Phenotypic Variation and Non-Allelic Interaction in the Gene-for-Gene Relationship Between Arabidopsis Thaliana and Peronospora Parasitica (Downy Mildew).

Abstract

Considerable variation was evident among interaction phenotypes after inoculation of a range of accessions with different isolates. Considering only phenotypes that could be readily discriminated among segregating individuals, alleles controlling the genotype-specific variation were identified at 19 RPP (resistance to P. parasitica) loci and most were mapped with varying degrees of resolution. RPP loci occur in several clusters and at least 2 clusters also contain loci identified by involvement in recognition of other pathogens. A cluster on chromosome III is of interest because it consists of RPP loci whose expression is associated with a range of phenotypes. Although non-allelic additive variation is not usually considered a feature of a gene-for-gene relationship, epistatic genetic variation being considered the rule (the observed phenotype being that associated with the allele conditioning the highest manifestation of incompatibility), examples were found where this assumption was not substantiated. Additive variation was evident in the altered phenotype associated with expression of some RPP alleles in combination with mutant alleles of other loci involved in plant defence.