Major QTLs for disease resistance and other traits identified in recombinant inbred lines from tropical maize hybrids

Major QTLs (quantitative trait loci) with large genetic effects often provi de the basis for rapid genetic gains with quantitative traits like disease anti pest tolerance. This study sought to identify major QTLs in maize thro ugh the creation and use of recombinant inbred lines (RILs) based uniquely on hybrids of elite tropical and temperate inbreds. Nine single crosses inv olving ten inbreds served as the source of 1072 RILs created through six cy cles of single seed descent in the absence of selection in Hawaii. About 30 sublines of each of the ten parental inbreds were bred to estimate means a nd variances of quantitative traits under study. These parameters were then used to predict RIL segregations of major QTLs based on normal probability distributions, designated here the RIL-NP method. Segregations were also t ested for fit to expected ratios by the use of maximum likelihood estimator s. The nine sets of RILs were grown selectively under disease epiphytotics at experimental stations in the United States, Korea, Mexico, Nigeria, and the Philippines. Major QTLs apparently acting monogenically (segregating 1: 1 in RILs) were identified to control general resistance to the following d iseases: Southern rust: Common rust, Northern le:lf blight, Southern leaf b light, Bacterial leaf blight, Stewart's bacterial wilt, Maize mosaic virus and Maize streak virus. Digenic segregations with additive gene action appe ared to characterize QTLs governing resistance to Striga witchweed and to E uropean corn borer. Major QTLs were also observed for polymorphisms in ear height, plant height, maturity, tassel branch number and central tassel-spi ke length. Examples are cited of molecular mapping based on these RILs. The potential use of major QTLs in marker-assisted selection is discussed in r elation to the transfer to temper;Ice germplasm of tolerances to disease, i nsect and stress from the largely untapped tropical germplasm.