Evolutionary Ecology of Virus Emergence
Viruses emerge by acquiring mutations permitting infection of new hosts. However, emergence cannot be sustained unless a virus’s growth rate in the new host population is greater than their death rate. It is expected that in most cases, this condition is not met, and the virus is said to exist in a sink habitat. The virus must adapt to the new host or go extinct.
In some situations, viruses can escape sinks. For example, migration of viruses from other habitats may permit a viruses population to persist in a sink long enough for adaptation to take place. In a study published in Evolution, we showed that intermediate levels of migration fostered virus adaptation in a sink habitat.
In this figure, the open circles represent the fitness after 30 passages of evolution for populations receiving migrants from a population evolved on the same host. By contrast, control populations (filled circles) did not experience any increases in fitness.
In another study published in Proceedings of the Royal Society B, we showed that the source of migrants did not matter (much). As long as the migrating viruses could infect the novel host, they contributed to virus population growth and adaptation.
In this figure, the open circles represent populations of Φ6 receiving migrants from a source population evolved on the same host type. The filled circles show the fitness trajectories of Φ6 populations receiving migrants from a source population evolved on a different host type (thus are not expected to contribute mutations advantageous on the present host type). The open squares constitute control populations.
In more recent work, we showed that Φ6 adaptation to a novel host in communities containing a mixture of hosts is proportional to the frequency of novel hosts in that community. Following 20 passages in mixed host cultures, we obtained the following results.
In both figures, reduction in host OD (y-axis) is used as a proxy for fitness. On the right, greater initial frequencies of the novel host ERA are correlated with greater adaptation to ERA. By contrast, as the figure on the left shows, increased initial frequencies of ERA are correlated with decreased adaptation to the canonical host PP.