Mating enhances immune function of Drosophila melanogaster populations against bacterial pathogens

Immunity and reproduction are two important processes that affect fitness of an organisms. Sexual activity has been previously shown to determine the degree to which a population is able to survive various infections. While many studies have demonstrated a trade-off between immune function and reproduction, many studies have found synergistic relation between the two fitness determinants. Besides it is generally hypothesised that sexes may differ in immunity due to relative cost they incur during reproduction with males losing in immunity to rather increase their reproductive success. In this study, we test the effect of immune function on the survival of mated and virgin replicates of a large outbred baseline D. melanogaster population that was infected with four different bacterial infections. We find enhanced survival in mated flies than virgin flies in response to all four bacterial infections across all replicates. While investigating sexual dimorphism in immune function, we find no difference in sexes in their ability to survive the imposed bacterial infections. Synergistic interaction between reproduction and immunity may exist if it improves Darwinian fitness of either sexes of a population under selection, and are not necessarily limited by each other due to trade-off over finite resources.