Abstract

Life history tradeoffs between reproductive output and immune performance are commonly observed throughout the animal kingdom but are rarely understood mechanistically. In the insect Drosophila melanogaster, reproductive investment reduces resistance to bacterial infection. We have found that the reduction in immunity can be traced to the cost of producing eggs, and that mating-induced changes in hormone balance regulate the differential investment in fecundity versus immunity. However, rather than arising through an adaptive shift in allocation of energetic resources from one process to the other, the tradeoff seems to originate in physiological constraints and overburdening of the fat body tissue. We observe natural genetic polymorphism for the magnitude of post-mating immune suppression but it is largely uncorrelated with reproductive output, potentially limiting the opportunity for evolutionary optimization. Thus, while reproduction and immune performance are observed to trade off in D. melanogaster as would be expected under classical life history theory, the mechanism and evolutionary potential may not conform to classical expectations.