State-dependent parasitism by a facultative parasite of fruit flies.

Parasites can evolve phenotypically plastic strategies for transmission such that a single genotype can give rise to a range of phenotypes depending on the environmental condition. State-dependent plasticity in particular can arise from individual differences in the parasite's internal state or the condition of the host. Facultative parasites serve as ideal model systems for investigating state-dependent plasticity because individuals can exhibit two life history strategies (free-living or parasitic) depending on the environment.