Genetic Analysis of the Gene-for-Gene Interaction Between Lettuce (Lactuca sativa) andBremia lactucae

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Abstract
Possible complexities of the gene-for-gene theory of host-parasite specificity were investigated in a genetic study of the interaction between Lactuca sativa (lettuce) and Bremia lactucae (down mildew). Crosses between pathogen isolates were made to test whether virulence loci matching a single host resistance gene were always allelic, whether dominant inhibitor loci or other modifier genes affected the expression of avirulence loci and whether avirulence loci were linked. The segregation data corresponded closely to the predictions of the gene-for-gene theory. Specific virulence to match resistance genes in letetuce was determined at the same loci in isolates of geographically diverse origins. Complete inhibition of avirulence loci by inhibitor genes appeared to be rare in B. lactucae, but partial modification of incompatible interactions between particular cultivars and isolates were observed. No tight linkage was detected between loci controlling avirulence. Previous results that were apparently inconsistent with a gene-for-gene interaction were often explained by the presence of uncharacterized resistance genes or by polyploidy in some pathogen isolates. The action of genes modifying avirulence was difficult to characterize unambiguously. The implications of the data for molecular studies of gene-for-gene interactions are discussed.