The Butterfly Wing Colouring Book

In my book Dazzled and Deceived: Mimicry and Camouflage, published in 2009, I tried to speculate on the mechanism behind the wing patterns of butterflies. There was plenty of work going on in laboratories across the world, especially in Cambridge, England and Cornell, USA, but sequencing was still expensive then and biologists didn’t have the most sophisticated gene editing tools. In the book I outlined a “colouring book” theory of patterning and colouring. There was evidence for this but it was far from conclusive.

I wrote:

“Painting is a useful analogy: on butterfly wings, nature is painting with the invisible hand of natural selection. What we know of butterfly wing patterns suggests that two aspects of painting are involved in nature’s patterning process. If you want to make a painting, you can either just paint and let the edge of each brush stroke dictate the shape, or you can make a sketch of the outlines first. If you make a sketch first, then painting, at its crudest, can be a colouring book exercise — just fill in the outlines. That’s simple — it’s the way kids do it.

If you don’t want make a sketch first, you run into the problem that it’s quite hard to know where the paint should stop to create the patterns you want. From the earliest days of the twentieth century, when biologists thought about pattern formation they imagined that washes of chemicals might spread across the wings during development and just stop where they ran out of steam. Or two gradients might meet and form patterns from the way they interacted. But perhaps there was a pre-patterned sketch that the colour merely filled in? A sort of black-and-white in which the areas to be coloured in later were marked out?”

On 18 September 2017, Nature reported that Bob Reed’s lab at Cornell have used CRISPR/Cas9 gene editing to control the wing patterns of several different species. The most startling result is the sudden appearance of blue iridescence in an edited Buckeye butterfly. Pigment colour and iridescent structural colour are achieved by totally different mechanisms. To create iridescence requires, not pigments, but intricately sculpted structures at a similar size to the wavelength of coloured light. Butterfly wing scales are already structured like this so in this case the loss of pigment in the edited butterfly seems to have revealed an underling capacity for iridescence.

In the Nature article, under the title ‘CRISPR reveals genetic master switches behind butterfly wing patterns’, the standfirst read:

“One gene draws the lines while a second fills in the colours.”

I couldn’t have put it better.

It’s a rare pleasure — no, almost unheard-of — for an armchair scientist to be vindicated like this. I’m sure it’s now going to get much more complicated but for now I’ll take this moment.