Iridescent colour in the butterfly Heliconius erato
Author: Mel Brien, ACCE PhD student at APS, Sheffield.
Structural colours are some of the brightest, most reflective colours in nature. They have different functions including visual communication, mate choice and thermoregulation. In contrast to many other colours which are produced by chemical pigments, structural colours are produced from physical nanostructures which reflect light in different ways.
The butterfly Heliconius erato is found across Central and South America, and it has a huge diversity in its wing colour patterns.
Because of the plants they feed on as caterpillars, they are toxic to predators, and they use these colour patterns as warning signals to the predators, who learn to avoid them. Most of these wing colour patterns are formed by pigments, and we know that these are controlled by a small number of genes.
West of the Andes in Ecuador you can find Heliconius erato cyrbia, which has a bright blue iridescent structural colour and very little is known about the genetic basis of this iridescent colour. As you move further north through Colombia, Erato subspecies gradually lose the blue colour and become matt black.
In our new paper, out in a special issue of the Journal of the Royal Society Interface Focus, we used experimental crosses between subspecies of Heliconius erato with and without iridescent colour to look at the genetics behind this trait. These crosses were carried out with biologists and staff at the Mashpi Reserve in Ecuador. Variation in the brightness of the blue colour in the offspring shows that it is controlled by multiple genes. Female offspring which had an iridescent father where much bluer than those which had an iridescent mother. This suggests that the trait is sex-linked – there are loci involved in controlling the colour on the Z sex chromosome.
In collaboration with Andy Parnell from the University of Sheffield Physics department, we took butterfly wings to the ESRF – the European Synchrotron Radiation Facility in France – to take X-ray scattering measurements which allow us to look at variation in the nanostructures. Scales are made up of longitudinal ridges and the spacing between these affects the intensity of the colour. Those butterflies with narrower spacing between these ridges had brighter levels of iridescent blue colour, and it seems that this spacing is also controlled by multiple genes.
The colour patterns of Heliconius erato also mimic the patterns of another Heliconius species, Heliconius Melpomene, despite these being quite distantly related. Future work will look if the same genetic patterns are found in this mimetic species.