Host resistance and the evolutionary response of plant viruses.
Plant virus diseases severely constrain agricultural production world-wide, especially in less developed countries. The use of resistant cultivars may put a selection pressure on the plant virus to evolve towards more aggressive types. We used a modelling approach to show the impact of resistance, expressed through different mechanisms, on selection for plant virus strains with a higher multiplication rate. Begomoviruses (Geminiviridae: Begomovirus) inducing leaf curl diseases on tomato were used as key examples. A model for the epidemiology of the plant-virus-vector system was combined with a model for the within-plant dynamics of the virus. Four types of resistance were defined, each expressing resistance through one or more model parameters. The evolutionary stable strategy (ESS) approach was used to study the effect of resistance on the evolution of within-plant virus multiplication rate. Resistance expressed through reduced virus acquisition by the vector, and resistance expressed through reduced inoculation of the plant, do not put a selection pressure on the virus to evolve towards a higher multiplication rate. Resistance expressed through reducing within-plant virus titre and symptom reducing resistance do put a selection pressure on the virus to evolve towards a higher multiplication rate. Synthesis and applications. We have shown how to disentangle different types of plant resistance. We have also shown that each type of resistance puts a different type of selection pressure on the virus. Virologists and plant breeders can use these results to develop methods to characterize the combination of resistance mechanisms in the cultivars they breed and, on the basis of this, determine whether the cultivar will put a selection pressure on the virus to evolve more harmful types. This new approach to plant resistance against virus diseases has been made possible by combining ecological insight in virus-vector-plant interactions with ESS calculations.