Nonrandom Association of Avirulence Loci in Four Disease Resistance Gene System. II. Replacement between Host Groups Having Different Number of Susceptibility Genes.

Abstract

The influence of replacement between host genotypes with different S groups (groups with a different number of resistance or susceptibility genes) on the types and patterns of nonrandom associations between avirulence loci in the pathogen was examined by a method of simulation. Through all combinations of S groups, nonrandom association patterns are largely influenced by various factors, and degrees of influence are in the following order. 1) Direction of transfer of values from donor to receptor (replacement of varieties with different genotypes from old to new), 2) combinations of S groups (groups having the different number of susceptibility genes), 3) fitness values given to virulence genes when various values with the same average are given, 4) generations of vegetative reproduction, and 5) the number of resistance loci with complementary resistance and susceptibility genes. When a large area was replaced by a new host genotype with one or three different genes, half of the possible six nonrandom associations were cross (C)-type and the remainder were noncross (N)-type. In these cases, combinations of an avirulence gene corresponding to a resistance gene in the new host genotype with other avirulence loci in the pathogen show N-type nonrandom associations. If a receptor has two resistance genes, combinations including one of avirulence loci corresponding to resistance genes in the receptor induce C-type nonrandom associations and combinations including the two loci show N type. Accordingly, the ratio of N:C was 2:4. When a receptor has three resistance genes, the ratio is 3:3. This can explain why two types were found in fields where two or more statistically significant nonrandom associations were found.