Optimization of the marker-based procedures for pyramiding genes from multiple donor lines: II. Strategies for selecting the objective homozygous plant

For extended application of marker-based plant breeding, strategies are discussed for selecting a high-degree gene-pyramided line from among progeny of a multiparentally produced heterozygous plant (root genotype). A strategy with combined use of haplo-diploidization and crossing between selected plants will be highly efficient; selection starts with haplo-diploidized plants raised from the,root genotype, and in the absence of a plant with the, objective marker genotype, two plants with the best complementary genotypes are crossed to produce a hybrid, which in turn is haplo-diploidized for the next round of selection. In this strategy, even a plant having as many as 20 target markers can be, obtained at an almost perfect certainty in, about three rounds of selection with a maximum of 200 tested plants per round. When haplo-diploidized plants are unavailable, a plant with the most promising marker genotype should be selected and self-fertilized in each generation, or in the absence of any promising plant, two plants with the best complementary genotypes are crossed for the next round of selection. In this strategy, the number of tested plants in the first two generations counts when the markers are codominant, whereas the rounds of selection counts when the markers are dominant. Of various supplementary measures for this strategy, backcrossing the root genotype with one of the donors could be useful when the donor has more than 70% of all targeted markers.