Evolution of phenotypic plasticity: genetic differentiation and additive genetic variation for induced plant defense in wild arugula Eruca sativa.

Phenotypic plasticity is the primary mechanism of organismal resilience to abiotic and biotic stress, and genetic differentiation in plasticity can evolve if stresses differ among populations. Inducible defense is a common form of adaptive phenotypic plasticity and long-standing theory predicts that its evolution is shaped by costs of the defensive traits, costs of plasticity, and a trade-off in allocation to constitutive versus induced traits. We used a common-garden to study the evolution of defense in two native populations of wild arugula Eruca sativa (Brassicaceae) from contrasting desert and Mediterranean habitats that differ in attack by caterpillars and aphids. We report genetic differentiation and additive genetic variance for phenology, growth, and three defensive traits (toxic glucosinolates, anti-nutritive protease inhibitors, and physical trichome barriers) as well their inducibility in response to the plant hormone jasmonic acid. The two populations were strongly differentiated for plasticity in nearly all traits. There was little evidence for costs of defense or plasticity, but constitutive and induced traits showed a consistent additive genetic trade-off within each population for the three defensive traits. We conclude that these populations have evolutionarily diverged in inducible defense and retain ample potential for the future evolution of phenotypic plasticity in defense.