High virulence does not necessarily impede viral adaptation to a new host: A case study using a plant RNA virus

Abstract

BackgroundWhen between-host selection pressures predominate, theory suggests that high virulence could hinder between-host transmission of microparasites, and that virulence therefore will evolve to lower levels that optimize between-host transmission. Highly virulent microparasites could also curtail host development, thereby limiting both the host resources available to them and their own within-host effective population size. High virulence might therefore curtail the mutation supply rate and increase the strength with which genetic drift acts on microparasite populations, thereby limiting the potential to adapt to the host and ultimately perhaps the ability to evolve lower virulence. As a first exploration of this hypothesis, we evolved Tobacco etch virus carrying an eGFP fluorescent marker in two semi-permissive host species, Nicotiana benthamiana and Datura stramonium, for which it has a large difference in virulence. We compared the results to those previously obtained in the typical host, Nicotiana tabacum, where we have shown that carriage of eGFP has a high fitness cost and its loss serves as a real-time indicator of adaptation.\n\nResultsAfter over half a year of evolution, we sequenced the genomes of the evolved lineages and measured their fitness. During the evolution experiment, marker loss leading to viable virus variants was only observed in one lineage of the host for which the virus has low virulence, D. stramonium. This result was consistent with the observation that there was a fitness cost of eGFP in this host, while surprisingly no fitness cost was observed in the host for which the virus has high virulence, N. benthamiana. Furthermore, in both hosts we observed few lineages with increases in viral fitness, and host-specific convergent evolution at the genomic level was only found in N. benthamiana.\n\nConclusionsThe results of this study do not lend support to the hypothesis that high virulence impedes microparasites evolution. Rather, they exemplify that jumps between host species can be game changers for evolutionary dynamics. When considering the evolution of genome architecture, host species jumps might play a very important role, by allowing evolutionary intermediates to be competitive.