Transmission rates and adaptive evolution of pathogens in sympatric heterogeneous plant populations

Diversification in agricultural cropping patterns is widely practised to delay the build–up of virulent races that can overcome host resistance in pathogen populations. This can lead to balanced polymorphism, but the long–term consequences of this strategy for the evolution of crop pathogen populations are still unclear. The widespread occurrence of sibling species and reproductively isolated sub–species among fungal and oomycete plant pathogens suggests that evolutionary divergence is common. This paper develops a mathematical model of host–pathogen interactions using a simple framework of two hosts to analyse the influences of sympatric host heterogeneity on the long–term evolutionary behaviour of plant pathogens. Using adaptive dynamics, which assumes that sequential mutations induce small changes in pathogen fitness, we show that evolutionary outcomes strongly depend on the shape of the trade–off curve between pathogen transmission on sympatric hosts. In particular, we determine the conditions under which the evolutionary branching of a monomorphic into a dimorphic population occurs, as well as the conditions that lead to the evolution of specialist (single host range) or generalist (multiple host range) pathogen populations.