Simulation Experiments on efficiencies of gene introgression by backcrossing
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Designing a highly efficient backcross (BC) marker-assisted selection (MAS) experiment is not a straightforward exercise, efficiency being defined here as the ratio between the resources that need to be invested at each generation and the number of generations required to achieve the selection. This paper presents results of simulations conducted for different strategies, using the maize genome as a model, to compare allelic introgression with DNA markers through BCs. Simulation results indicate that the selection response in the BC1 could be increased significantly when the selectable population size (N-sl) is <50, and that a diminished return is observed when this number >100. Selectable population size is defined as the number of individuals with favorable alleles at the target loci from which selection with markers can be carried out on the rest of the genome at nontarget loci, simulations considered the allelic introgression at one to five target loci, with different population sizes, changes in the recombination frequency between target loci and flanking markers, and different numbers of genotypes selected at each generation. For an introgression at one target locus in a partial line conversion, and using MAS at nontarget loci only at one generation, a selection at BC3 would be more efficient than a selection at BC1 or BC2, due to the increase over generations of the ratio of the standard deviation to the mean of the donor genome contribution. With selection only for the presence of a donor allele at one locus in BC1 and BC2, and MAS at BC3, lines with <5% of the donor genome can be obtained with a N-sl of 10 in BC1 and BC2, and 100 in BC3. These results are critical in the application of molecular markers to introgress elite alleles as part of plant improvement programs.
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