Adaptation to altitude in Heliconius butterflies

Local adaptation is an important process for studying recent evolutionary change. The environment changes drastically along steep clines, such as mountains, and diverse sets of challenges are predicted to drive local adaptation. Thus, these clines represent ideal settings to identify the traits and genomic mechanisms that allow some organisms to succeed across wide geographical ranges, which is a major goal of evolutionary biology. In this thesis I explore the environmental variables, phenotypic traits, and genomics underlying adaptation to altitude in the Heliconius butterfly genus. Firstly, using a collection of over 3500 wings I discovered that wing morphology varies predictably across elevations, with species and populations in the highlands having rounder wings than those in the lowlands. This study also highlighted that life-history, whether larvae are gregarious or solitary, determines the direction of wing sexual size dimorphism across species. Secondly, to understand the microclimates experienced in the wild by Heliconius butterflies, I measured hourly temperature and humidity for a full year in 28 sites across elevations and microhabitats on both sides of the Andes. The canopy greatly buffered the climate within the forest, but publicly available datasets failed to accurately predict these temperatures. Further, I found that species inhabiting higher altitudes were less tolerant to heat in the wild, while common-garden reared individuals of H. melpomene were equally tolerant after one generation, showing plasticity for this trait. Thirdly, with a dataset of over 600 whole-genome sequenced H. erato and H. melpomene individuals from four elevational clines, I found many parallel signatures of local adaptation to high altitude across clines and sides of the Andes, especially within H. erato. Finally, I studied the genomic basis of one of the traits I identified as being potentially involved in adaptation to altitude, wing shape. By combining common-garden rearing of highland and lowland populations of H. erato and H. melpomene and 666 whole-genome sequences from a published study, I found that wing aspect ratio is highly heritable, and identified relevant candidates for future functional studies. Overall, this work highlights that butterflies readily adapt to their local environment and that they do so in a more convergent fashion than previously thought. This thesis lays the groundwork for an exciting new branch of study for Heliconius research, where we combine the long fascination for their natural history and speciation, with new approaches to study how they adapt to the environment.

University of Cambridge Earth System Science NERC Doctoral Training Partnership