Cost of host radiation in an RNA virus.

Although host radiation allows a parasite to expand its ecological niche, traits governing the infection of multiple host types can decrease fitness in the original or alternate host environments. Reasons for this reduction in fitness include slower replication due to added genetic material or modifications, fitness trade-offs across host environments, and weaker selection resulting from simultaneous adaptation to multiple habitats. We examined the consequences of host radiation using vesicular stomatitis virus (VSV) and mammalian host cells in tissue culture. Replicate populations of VSV were allowed to evolve for 100 generations on the original host (BHK cells), on either of two novel hosts (HeLa and MDCK cells), or in environments where the availability of novel hosts fluctuated in a predictable or random way. As expected, each experimental population showed a substantial fitness gain in its own environment, but those evolved on new hosts (constant or fluctuating) suffered reduced competitiveness on the original host. However, whereas evolution on one novel host negatively correlated with performance on the unselected novel host, adaptation in fluctuating environments led to fitness improvements in both novel habitats.