Life-history trade-offs and stress resistance in Drosophila melanogaster populations adapted to pathogenic bacterial infection

Abstract

Evolution of increased immune defence is often limited by costs: correlated changes in other traits (viz. life-history traits) that otherwise reduce the fitness of the host organisms. Experimental evolution studies are useful for understanding the evolution of immune function and correlated changes in other traits. We experimentally evolved replicate Drosophila melanogaster populations to better survive infection challenges with an entomopathogenic bacteria, Enterococcus faecalis. Within 35 generations of directional selection, selected populations showed a marked increase in post-infection survival than ancestrally paired controls. We next measured various life-history traits of these populations. Our results show that the selected populations do not differ from control populations for larval development time and body weight at eclosion. No difference is also observed in case of fecundity and longevity (following the acute phase of infection), either when the flies were subjected to infection or when the flies were uninfected, although infected flies from all populations die much earlier than uninfected flies. Selected populations are either equally good or occasionally better as the control populations at surviving abiotic stressors (starvation and desiccation), although infected flies from all populations are more susceptible to stress than uninfected flies. Therefore, we conclude that (a) D. melanogaster populations can rapidly evolve to be more immune to infection with E. faecalis, (b) the evolution of increased defence against E. faecalis entails no life-history cost for the hosts and (c) evolving defence against a biotic threat (pathogen) does not make flies more susceptible to abiotic stressors.