Insecticide resistance evolution and assisted gene flow interact to shape the evolution of plasticity

Adaptive phenotypic plasticity can bolster fitness in changing environments, but the extent to which plasticity evolves rapidly, and which forces shape this evolutionary trajectory, is largely unknown. To empirically study the evolution of plasticity we first conducted a replicated field experiment in which Drosophila melanogaster populations adapted to insecticide exposure and a subset of these populations received high diversity assisted gene flow. We then reared individuals from each population across temperature and insecticide treatments in common garden to test the following questions: 1. Has prior selection and rapid adaptation of insecticide resistance led to evolved shifts in plasticity relative to naïve populations? 2. Does gene flow from genetically diverse populations contribute to adaptive plasticity evolution relative to gene flow-restricted low diversity populations? Both gene flow and prior evolution of resistance influenced the evolution of plasticity for multiple traits and were often maladaptive for resistant and gene flow-restricted populations, suggesting a trade-off between trait and plasticity evolution. Assisted gene flow minimized maladaptive plasticity potentially through relaxation of underlying epistatic or pleiotropic constraints. Together, these results demonstrate the dynamic interactions between trait evolution, the evolution of plasticity, and forces that shape genetic diversity with implications for conservation of threatened populations.