The proposal that during their evolution a host and its parasite develop complementary genetic systems, with each gene that provides the host with resistance matched by a gene in the parasite that confers susceptibility. The interacting genes from the two species are called corresponding genes, since for each gene that conditions resistance in the host there is a corresponding gene that conditions avirulence in the parasite, and the products of the two genes interact. The product of the resistance gene serves as a receptor for a ligand produced by the parasite, directly or indirectly through expression of an avirulence gene. The binding of receptor and ligand is the recognition event that elicits through cellular signal transduction (q.v.), a cascade of defense responses that constitute the resistant phenotype. See Chronology, 1955, Flor; coevolution, Linum usitatissimum, Melampsora lini.
Subjects: Genetics and Genomics.