Summary:

The importance of historical contingency in determining the evolvability of virus has been largely unappreciated. Identifying the constraints imposed by past adaptation is, however, of importance for understanding the evolution of host usage dynamics by multi-host viruses or the emergence of escape mutants that persist in the absence of antiviral treatments. To address this issue, we undertook an experimental evolution approach. Sixty lineages of Tobacco etch potyvirus that differ in their past evolutionary history and degree of adaptation to Nicotiana tabacum, and thus in their position on the adaptive landscape, were allowed to adapt to this host for 15 rounds of within host multiplication and transfer. The degree of adaptation to the new host as well as to the original ones was evaluated and the consensus sequence of each evolved lineage was obtained. Past evolutionary history did not determine the phenotypic outcome of this common host evolution phase. By contrast, historical contingency left footprints at the genotypic level: the majority of host-specific mutations present at the beginning of this experiment were retained in the end-point populations and may have affected which new mutations were consequently fixed. Further divergence between the sequences occurred despite a shared selective environment. Our results imply that viruses are not easily trapped into suboptimal phenotypes and that TEV (re)emergence is not evolutionarily constrained.