Pathogens can show high levels of diversity at a variety of scales, from within a single host to among different populations of hosts. The impact of this diversity on the host-pathogen relationship and how it changes temporarily and spatially are poorly understood; however, diversity is predicted to impact the evolution of host resistance and pathogen virulence. We have examined how pathogen diversity alters the evolution of resistance in a multi-generational, experimental evolution study. Using a wild-type insect virus (Autographa californica multiple nucleopolyhedrovirus, AcMNPV) and a highly susceptible natural host, the cabbage looper (Trichoplusia ni), we isolated and characterized AcMNPV variants using in vivo dilution cloning, RFLP analysis and pathogenicity bioassays. Baculoviruses are highly pathogenic obligate pathogens that attack the larval stage of Lepidoptera. We then selected for resistance in replicate populations over five generations using inocula with different variant diversities and demonstrating a range of virulence: single variants, four variant mixtures and the original, mixed wild-type isolate.
Results/Conclusions
Virus-related mortality decreased over time in all treatments. We found that the rate of resistance evolution was lower in diverse pathogen infections, which also had higher virulence and higher associated costs (growth rate, pupal weight and fecundity). Individual variants showed a wide range of pathogenicity and very different trajectories in resistance evolution. Our findings provide insight into the role of pathogen diversity in the evolution of host-pathogen relationships. In addition, microbial insecticides are important tools in sustainable pest management and the results suggest mechanisms whereby the evolution of resistance to microbial insecticides can be managed.