Viruses sometimes can evolve the ability to infect new hosts, but the determinants of host range and of differential fitness on different hosts are not always clearly identified or separated. Ecological and evolutionary factors as well as genetic constraints can all influence host switching capability as well as fitness within new hosts. Experiments addressing these may pave the way for a clearer understanding of emerging infectious diseases. However, conclusions may be limited by considering only some of these factors or by the range of approaches taken to study the question.

Here, the focus is on how transitions between hosts affect viral adaptation overall. We have used the model bacteriophage ΦX174 and three different host bacterial strains. The fitness of each population, propagated for a relatively short period on a given bacterial strain, is measured using qPCR in liquid culture with the same and with other hosts. This allows us to characterize fitness improvements on hosts used for propagation and to identify pleiotropic effects in other hosts. Genotypes are determined in lysates and in isolates and are compared with changes observed in the literature to better characterize mutations occurring during growth in liquid culture. The fitness effects of some transfers between hosts are also explored.

ΦX174 shows significant host range flexibility, but our approach allows us to better characterize the fitness costs often linked with host switching. We hope that our work contributes to a better understanding of the general constraints affecting the evolution of parasite populations as they adapt to the complexities of a novel host environment.