Evolution of the host range of the bacteriophage &phis;6

The emergence of novel viral disease into naive host populations is a specific case of biological invasion. One critical component of viral disease emergence is the evolution of a virus' host range -- the ability of a virus to utilize different hosts. In this dissertation I use the bacteriophage &phis;6 as a model system to investigate the evolution of virus host range.;The expansion of a virus' host range is often the first step in emergence. I found that &phis;6 expands its host range by acquiring mutations that allow viral attachment to a novel host, that many mutations can expand host range, and that the majority of these mutations cause a decrease in the fitness of &phis;6 on its original host. I also examined whether the genetic similarity between the original host of &phis;6 and a novel host was predictive of characteristics of host range mutations that are important to emergence. I found that as host genetic similarity decreases, fewer mutations are capable of expanding &phis;6's host range, the fitness costs these mutations cause &phis;6 on its original host decrease, and the fitness of these host range mutants on a novel host decreases. Taken together, these results suggest that as host genetic similarity decreases, viruses should be less successful in colonizing and persisting on a novel host.;Once a virus can grow on a novel host, viral adaptation is often necessary for persistence on this host. I adapted replicate populations of an expanded host range mutant of &phis;6 to a novel host until each population acquired a single adaptive mutation. I found that all of these mutations greatly increased the fitness of &phis;6 on this novel host, that they occur in many genes in the &phis; genome, and that most of these mutations do not reduce the fitness of &phis;6 on its original host. In conjunction with the data I collected on host range expansion, these results suggest that the expansion of a virus' host range will often be the limiting step in successful emergence.