Disentangling local adaptation and phenotypic plasticity in traits associated with altitude and temperature in widespread tropical butterflies.

Climatic stratifications, in particular differences in temperature, occur along altitudinal clines. Understanding divergence across these regions can give insight into speciation and diversification, as well as how species may respond to climate change scenarios. Most past research has focused on temperate regions, yet year-round stable temperatures in the tropics make altitudinal temperature variation more pronounced and increase the likelihood of local adaptation across relatively narrow gradients. Here we investigate how genetics and the environment influence a wide range of traits in two widespread neotropical butterfly species, Heliconius erato and Heliconius melpomene, which occur across an altitudinal cline in the Andes. Using "common garden" rearing of over 1,000 offspring from over 70 wild females caught along an altitudinal gradient, as well as rearing of populations from either end of the altitudinal range in their reciprocal temperature environments, we investigate genetic, environmental and gene-by-environment interaction effects on developmental, morphological, and thermal tolerance traits. We find parallel divergence in wing colour in both species, with darker colours at higher altitude, consistent with a role in thermoregulation. We also find evidence for gene-by-environment interactions: In H. erato we found local differences in heat acclimation response, with increased heat knock-out times at higher rearing temperatures found only in low altitude populations, which are exposed to the hottest temperatures. We find evidence for heritable genetic variation in most traits measured, with positive implications for adaptation to climate change, although our results suggest that selection may not act in a straightforward way on these traits.