Mechanisms of virus evolution and emergence

Abstract

Experimental evolution offers biologists the “Beagle in a bottle”, a testing ground for evolutionary hypotheses. These hypotheses are tested by tracking of the laboratory evolution of microorganisms. One aspect that has received a good deal of attention in recent years has been the evolution of generalist and specialist pathogen strains, a process that is in the basis of viral emergence. Emerging viruses can be defined as the causative agents of infectious diseases whose incidence is increasing following its appearance in a new host population or whose incidence is increasing in an existing host population as a result of long-term changes in its underlying epidemiology. Over the years, we have been performing evolution experiments with tobacco etch (TEV) and turnip mosaic (TuMV) potyviruses that simulate the spill over from the reservoir to new hosts. A first group of experiments tackled the existence of trade-offs in TEV fitness and virulence among hosts species that differ in their degree of taxonomic relatedness with the reservoir. In a second set of experiments, we explored the role of within-host species genetic variability for susceptibility in the extent of TEV and TuMV local adaptation and of virulence. Experiments always result in a pattern of local adaptation, characterized by a higher infectivity and virulence on host(s) encountered during evolution. However, local adaptation not always pays a cost on the foreign hosts. Therefore, the expected cost of generalism, arising from antagonistic pleiotropy and other genetic mechanisms generating fitness tradeoffs between hosts, could not be generalized and strongly depend on the characteristics of each particular pathosystem, specially on the genetic relatedness between new and reservoir host species. Finally, we observed that selection for generalist or specialist strategies have a strong effect on the way virus alters the transcriptome of plants.