Dietary protein mediates terminal investment in egg quantity or quality following bacterial gut infection in Drosophila

Organisms have evolved a range of behavioural and physiological responses which minimize the impact of infection on fitness. When future reproductive potential is threatened, for example, as a result of pathogenic infection, the terminal investment hypothesis predicts that individuals will respond by investing preferentially in current reproduction. Terminal investment involves reallocating resources to current reproductive effort, so it is likely to be influenced by the quantity and quality of resources acquired through diet. Dietary protein specifically has been shown to impact both immunity and reproductive output in a range of organisms, but its impact on terminal investment during infection is unclear. We tested the effect of dietary protein on terminal investment in the fruit fly Drosophila melanogaster following oral exposure to the opportunist bacterial pathogen Pseudomonas aeruginosa. Oral exposure to bacteria triggered an increase in reproductive investment, but we find that the nature of the terminal investment strategy depended on the level of dietary protein. Flies feeding on a high protein diet increased the number of eggs laid when exposed to P. aeruginosa, while flies fed an isocaloric, lower protein diet did not increase the number of eggs laid but instead showed an increase in egg-to-adult viability following infection. We discuss the importance of considering diet and natural routes of infection when measuring non-immunological defenses.