Genetic dissection of an elite rice hybrid revealed that heterozygotes are not always advantageous for performance.

We introduced an experimental design that produced an "immortalized F(2)" population allowing for complete dissection of genetic components underlying quantitative traits. Data for yield and three component traits of the immortalized F(2) were collected from replicated field trials over 2 years. Using 231 marker loci, we resolved the genetic effects into individual components and assessed relative performance of all the genotypes at both single- and two-locus levels. Single-locus analysis detected 40 QTL for the four traits. Dominance effects for about one-half of the QTL were negative, resulting in little "net" positive dominance effect. Correlation between genotype heterozygosity and trait performance was low. Large numbers of digenic interactions, including AA, AD, and DD, were detected for all the traits, with AA as the most prevalent interaction. Complementary two-locus homozygotes frequently performed the best among the nine genotypes of many two-locus combinations. While cumulative small advantages over two-locus combinations may partly explain the genetic basis of heterosis of the hybrid as double heterozygotes frequently demonstrated marginal advantages, double heterozygotes were never the best genotypes in any of the two-locus combinations. It was concluded that heterozygotes were not necessarily advantageous for trait performance even among genotypes derived from such a highly heterotic hybrid.

[Analysis of QTL x environment interaction for rice panicle characteristics].

The development of molecular genetic linkage map has accelerated the identification and mapping of genomics regions controlling quantitative trait loci. A recombinant inbred line population derived from Zhenshan 97/Minghui 63 was grown in two years. Mixed linear model approach was used to jointly identify QTLs and QTL x environment interactions for five panicle characteristics. Ten, three, six, eight and seven QTLs were detected for spiketets per panicle, grains per panicle, seed setting, panicle length and panicle setting density, respectively. They collectively explained 29.13%, 19.2%, 29.46%, 26.39% and 35.76% of phenotypic variations for these traits, respectively. For the five traits, QTLs with increasing effects and decreasing effects could be simultaneously identified in high value parent and low value parent. QTLs for correlative traits clustering located in the similar regions. One QTL for panicle length, two QTLs for spikelets per panicle and three QTLs for seed setting performed significant interactions with environment. The contributions from interactions were slightly larger than that from the QTLs involved in the interactions. No QTL x environment interaction was detected for traits with high heritabilities such as grains per panicle and panicle setting density.