The host susceptibility to one pathogen can decrease, increase, or remain unaffected by virtue of the host evolving resistance towards a second pathogen. Negative correlations between a host susceptibility to different pathogens is an often-cited explanation for maintenance of genetic variation in immune function determining traits in a host population. In this study, we investigated the change in susceptibility of Drosophila melanogaster flies to various novel bacterial pathogens after being experimentally selected for increased resistance to one particular bacterial pathogen. We independently selected flies to become more resistant towards Enterococcus faecalis and Pseudomonas entomophila, and baring a few exceptions the evolved populations exhibited cross-resistance against the range of pathogens tested in the study. Neither the identity of the native pathogen nor the host sex was major determining factors in predicting the pattern of cross-resistance exhibited by the selected populations. We therefore report that a generalized cross-resistance to novel pathogens can repeatedly evolve in response to selection for resistance against a single pathogen.

Mounting an immune response requires a considerable energy investment by the host. This makes expression of immune phenotypes susceptible to changes in availability of resources. There is ample evidence in scientific literature to suggest that hosts become more vulnerable to infection by pathogens and parasites when access to nutrition is limited. Using populations of Drosophila melanogaster experimentally evolved to better survive bacterial infections, we explore if host selection history influences host response to resource deprivation in terms of immune function. We find that when reared on a suboptimal diet (both in larval and adult stages), adult flies from evolved populations are still more immune to infections compared to flies from control populations. Furthermore, we observe a sex-dependent effect of interaction between selection history and diet on immune function. We thus conclude that immune function of hosts adapted to pathogen challenge is less affected by resource limitation compared to non-adapted hosts.

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 challenge with an entomopathogenic bacteria, Enterococcus faecalis. Within 35 generations of forward selection, selected populations showed marked increase in post-infection survival compared to 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 are subjected to infection or when the flies were uninfected; although infected flies from all populations die much earlier compared to uninfected flies. Selected flies and control flies are also equally good 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) 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 resistant to abiotic stressors.

In the experiments reported in this manuscript, we explore the effect of bacterial infections on the reproductive output of Drosophila melanogaster females. Canonical view of host-pathogen interactions supposes two possible outcomes. Because of immune defence being an energy/resource intensive function, an infected female reallocates resources away from reproductive processes and towards immune defence, therefore compromising its reproductive output. Alternatively, faced with impending mortality, an infected female increases its reproductive output to compensate for lost opportunities of future reproduction. We tested if pathogen identity, infection outcome (survival vs. death), and/or time of death determines the reproductive output of females infected with three bacterial pathogens. Our results show that pathogen identity is a reliable predictor of population level response of infected females but does not reliably predict the behaviour of individual females. Additionally, females succumbing to infection exhibit greater variability in reproductive output, compared to both survivors and controls, but this variability is not explained by either the time of death or the identity of the infecting pathogen. Furthermore, survivors of infection have reproductive output similar to control females.

Sexual activity (mating) negatively affects immune function in various insect species, in both sexes. In the experiments reported in this manuscript, we tested if hosts adapted to regular pathogen challenges are less susceptible to mating induced immune suppression, using experimentally evolved Drosophila melanogaster populations selected for increased post-infection survival when infected with a Gram-positive bacterium, Enterococcus faecalis. Mating increased susceptibility of females to bacterial pathogens, but in a pathogen specific manner. Mating-induced increase in susceptibility was also affected by host evolutionary history, with females from selected populations exhibiting similar post-infection survival irrespective of mating status, while females from control populations became more susceptible to bacterial infections after mating. Post-infection survival of males, irrespective of their evolutionary history, was not affected by their mating status. We therefore conclude that hosts evolved to better survive bacterial infections are also better at resisting mating-induced increase in susceptibility to infections in Drosophila melanogaster.