Credit: Cartoon of boxing VSVs courtesy of Isabel Novella

The order of genes in the vesicular stomatitis virus (VSV) genome is crucial for viral fitness, according to a new study published in the Journal of Virology.

Many concepts in evolutionary and population genetics apply to RNA viruses, and VSV, the prototype of the Rhabdoviridae, is a model system for evolutionary studies. Gene expression is controlled at the level of transcription in VSV and is determined by gene order — the genes that are nearest to the single 3′ promoter are the most highly transcribed.

Considering this mechanism of gene regulation, it is not surprising that gene order (3′-N, P, M, G, L-5′) is well conserved in VSV. However, previous experiments that rearranged the order of the VSV P, M and G genes yielded viruses that seemed to grow as well, or better, in vitro than wild-type (WT) virus.

Novella et al. initially confirmed that both the growth kinetics and the amounts of virus produced by infected cells were comparable between WT and rearranged VSV viruses. Assessing the fitness of the rearranged viruses was not as straightforward. A monoclonal antibody-resistant mutant (MARM) of VSV was selected (RU) that was as fit, in competition assays, as the WT virus. Competition assays were followed by selection and enumeration of this marked surrogate WT virus.

Competition studies showed that the fitness of each rearranged virus was reduced compared with RU. Intriguingly, fitness of the rearranged viruses improved with increasing multiplicity of infection (MOI, the average number of virus particles that infect a single cell).

At high MOIs the likelihood of co-infection of a host cell with both WT and a rearranged virus is increased. The authors favour the possibility that the dependence of increased fitness of rearranged viruses on the MOI is due to complementation of less-fit rearranged viruses by wild-type isolates. The fate of rearranged and other mutant viruses during infection might rely on complementation in co-infection, which is thought to be common in virus spread though infected hosts.

Closer inspection of rearranged VSV viruses shows that the wild-type virus is always the most fit, so it seems that evolution has selected the fittest genome after all.