SIMULTANEOUS SELECTION FOR MAJOR AND MINOR RESISTANCE GENES

In many crop species, genes that confer resistance to diseases or insects often exhibit strong dominance and epistasis. As a result, it is generally considered difficult or impossible to accumulate, or pyramid, resistance genes with both large and small effects in a single population or cultivar. Availability of differential disease or insect cultures or molecular markers linked to major resistance loci may allow such loci to be pyramided efficiently, along with minor genes. A deterministic model was used to simulate recurrent selection by truncation in a population segregating at multiple loci with intra- and interlocus interactions similar to those that occur in systems of resistance genes. Selection at major loci generated large nonadditive genetic variances and gametic disequilibria. Phenotypic selection at minor loci was generally ineffective when major genes were also present; pleiotropic penalties on resistance genes caused minor resistance alleles to be eliminated from the population, illustrating the Vertifolia effect. The most effective method by which frequencies of both major and minor genes could be increased simultaneously was to use data from differential cultures or linked markers to discard all individuals with more than one major allele and then select the most resistance of the remaining individuals in each cycle, based on means of their selfed progenies. This methodology increased the frequency of desired genotypes to a level at which use of the population in a breeding program would be practical.