| dc.description.abstract | Because resources are obviously restricted, it is not possible to advance all the progenies generated to later generations, so there is need to reduce the number of populations that will be subject to intensive testing and selection. The Usefulness Criterion combines information on both the mean and the genetic variance. It has been suggested as a potential aid to selecting the most promising populations, to which the most intense evaluation and selection would be applied. This study aimed mainly at determining whether the gains predicted by the Usefulness Criterion could provide a suitable guideline for choosing the bi-parental populations with the best potential for producing inbreds to breed elite, high-yielding, drought-tolerant and disease-resistant hybrids. Specific objectives were: 1) determine whether using the Usefulness Criterion to select among segregating maize populations results in different populations being selected than if selection is based on the population means alone. 2) Develop selection indices for selecting within and among populations based on the yield of testcrosses, and on values for MSV, TLB & ASI in the inbreds and their testcrosses and 3) Characterize the segregation patterns for AD, SD, ASI, resistance to TLB and MSV in the inbreds and TC’s, and determine the relationship between inbreds and testcrosses for those traits.
To develop a strategy to achieve these objectives, 10 F2 populations from the heterotic group of the parents of the populations (Group B) were selfed and advanced to F3, screened against TLB and MSV pathogens, and testcrossed. From these F3 testcrosses (F3TC’s), 250 were selected for evaluation, based on F2 and F3 performance. These 250 were evaluated in 3 environments of Uganda (Namulonge, Bulindi and Serere) during season 2012A. Testcrosses were planted in one row plots in an α-lattice of 2 replications in each of the three environments.
Analysis of variance revealed consistent yield performance of most populations across three locations. However, maturity traits were inconsistent, especially for days to pollen shedding and the anthesis silking interval. ASI values and TLB and MSV incidence and severity scores were highly heritable among the TC’s and populations across locations. The study revealed that non-genetic effects were highly involved in the yield performance of the testcrosses (H = 0.35 for testcrosses, H = 0.10 for populations and H = 0.31 for testcrosses within populations). The populations exhibited similar levels of variability in yield due to the high similarity of parents. Therefore, the usefulness criterion could not provide much help in identifying the best populations, since the U value differs from the mean only because of variation within populations. However, there were greater differences in genetic variance within populations for kernel, maturity traits and disease resistance. These traits also correlated with yield, so the usefulness criterion differentiated the potential of the different populations for these traits, and indirectly for yield. In considering the usefulness criterion along with index selection, populations E99, E93, E74, E87 and E80 were identified as the best populations to use for deriving elite inbreds as parents of high-yielding and disease-resistant hybrids.
TLB resistance, AD, SD and ASI were quantitatively inherited with minor gene effects, though both qualitative and quantitative genes were involved in the inheritance of TLB resistance. A relationship between narrow-sense heritability in the inbreds and broad-sense heritability in the testcrosses was evident. This implies that selection can be done in early and subsequent generations of the populations with high narrow sense heritability. Populations showed high breeding values and effective transmission of genes from parents to offspring. A positive relationship between inbreds and TC’s in the index selection, and the high value of the means of TC’s from the first ten inbreds selected by the index, compared to the population’s mean, suggested that careful and effective index selection of inbreds may accurately indicate the best populations and could significantly improve the yield of the inbreds eventually selected at homozygosity. | en_US |
