DETERMINANTS OF DENSITY-DEPENDENT AND FREQUENCY-DEPENDENT FITNESS IN COMPETING PLANT-PATHOGENS

Using mathematical models, we investigated how infection and sporulati on characteristics of competing plant pathogens determine the density and frequency dependence of relative fitness. Two models, one for the infection stage and one for the sporulation stage of a pathogen's life cycle, describe reproductive output of pathogen strains in mixture as a function of the strains' population densities. Model parameters inc lude infection and sporulation efficiencies, carrying capacities on le aves for sporulating lesions and spore production, and coefficients of interstrain competitive effects in both life cycle stages. Although t he models were originally developed for rust fungi, they are generally applicable to any organism with distinct colonization (e.g., infectio n) and propagative (e.g., sporulation) life cycle stages. In this work , paired hypothetical strains were assigned equal baseline parameter v alues. Parameters were then altered one at a time for one or both stra ins, and relative fitness was calculated over a range of densities and strain frequencies. Except for infection efficiency, the fitness bene fit conferred by an advantage in a single parameter was always density dependent. Relative fitness was frequency dependent whenever inter-an d intrastrain competitive effects were not equal. These results sugges t that the fitness of pathogens in nature is rarely fixed, but, rather , may typically be highly dependent on the densities and frequencies o f all coexisting strains in a habitat.