Experimental evolution for improved post-infection survival selects for increased disease resistance in Drosophila melanogaster

Disease resistance (defined as the host capacity to limit systemic infection intensity) and disease tolerance (defined as the host capacity to limit infection-induced damage) are two complementary defense strategies that help the hosts maximize their survival and fitness when infected with pathogens and parasites. In addition to the underlying physiological mechanisms, existing theory postulates that these two strategies differ in terms of the conditions under which each strategy evolves in host populations, their evolutionary dynamics, and the ecological and epidemiological consequences of their evolution. Here we explored if one or both of these strategies evolve when host populations are subjected to selection for increased post-infection survival. We experimentally evolved Drosophila melanogaster populations, selecting for the flies that survived an infection with the entomopathogen Enterococcus faecalis, and found that the host populations evolved increased disease resistance in response. This was despite the physiological costs associated with increased resistance. We did not find evidence of any change in disease tolerance in the host populations. We have therefore demonstrated that in an experimental evolution set-up, where insect hosts must survive an infection with a pathogenic bacterium, the hosts evolve improved disease resistance but not disease tolerance.