Multi-host experimental evolution of Tobacco etch virus carrying eGFP

As a first exploration of the effects of virulence on microparasite evolution, Tobacco etch virus (TEV) was evolved in two host species for which it has a large difference in virulence: Nicotiana benthamiana and Datura stramonium. Moreover, to immediately gauge whether adaptive evolution might be occurring, we passaged a TEV variant expressing a fluorescent marker protein, the enhanced GFP (eGFP). Upon long-duration passages in TEV's typical host, Nicotiana tabacum, this exogenous sequence is quickly lost due to its strong fitness cost, and its loss is reliably indicated by a loss of eGFP fluorescence. We expected that adaptive evolution would occur more quickly in the host species for which TEV has lower virulence, D. stramonium, than in the host species for which it has high virulence, N. benthamiana. However, these two alternative host species ameliorated the fitness cost of eGFP, and loss was only observed in one lineage of D. stramonium. This was consistent with the observation that there was a fitness cost of eGFP in this host, while surprisingly no fitness cost was observed in N. benthamiana. Moreover, in both hosts we observed a low rate of adaptation in viral fitness as well as at the genomic level. The 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.