On the stability of sequences inserted into viral genomes

Willemsen, Anouk; Zwart, Mark P

After over half a year of evolution in two semi-permissive host species, with a large difference in virus-induced virulence, the eGFP insert appears to remain stable. A fitness costs of eGFP was only found in the host for which TEV has low virulence. In the hosts for which TEV has high virulence there was no fitness cost and viral adaptation was observed. This contradicts theories that suggest that high virulence could hinder between-host transmission. When considering the evolution of genome architecture, host species jumps might play a very important role, by allowing evolutionary intermediates to be competitive. The stability of an insert could change when considering insertions that might be beneficial for the virus. Using the TEV genome we simulated two HGT events, by separately introducing functional exogenous sequences that are potentially beneficial for the virus (Willemsen et al. 2017 ). In one case, the insertion was rapidly purged from the viral genome, restoring fitness to wild-type fitness levels. In another case, the inserted gene-the 2b RNA silencing suppressor from Cucumber mosaic virus-did not seem to have a major impact on viral fitness and was therefore not lost when performing experimental evolution. Interestingly this insertion duplicated the function of RNA silencing suppression function of another gene in the genome. When mutating this functional domain of the TEV gene, the inserted gene provided a replicative advantage. These observations suggest a potentially interesting role for HGT of short functional sequences in improving evolutionary constraints on viruses.