In a paper published in Philosophical Transactions of the Royal Society, B, we demonstrate the genetic basis of structural colour evolution in the co-mimic butterflies, Heliconius erato and Heliconius melpomene. These species both show variation in colour from black to blue, due to differences in nanostructures on their scale wings, across a geographical cline in South to Central America. This brings together the work of three former and current PhD students in the group.
We performed crosses between blue and black subspecies of both species and used these to identify regions of the genome responsible for colour variation. Firstly, Melanie Brien used photographs to quantify colour variation between individuals. Juan Enciso-Romero, then used x-ray scattering data from the European Synchrotron Radiation Facility to quantify variation in the scale structures.
In Heliconius erato we largely find a single genomic region that controls variation in both blue colour and scale structure. This is on the sex-determining Z chromosome (males have two copies of this chromosome while females just have one)
In Heliconius melpomene, by contrast, we find that quantified colour differences are associated with variation on chromosome 3, while variation in scale structure is associated with chromosome 7. These results suggest that the two species have convergently evolved similar colour using different genetic mechanisms.
To try to find which genes in these genomic regions were controlling structural colour, Victoria Lloyd then compared the level of expression of genes within this mapped genomic regions. Using developing wing tissue from blue and black butterflies, she looked for genes that were expressed at a higher level in the blue wings. Interesting candidates include genes that are part of or interact with the cell cytoskeleton, which might have a role in shaping the structures.
This work is a first step in understanding how biological systems make and control structural colours. Something that humans have been trying to replicate for decades, with limited success.