Contrasting evolution of virulence and replication rate in an emerging bacterial pathogen

Host resistance through immune clearance is predicted to favour pathogens that are able to transmit faster and are hence more virulent. Increasing pathogen virulence is, in turn, typically assumed to be mediated by increasing replication rates. However, experiments designed to test how pathogen virulence and replication rates evolve in response to increasing host resistance, as well as the relationship between the two, are rare and lacking for naturally-evolving host-pathogen interactions. We inoculated 55 isolates of Mycoplasma gallisepticum collected over 20 years from outbreak, into house finches (Haemorhous mexicanus) from disease-unexposed populations, which have not evolved protective immunity to M. gallisepticum. We show using three different metrics of virulence (body mass loss, symptom severity and putative mortality rate) that virulence has increased linearly over >150,000 bacterial generations since outbreak (1994-2015). By contrast, while replication rates increased from outbreak through to the initial spread of resistance (1994-2004), no further increases have occurred subsequently (2007-2015). Finally, as a consequence, we found that any potential mediating effect of replication rate on virulence evolution was restricted to the period when host resistance was initially increasing in the population. Taken together, our results show that pathogen virulence and replication rates can evolve independently, particularly after the initial spread of host resistance. We hypothesize that the evolution of pathogen virulence can be driven primarily by processes such as immune manipulation after resistance spreads in host populations.