Rice Genetics IV - IRRI books - International Rice Research Institute

Gene action of QTLsIn QTL mapping studies, determination of gene action associated with QTLs hasbeen one of the major objectives, which is required for the application of MAS ofQTLs to breeding programs. Early QTL mapping studies using F 2 or backcross populationshave generated some results on gene action of mapped QTLs in rice (Li et al1995a,b, Lin et al 1996, Xiao et al 1995). However, the information of mapped QTLsshowing nonadditive gene action has been particularly lacking largely because of theshifting trend of using permanent homozygous populations such as RI/DH populationsand introgression lines for QTL mapping. Here, a striking result from the fiverelated mapping populations was obtained, which appears to be unexpected from theclassical quantitative genetics theory.Table 11 shows that, for most traits, QTLs showing additive gene action and QTLshaving nonadditive gene action appear to belong to different groups of genes and fewloci show both additive and nonadditive (partial or complete dominance) gene action.For example, most QTLs affecting the trait performance of the backcross and testcrossF 1 hybrids showed either additive gene action (32.5% for M-QTLs and 17.5%for E-QTLs) or overdominance action (57.9% for M-QTLs and 78.5% for E-QTLs),and few loci (10.5% for M-QTLs and 3.9% for E-QTLs) showed partial or completedominance (Table 11). The only exception was the QTLs affecting heading date whereQTLs of partial or complete dominance accounted for 40.0% and 14.3% of the totalQTLs identified, respectively. These results, in conjunction with those in previoussections, have led us to conclude that epistasis and overdominance are the primarygenetic basis of heterosis for most yield-related traits in rice (Li et al 2001a, Luo et al2001).Table 11. Number of main-effect QTLs (M-QTLs) and epistatic QTLs (E-QTLs) showing differentgene action identified in the five related rice mapping populations of the Lemont/Teqing cross.M-QTLsE-QTLsTrait Additive with complete Over- with complete Overorpartial dominance Additive or partial dominanceadominancedominanceHeading date 5 (2) 4 1 4 (8) 3 14Plant height 3 (4) 2 4 2 (8) 1 14Panicles plant –1 2 (4) 0 3 5 (11) 0 20Spikelets panicle –1 3 (4) 0 4 2 (12) 0 24Spikelet sterility 4 (2) 1 4 6 (12) 1 12Panicle length 3 (1) 0 4 3 (9) 4 8Grains panicle –1 2 (4) 1 8 2 (12) 0 19Grain weight 1 (3) 0 4 11 (12) 0 19Biomass 1 (4) 0 6 4 (12) 0 24Grain yield 1 (4) 0 6 1 (12) 0 25Mean 2.5 (3.2) 0.8 4.4 4.0 (10.8) 0.9 17.9Standard deviation 1.4 (1.1) 1.3 1.9 2.9 (1.8) 1.4 5.7aThe numbers in parentheses are results from the recombinant inbred population and the remaining arethe mean data from two backcross and two testcross F 1 populations.166 Zhi-Kang Li