Host-jumping, demographic stochasticity and extinction: Lytic viruses

Question: We envision a lytic virus invading a novel-host population, when rarity of productive infections suggests a role for demographic stochasticity. We ask how functionally constrained viral trait combinations that reduce the chance of extinction (promoting invasion) might differ from traits increasing the expected growth rate of the viral population. Mathematical methods: To focus on random variation in viral reproduction (burst size), we develop a branching process and derive the probability generating function for the number of new infections per infection. The generating function permits comparison of the extinction probability and mean growth rate for any viral life history. Then we turn to random variation in viral generation length, which sums time spent as a free virion with time reproducing within a host. We simulate this process to compare extinction frequency and mean growth rate for different combinations of viral traits. Key assumptions: We assume infections are rare as invasion of the novel host begins, and neglect density dependence. We emphasize pleiotropic constraints, functional dependencies between viral traits governing quantity and quality of viral reproduction, and survival of free virions. Conclusions: When pleiotropic interaction affects the burst-size distribution, with generation time fixed, extinction-resistant phenotypes increase offspring quantity, at the expense of either increased error during replication or reduced survival outside of a host, compared with growthrate maximizing phenotypes. When pleiotropic interaction affects the random waiting time until lysis, extinction-resistant phenotypes delay lysis to gain either increased survival outside of hosts or larger bursts, at the expense of slower reproduction within hosts, compared with growth-rate maximizing phenotypes.