RECOMBINATION AND SELECTION IN POPULATIONS OF PLANT-PATHOGENS

A theoretical model has been used to study the dynamics of the frequen cies of the following: a virulence gene which is selected by part of t he host plant population; an unnecessary virulence gene, which is not required for infection of the host; and the gametic disequilibrium bet ween the two genes. If the two genes are not initially in gametic equi librium, the frequency of the unnecessary virulence may be altered gre atly by hitch-hiking selection, because of the increased frequency of the selected virulence. The hitchhiking effect is strongest if reprodu ction is entirely asexual, but can still be significant if the frequen cy of recombination is less than the fraction of the host population w hich consists of selectively resistant plants. The frequency of recomb ination may be reduced if reproduction is partly clonal, rather than f ully sexual, or if the two genes are linked. Selection against unneces sary virulence may give rise to complex dynamics of both virulence all eles; in particular, the frequency of an unnecessary virulence can ris e substantially, by hitch-hiking selection, even if there is some sex or recombination. The direction in which the unnecessary virulence's f requency changes depends on the sign of the gametic disequilibrium bet ween it and the selected gene, and on the existence of selection again st unnecessary virulence. If there is no such selection, the long-term dynamics of genotype frequencies in a largely asexual pathogen popula tion may be unpredictable. Consequently, disease control strategies ba sed on planned replacements of one resistance gene by another are unli kely to be effective.