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CROP BREEDING & GENETICS

Optimization of the Marker-Based Procedures for Pyramiding Genes from Multiple Donor Lines: II. Strategies for Selecting the Objective Homozygous Plant

^a Marker-Assisted Rice Breeding Research Team, National Institute of Crop Science, Tsukuba 305-8518, Japan
^b Dep. of Biotechnology, Kyoto Sangyo Univ., Kyoto 603-8555, Japan

^* Corresponding author (yonezaw{at}cc.kyoto-su.ac.jp).

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.

Abbreviations: CGP, a pair of plants with complementary marker genotypes • CPP, a pair of plants with complementary marker phenotypes • CRO, expected number of crossings performed in a selection strategy • GEN, expected rounds of selection performed in a selection strategy • GNP, expected number of plants tested per selection round • HF₂, a selection strategy with combined use of F₂ enrichment and haplo-diploidization • IG, a plant with the objective homozygous marker genotype • PG, a plant with promising marker genotype • PP, a plant with promising marker phenotype • PRO, the probability of success; RHC, a selection strategy with recurrent use of haplo-diploidization and crossing between the best complementary genotypes • RSC, a recurrent selection strategy with selfing or crossing of selected plants • SH, a selection strategy with a single round of haplo-diploidization and selection • TPN, expected total number of tested plants in a selection strategy