Introduction

Those requiring sources for the origins and development of the concept of the gene-for-gene relationship should see Briggs (1933), Flor (1940, 1945), Black et al (1953), Person (1959), Vanderplank (1963), Habgood (1970), and Robinson (1976, 1986). (I am indebted to Steve Slopek, personal communication, 1993, for discovering the early, and previously unrecognised, percipience of Briggs).

In plant parasitism, the gene-for-gene relationship is an approximate equivalent of the antibodies and antigens in mammals. However, this is only an analogy and it must not be stretched too far. The main difference is that each resistance in the plant host is inherited, and it is consequently present before infection, rather than being physiologically acquired after infection, as happens with antibodies. The inheritance of each resistance is controlled by a single Mendelian gene. And, for each resistance gene in the host, there is a corresponding, or matching, gene for parasitism in the parasite. Each resistance gene is thus the approximate botanical equivalent of an antibody, and each parasitism gene is the approximate botanical equivalent of an antigen. However, there is an additional reason why the analogy should not be stretched too far. If the genes of the parasite match the genes of the host, the resistance mechanism is not triggered, the resistance does not operate, and parasitism can occur. If the parasite genes do not match the host genes, the resistance mechanism is triggered, the resistance does operate, and parasitism cannot occur. This is the converse of the relationship between antigens and antibodies.

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