Why do pathogens carry avirulence genes?

In the gene-for-gene hypothesis, Flor [19] proposed the existence of avirulence alleles of Virulence genes, and that the Virulence genes actively conditioned cultivar-specific virulence on hosts. Yet cultivar-specific virulence genes have not been found, and many of the avirulence (avr) genes cloned to date (over 40) do not appear to condition pathogenicity in general, virulence or anything else of value to the microbe. There is strong indirect evidence that many, if not nearly all, Avr proteins are secreted from pathogenic bacteria, enter plant cells and signal a response phenotype, usually associated with a hypersensitive defense response (HR). Why the bother for avirulence? Evidence is accumulating that most avr genes are, or once were, pathogenicity (pth) genes found in biotrophic pathogens that determine (d) host range, not in a cultivar-specific manner, but in a host species-specific manner. At least some of these genes appear to function for pathogenicity by encoding protein signals that are "injected" into plant cells by the hrp system, resulting in programmed host cell death, a characteristic normally associated with necrotrophs. A growing body of evidence indicates that most microbial genes conditioning pathogenicity, including the hrp, Pth and avr genes, are present because of horizontal gene transfer, often involving movement of large gene clusters on "pathogenicity islands". Since horizontal transfer is a stochastic process, many avr genes are likely to be maladapted pth genes, following their horizontal transfer to strains in which their function may be either gratuitous or detrimental. The structure of some of these genes may allow rapid adaptive selection for pathogenic function. (C) 1999 Academic Press.