The genetics and evolution of iridescent structural colour in Heliconius butterflies

The study of colouration has been essential in developing key concepts in evolutionary biology. The Heliconius butterflies are well-studied for their diverse aposematic and mimetic colour patterns, and these pigment colour patterns are largely controlled by a small number of homologous genes. Some Heliconius species also produce bright, highly reflective structural colours, but unlike pigment colour, little is known about the genetic basis of structural colouration in any species. In this thesis, I aim to explore the genetic basis of iridescent structural colour in two mimetic species, and investigate its adaptive function. Using experimental crosses between iridescent and non-iridescent subspecies of Heliconius erato and Heliconius melpomene, I show that iridescent colour is a quantitative trait by measuring colour variation in offspring. I then use a Quantitative Trait Locus (QTL) mapping approach to identify loci controlling the trait in the co-mimics, finding that the genetic basis is not the same in the two species. In H. erato, the colour is strongly sex-linked, while in H. melpomene, we find a large effect locus on chromosome 3, plus a number of putative small effect loci in each species. Therefore, iridescence in Heliconius is not an example of repeated gene reuse. I then show that both iridescent colour and pigment colour are sexually dimorphic in H. erato and H. sara, pointing to differing selection pressures on the sexes. Structural colour, and to a lesser extent pigment colour, are condition dependent, suggesting the trait could be used as a signal of condition in mate choice. Together this work provides an understanding of the evolution of structural colour in Heliconius, in terms of its genetic control and its function as a signal and mimetic warning pattern.