Resumen:
Chemical defences against predators underlie the evolution of aposematic coloration
and mimicry, which are classic examples of adaptive evolution. Surprisingly little is
known about the roles of ecological and evolutionary processes maintaining defence
variation, and how they may feedback to shape the evolutionary dynamics of species.
Cyanogenic Heliconius butterflies exhibit diverse warning color patterns and mimicry,
thus providing a useful framework for investigating these questions. We studied
intraspecific variation in de novo biosynthesized cyanogenic toxicity and its potential
ecological and evolutionary sources in wild populations of Heliconius erato along
environmental gradients, in common-garden broods and with feeding treatments. Our
results demonstrate substantial intraspecific variation, including detectable variation
among broods reared in a common garden. The latter estimate suggests considerable
evolutionary potential in this trait, although predicting the response to selection is
likely complicated due to the observed skewed distribution of toxicity values and
the signatures of maternal contributions to the inheritance of toxicity. Larval diet
contributed little to toxicity variation. Furthermore, toxicity profiles were similar
along steep rainfall and altitudinal gradients, providing little evidence for these factors
explaining variation in biosynthesized toxicity in natural populations. In contrast,
there were striking differences in the chemical profiles of H. erato from geographically
distant populations, implying potential local adaptation in the acquisition mechanisms
and levels of defensive compounds. The results highlight the extensive variation and
potential for adaptive evolution in defense traits for aposematic and mimetic species,
which may contribute to the high diversity often found in these systems.
