Parental effects of pathogenic bacterial infections in Drosophila melanogaster: trans-generational immune priming without apparent costs

Parental experience with pathogens and parasites can shape offspring susceptibility to infections, a phenomenon commonly referred to as trans-generational immune priming (TgIP) in insects. TgIP can influence the evolution of both host immune traits and pathogen virulence. The host ecology and its environmental pathogen load can in turn shape the evolution of TgIP. Therefore, the functional limits of TgIP and the conditions under which it is observed need to be explored in a variety of species. Majority of empirical studies addressing this phenomenon focus on non-pathogenic infections of parents, implemented via either the use of pathogen substitutes (viz., liposaccharide molecules), or the use of sub-lethal infection doses, by using inactivated pathogens to infect the parents. We tested if exposing parents to pathogenic bacterial infections affects offspring susceptibility to homologous (same pathogen as the one used to infect the parents) and heterologous (different pathogen than the one used to infect the parents) infection challenges in Drosophila melanogaster. We found that parental infection increases offspring survival following a homologous challenge. Similar effects were seen in the case of heterologous challenges too, with the breadth of cross-reactivity being dependent on the pathogen used to infect the parents. Parental infection also improves offspring capacity to restrict systemic pathogen proliferation following a homologous challenge, suggesting increased disease resistance. Parental infection has no effect on offspring reproduction, either in presence or absence of infection, suggesting an absence of associated costs and limited benefit of trans-generational immune priming. Overall, we have demonstrated that parental infection helps offspring better counter infection challenges in D. melanogaster without incurring any measurable costs.