Fitness Determinations in Vesicular Stomatitis Virus

Abstract

Fitness is defined as the overall replicative ability. Testing whether a mutation (or combination of mutations) has an effect on fitness often relays on determining virus production as a surrogate measurement. However, viruses do not usually replicate in a void, and evolutionary speaking, it is key to determine replicative ability compared to other viruses, e.g. the relative fitness. John Holland developed a method for vesicular stomatitis virus based on the use of a neutral genetic marker that allows to distinguish two competitors and to measure accurately the relative ratio between the two during competition (Holland et al., 1991). The marker is a mutation in the external G glycoprotein that has no effect on the virus other than conferring resistance to a monoclonal antibody, I1. To measure fitness a marked test strain is mixed with a reference unmarked strain and the mixture is allowed to infect a cell monolayer. Ratios before and after competition are measured by plaque assay in the presence and absence of I1 antibody, and changes in ratio give the fitness value.