Genetic Equilibria in Host-Pathogen Systems

A model is presented to illustrate the processes by which genetic equilibria could arise in host-pathogen systems. The model is based on the assumption that genes for virulence in the pathogen population are selected against when they are not required for pathogenicity. Genes for resistance are assumed to be selected against, in environments in which they do not protect against the pathogen, either because the pathogen is not present or because all biotypes of the pathogen possess genes for virulence that enable them to overcome the effects of the resistance genes in the host. These assumptions are based on observations that simple pathogenic races usually predominate over simple varieties, and that host populations grown for extended periods of time in the absence of their pathogens tend to lose their resistance to those pathogens. If the model is valid, multiline varieties should not be superior to pure line varieties in disease control unless simple races of the pathogen suppress the reproduction of complex races, or unless races with genes for virulence reproduce most efficiently on host varieties with the corresponding genes for resistance. If these phenomena do not occur, the establishment of an equilibrium among pathogenic races would not necessarily result in lower rates of pathogen reproduction than could be achieved with the use of pure line varieties susceptible only to the most complex races. It is also suggested that a multiline variety should be more effective if it consists of a series of varieties with different genetic backgrounds rather than a series of backcross lines from the same recurrent parent variety.