Phenotypic plasticity in chemical defence of butterflies allows usage of diverse host plants.

Host plant specialization is a major force driving ecological niche partitioning and diversification in insect herbivores. The cyanogenic defences of plants keep most herbivores at bay, but not the larvae of s butterflies, which can both sequester and biosynthesize cyanogenic compounds. Here, we demonstrate that both and have remarkable plasticity in their chemical defences. When feeding on species with cyanogenic compounds that they can readily sequester, both species downregulate the biosynthesis of these compounds. By contrast, when fed on plants that do not contain cyanogenic glucosides that can be sequestered, both species increase biosynthesis. This biochemical plasticity comes at a fitness cost for the more specialist , as adult size and weight for this species negatively correlate with biosynthesis levels, but not for the more generalist . By contrast, has increased performance when sequestration is possible on its specialized host plant. In summary, phenotypic plasticity in biochemical responses to different host plants offers these butterflies the ability to widen their range of potential hosts within the genus, while maintaining their chemical defences.