Rice Genetics IV - IRRI books - International Rice Research Institute

QTL mapping in rice:a few critical considerationsZhi-Kang LiAdvances in DNA markers and molecular linkage maps have stimulated anew area of molecular quantitative genetics through mapping of quantitativetrait loci (QTLs). Several important questions regarding the types and numberof QTLs, QTL × environment (QE) interactions, molecular dissection oftrait correlation, and gene actions of QTLs are addressed based on resultsfrom many previous QTL mapping studies in rice and examples from two wellstudiedmapping populations and their related progenies. Two major types ofQTLs, main-effect QTLs and epistatic QTLs, are recognized. Many QTLs arefound to affect specific quantitative traits and they are widely distributed inthe genome, though only a limited number of these loci are detectable in amapping population largely because of epistasis and genotype × environment(GE) interactions. The effects (both main and epistatic) of individualQTLs affecting specific phenotypes may vary considerably. Most QTLs appearto be epistatic and complementary interaction appears to be the mostcommon form of epistasis. Most QTLs tend to show varied degrees of QEinteractions as a result of differential gene expression to biotic and abioticstresses in different environments. QTLs differ greatly in their GE interactionsand epistasis plays an important role in QE interactions. QTLs showingdifferent gene actions appear to belong to different groups of genes andthose exhibiting both additive and nonadditive gene actions are few. A newstrategy is proposed for simultaneous QTL identification and transfer, andallele discovery through the development of introgression lines and use ofDNA markers.Most important traits dealt with by plant breeders are quantitative in nature. The classicalmultiple-factor hypothesis considered the continuous variation of quantitativetraits as the collective effects of many genes, each with a small effect (Nilsson-Ehle1909, Mather and Jinks 1982). Although numerous quantitative genetic studies haverevealed the relative importance of genes with different actions on quantitative traits,the methodology itself does not allow resolution of continuous trait variation intoindividual underlying Mendelian factors. Advances in DNA markers since the lateQTL mapping in rice: . . . 153