Evolutionary Recovery of a Recombinant Viral Genome

It is well appreciated that the evolutionary divergence of genes and genomes from a common ancestor ultimately leads to incompatibilities if those genomes are hybridized. Far less is known about the ability and nature of compensatory evolution to yield the recovery of function in hybrid genomes. Here the major capsid gene of the bacteriophage T7 (40-kb dsDNA) was replaced with the homologous gene of either T3 or K11, each 22% different at the protein level from the T7 homolog. Initial fitness was moderately impaired for the T3 exchange, but the K11 exchange was not viable without a compensatory change in the T7 scaffolding protein. Subsequent adaptation of the transgenic phages led to nearly complete fitness recoveries. Compensatory changes were few, mostly in the transgene and its main interacting partner, the scaffolding protein gene. The large magnitude of fitness recovery with relatively few mutations suggests that the fitness costs of hybridizations and horizontal gene exchanges between moderately diverged genomes can potentially be short-lived through compensatory evolution.