Phenotypic plasticity in chemical defence allows butterflies to diversify host use strategies

Hostplant specialization is a major force driving ecological niche
partitioning and diversification in insect herbivores. The cyanogenic
defences of Passiflora plants keeps most herbivores at bay, but not larvae
of Heliconius butterflies, which can both sequester and biosynthesize
cyanogenic compounds. Here, we demonstrate that both Heliconius cydno
chioneus, a host plant generalist, and H. melpomene rosina, a specialist,
have remarkable plasticity in their chemical defence. When feeding on
Passiflora species with cyanogenic compounds they can readily sequester,
both species downregulate the biosynthesis of these compounds. In
contrast, when fed on Passiflora plants that do not contain cyanogenic
glucosides that can be sequestered, both species increase biosynthesis.
This biochemical plasticity comes at a significant fitness cost for
specialist like H. m. rosina, as growth rates for this species negatively
correlate with biosynthesis levels, but not for a generalist like H. c.
chioneus. In exchange, H. m rosina has increased performance when
sequestration is possible as on its specialised hostplant. In summary,
phenotypic plasticity in biochemical responses to different host plants
offers these butterflies the ability to widen their range of potential
host within the Passiflora genus, while maintaining their chemical
defences.