Genetic limitations on models of specific interactions between a host and its parasite

Abstract

Specific interaction between a resistance gene (R gene) of the host and an avirulence gene (A gene) of the parasite is the basis of the gene-for-gene relationship. R and A genes are conditional genes, capable of associating with either of two phenotypes depending on the presence or absence of the other. Although avirulence usually segregates as a dominant character, microevolutionary history suggests that in its primary function an A gene associates not with avirulence but with virulence, also that R:A interaction generates a “stop signal,” which prevents disease development, and finally that the primary function of a recessive a-gene, when it replaces a dominant A gene, is one of negating the stop signal. Where several gene-for-gene relationships operate simultaneously, a single R:A interaction is sufficient to prevent disease development. R and A genes occupied in R:A interaction are thus epistatic over those which are not. Because of the characteristics of the gene-for-gene relationship, definite limitations are imposed on genetic study. In any genetic study there may be, because of these limitations, contributing genes in both the host and the parasite which are not detected.