Rice Genetics IV - IRRI books - International Rice Research Institute

hybrids rather than progenies of self-fertilization. Fourth, the molecular marker dataneed to be collected from only the 240 RILs no matter how many crosses are included inthe population. Additionally, this population provides opportunities for genetic mappingof heterosis per se rather than analyses based on performance measurements of thetrait if the hybrids and the parents for each cross are planted side by side in the field,which allows for measuring the level of heterosis for every cross.The field trials of 360 crosses were conducted in the rice-growing seasons of 1998and 1999 following a randomized complete block design with two replications withineach year. Each block consisted of four rows with 12 plants per row: two rows of thehybrid and one row for each of the parents. Hybrid plants of the crosses were identifiedby morphological comparison with the parents and SSR (simple sequence repeats) assay.Data for agronomic performance were scored in the same manner as in the F 2:3analysis.A total of 217 molecular markers, including 171 RFLPs and 46 SSRs, were used forassaying the 240 RILs. A linkage map was constructed based on this data set that providedthe framework for QTL mapping. We have currently completed part of the analysis for thedata collected in 1998, which will be presented briefly in the following sections.Single-locus QTLs for yield and yield component traitsA total of 28 QTLs were detected for yield and the three traits that were componentsof yield (Table 3). This number is much larger than the numbers detected for a singleyear in the F 2:3 analysis. The most pronounced differences were the number of QTLsfor tillers per plant and for grain weight; in both cases the numbers detected in thisstudy were substantially greater than in the F 2:3 analysis. Overdominance was detectedat three of the five QTLs for yield and two of the five QTLs for grains perpanicle, but was not detected in the other two traits. This trend is also similar to thelevel of dominance observed in the F 2:3 study.Digenic interactionsThe data set was also subject to the two-way ANOVA for digenic interactions, asoccurred in the F 2:3 analysis. A search of the 23,239 possible two-locus pairs that werepossible with the data set resulted in 96 to 224 two-locus pairs that showed significantinteractions at the 0.001 probability level (Table 4). Partitioning of the interactionsusing orthogonal contrasts revealed that all three types of interaction (i.e., AA, AD orDA, and DD) were involved in the interactions. AA interactions occurred much morefrequently than AD ones, which in turn were more frequent than DD interactions. Thistrend is also similar to the results of F 2:3 analysis. It should be noted that, although thescreening was performed statistically at a much higher stringency than was done inthe F 2:3 analysis, the number of epistatic interactions that were detected was approximatelyof the same order of magnitude as those detected in the F 2:3 analysis.In summary, the analyses of the data from both the F 2:3 and immortalized F 2 populationsdetected the existence of a large number of two-locus epistatic interactions inthe rice genome. Epistasis clearly plays a significant role in the inheritance of quantitativetraits as well as in the genetic basis of heterosis. Thus, the relationship betweenGenetic and molecular basis of heterosis in rice 179