Alternative strategies in fungal race-specific parasitism.

Based on the gene-for-gene relation in race-specific resistance versus virulence, racial complexity of a pathogen population can be revealed by using host lines each with a single gene for resistance as detector. Such inventories of cereal rusts have shown: i. Genes for virulence may have pleiotropic effects acting on general fitness and their relative prevalence. ii. Genes for virulence are, as most other genes, dependent on genetic background for their general fitness. iii. Specific and general gene erosion in a pathogen population submitted to the assortative function of a race-specific host selection pressure is proportional to the degree of existing recombination and thus ultimately upon mode of reproduction (sexual or asexual). iv. Genetic storage capacity is dependent on ploidy constitution. v. Host alternation for safe annual survival favours a genetic system able to store temporarily unnecessary genes for virulence. - Due to shifting circumstances, pathogens like rusts will even inside the same forma specialis show different strategies. The trend may lead to a process of stabilizing selection and dependence on immediate and provisional flexibility just as typical of true haploids. It may lead to a pattern of preparedness: i.e. accumulation of 'unnecessary' genes for virulence. In the latter case, the modern concept of gene diversification in breeding for disease resistance is less effective. In the former case, gene accumulation can also work.