One of the most interesting aspects of virology is the obligatory dependence of the virus upon its host. This dependence means that viruses have an intricate relationship with their host and have evolved to utilise the components of a specific host, although many viruses are happy to replicate in other organisms. For some it’s a necessity; arboviruses such as West Nile virus (WNV) must replicate in both vertebrate and invertebrate (in the case of WNV a mosquito) hosts to complete their lifecycles. Inevitably, this means that viruses can jump the species barrier leading to new (and potentially dangerous) scenarios where viruses behave differently in the new host. HIV for instance is thought to originally have crossed the species barrier into humans from chimpanzees.
Different hosts means different evolutionary pressures on a virus; factors such as bottle-necks and rates of transmission as well as immunological pressure will all impact upon the virus. A recent paper in PLoS Pathogens looks at how rabies virus evolves at different rates in different species of bat, and how the biology of bats impacts the subsequent evolution ofthe virus.
It seems that, once a virus is established in a particular species, the evolutionary rate remains fairly constant, although some virus lineages were able to evolve at a rate 5-22 times greater than that of others. Whilst the virus lineages themselves can show variation, they looked at whether there was variation in the rates of evolution in different species of bats and their ecology. Interestingly, the slower evolving lineages tended to be those evolving in bats of temperate zones, where the viruses were found to evolve at a rate nearly four-fold lower than that in tropical and sub-tropical areas. It seemed that the rate of evolution correlated not with a particular bat, but with where the bat was living. So do colder temperatures mean a lower metabolic rate and thus a reduced virus evolutionary rate? Apparently not. It appears it’s probably more to do with transmission rates and the impact that this has upon virus evolution.
This all reminds me of a paper last year showing that different West Nile virus genomes predominated in different hosts (vertebrate vs. invertebrate) and that this affected the fitness of the virus in the alternative host (1). All in all, these kinds of study show just how complex viral evolution can be; can this all be explained by an evolutionary model based upon the concept of quasispecies?