9311B) and Wu-shan-si-miao (WSSM), and a backcross F₁ (BC₁F₁) population developed by crossing the RILs with Quan9311A (Q9311A), combined with phenotyping in two environments. The F₁ hybrid (Q9311A × WSSM) showed various degrees of heterosis for milling and appearance quality. A total of 142 main-effect QTL (M-QTL) and 407 pairs of epistatic QTL (E-QTL) were identified for five milling and appearance quality traits and grain yield per plant (GYP) in the RIL, BC₁F₁ and mid-parental heterosis (H_MP) populations. Differential detection of QTL in three populations revealed that most additive loci detected in the RILs did not show heterotic effects, but some of them did contribute to BC₁F₁ trait performance. Unlike heterosis of GYP, single-locus overdominance and epistasis were the main contributors to heterosis for milling and appearance quality. Epistasis contributed more to the heterosis for milling quality than to that for appearance quality. Three (four) QTL regions harboring opposite (consistent) directions of favorable allele effects for GYP and grain quality were identified, indicating the presence of partial genetic overlaps between GYP and grain quality. Three strategies are proposed to develop hybrid rice with high yield and good grain quality: 1) pyramiding favorable alleles with consistent directions of gene effects for GYP and grain quality at the M-QTL on different chromosomes; 2) introgressing favorable alleles for GYP and grain quality into the parents and then pyramiding and fixing these additive effects in hybrids; and 3) pyramiding overdominant and dominant loci and minimizing or eliminating underdominant loci from the parents.