Costs of resistance-breaking and virus genetic diversity

Breeding resistance into crops is a main strategy of disease control, as it is effective, target-specific and environmental friendly. However, resistance may not be durable because pathogens evolve to break resistance. The evolution of resistance breaking may be hindered by fitness penalties associated to resistance-breaking mutations, a topic much explored with plant viruses. Focusing on viruses that infect pepper crops we have shown in the past that mutations that determine resistance-breaking may have effects on different components of the virus fitness, which may delay resistance breaking and increase resistance durability. We show now that resistance-breaking genotypes with high fitness penalties may survive in the virus population if reversion to non-resistance breaking is difficult and/or costly. Analyses of revertants to non-resistance-breaking phenotypes showed that the genetic diversity of the virus in plant tissues undergoing a resistance reaction was higher than in susceptible tissues. In tissues undergoing resistance new virus genotypes overcoming plant defences appeared that could infect and colonise new host genotypes or species. The results of this study contribute to better understanding the evolutionary dynamics of resistance breaking and host-range expansions.