Host-switching is an important mechanism for RNA virus diversification that can foster the evolution of new pathogens with novel virulence characteristics. RNA viruses have a high polymerase error rate and short generation time and, therefore, have a great capacity to adapt to new environments. When an RNA virus is introduced to a “new” host species, the environment for virus infection and replication change, initiating selection for a virus phenotype most fit for the new species. We tested the hypothesis that host-switching is a mechanism for the diversification of alphaviruses in the western equine encephalomyelitis (WEE) virus complex by serially passaging a virus in three insect vector species: the mosquitoes Culex tarsalis and Culiseta melanura and the cimicid bug Oeciacus vicarius. The WEE complex consists of six closely related viruses that are transmitted by different insect species. All three species we used are primary vectors of viruses in the WEE complex. After ten serial passages of WEE virus in triplicate for each insect species, we observed a distinct change in virus phenotype. Virus phenotype was characterized by the percent of insects that transmitted virus, titer of virus transmitted, percent of insects infected, titer of virus in the insect bodies, virus plaque morphology and one-step growth curves. Results indicated that WEE virus increased its fitness for each insect species passage series by increasing the percent of insects that transmitted virus, titer transmitted, percent of insects infected, and titer in insect bodies. The rapid and predictable adaptation of WEE virus for the three insect species demonstrates the ability of an RNA virus to change and successfully infect and replicate in a new host species. Understanding the patterns, processes, and predictability of virus change in response to host-switching will enable us to explain the emergence of new diseases.
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA