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 ofPassiflora plants keep most herbivores at bay, but not the larvae ofHeliconiu s butterflies, which can both sequester and biosynthesize cyanogenic compounds. Here, we demonstrate that bothHeliconius cydno chioneus andH. melpomene rosina have remarkable plasticity in their chemical defences. When feeding onPassiflora species with cyanogenic compounds that they can readily sequester, both species downregulate the biosynthesis of these compounds. By contrast, when fed onPassiflora 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 specialistH. m. rosina , as adult size and weight for this species negatively correlate with biosynthesis levels, but not for the more generalistH. c. chioneus . By contrast,H. m rosina 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 thePassiflora genus, while maintaining their chemical defences.