Trade-offs in host range evolution of plant viruses

Understanding host range evolution in plant viruses is central for the control of viral diseases: changes in host range resulting in host switches or host range expansions are at the root of virus emergence either in new host species or in host genotypes previously resistant to the virus. Theory predicts that different selection pressures in different hosts will result in fitness trade-offs across hosts, leading to specialization. This paper reviews the evidence in support of this prediction. A large fraction of plant viruses are multihost parasites: evidence for across-species trade-offs has been reported both from the analysis of field virus populations and, more often, from passage experiments resulting in host adaptation. More abundant evidence for across-host trade-offs derives from the analysis of the performance of resistance-breaking virus genotypes in resistant and susceptible host genotypes. Data indicate that overcoming dominant resistance is coupled to severe fitness costs, while overcoming recessive resistance may be associated with fitness costs according to the specific virus genotype. There is ample evidence that antagonistic pleiotropy is a major cause of the reported across-host trade-offs.