Reciprocal costs of infection and reproduction in <i>Drosophila melanogaster</i>

Trade-offs occur when an organism has to allocate limited resources to multiple biological processes. How organisms allocate their resources and whether one trait gets priority over another is poorly understood. Prior work has shown that reproductive investment reduces the capacity of <i>Drosophila melanogaster</i> to mount an effective immune response against subsequent bacterial infection. However, it has not been tested whether the observed trade-off was unidirectional with reproductive fitness given primacy over immunity, or whether it might also occur in the reciprocal direction with an active prior immune response reducing reproductive output. In this work, we delivered bacterial infection to female <i>D. melanogaster</i> prior to mating and tested whether reproductive capacity became reduced. We found that infected females produced the same number of eggs as uninfected females, but the eggs from infected females exhibited lower survivorship to adulthood. Additionally, we found that mating destabilizes chronic bacterial infections, stimulating additional host death and increasing variance in pathogen burden. Together, our results suggest the cost of reproduction and infection in <i>Drosophila</i> females is reciprocal, regardless of the order in which they occur.