John J. Dennehy (Yale University), Nicholas A. Friedenberg (Dartmouth College), Robert D. Holt (University of Florida), and Paul E. Turner (Yale University), “Viral ecology and the maintenance of novel host use”
Infectious diseases often mysteriously appear and disappear in populations. On some occasions small outbreaks rapidly die out; on others, epidemics ensue. Dennehy et al. explore the dynamics of pathogen emergence using viruses and their host bacteria as a study system. A mathematical model was constructed from basic demographic data to predict transmission rates required for persistence of viruses on their native and novel hosts. The model was validated with serial passage experiments revealing a range of transmission rates where virus populations were sustained on the native host, but went extinct on the novel host. In this critical region, periodic exposure to native hosts allowed the viruses to survive on novel hosts, an unanticipated result. The mechanism behind this phenomenon may be a â€œhost legacyâ€ effect. Dennehy et al. observe that viruses previously reared on the native host showed greater productivity on the novel host than did viruses previously reared on the novel host. This experimental demonstration of a host legacy effect has important implications. The capacity of a virus to propagate upon a novel host apparently is conditional on the recent experience of preceding generations. This is intrinsically interesting, suggesting a kind of complexity in pathogen population dynamics that has not been widely regarded. Second, given this host legacy effect, the total viral population size experiencing the novel environment is greater than would otherwise be expected. Because the amount of genetic variation that can be exposed to selection via novel mutations should scale with population size, this provides a more fertile ground for adaptive evolution to the new host.
Filed under: Bacteria