Dose- and outcome-dependent effects of bacterial infection on female fecundity in Drosophila melanogaster

The virulence of a pathogenic infection challenge can manifest in the form of increased host mortality rates and/or reduced host fecundity. Reduced fecundity in infected hosts can result from resource-allocation trade-offs: increased investment in immune defences depletes the common pool of resources, which are also required for reproduction. Alternatively, reduced fecundity may result from damage to host organs, especially reproductive organs, caused by the infection. We infected Drosophila melanogaster females with the bacterial pathogen Enterococcus faecalis at different infection doses and found that increasing the infection dose led to greater suppression of fecundity, without a corresponding increase in mortality rates. We further found that the reduction in host fecundity is contingent on the infection outcome (whether the host lives or dies after being infected), particularly in flies infected with a high dose of bacteria. Interestingly, survivors of the infection challenge had exhibited comparable fecundity irrespective of the dose used to infect them, whereas amongst females that died after infection, a higher infection dose led to a lower fecundity. We therefore propose that our results indicate that fecundity suppression in E. faecalis -infected females is likely caused by host organ damage rather than by diversion of limited resources towards immune function.