The peppered moths of 19th-century England are a textbook example of natural selection. After the onset of the Industrial Revolution, pollution killed off pale lichens on tree trunks and left them covered in soot. As a result, the common pale gray form became more exposed to their bird predators, and they declined rapidly. At the same time, the rare black variant increased steadily since they were now better camouflaged.
One thing was still unclear: What mutation gave rise to the previously unknown dark moths? According to two studies published in Nature this week, a gene called cortex and its associated mutation controls the darkening of peppered moth wings. Furthermore, the same gene that darkens moths also colors butterflies.
University of Liverpool’s Ilik Saccheri and colleagues discovered that a "jumping gene" inserted within cortex is responsible for the moths’ color change. Jumping genes, also called transposable elements, are chunks of mobile DNA that can change their position within a genome, and subsequently, alter the expression of other genes. The gene cortex helps regulate cell division in the insect order Lepidoptera, which includes moths and butterflies.
The insertion of the large transposable element into cortex occurred back in 1819, but it took about 30 years for the mutation to become common enough to be noticed. The first documented sighting of a black peppered moth was in 1848 in Manchester in northern England – though it could have gone undetected at low frequencies before then.
In another study, a team led by Nicola Nadeau from the University of Sheffield and Chris Jiggins from the University of Cambridge discovered that the expression of cortex varies with color patterning in butterflies from the genus Heliconius. That means the gene that blackens British moths also influences the extremely bright and colorful patterns of tropical butterflies.
"What's exciting is that it turns out to be the same gene in both cases. For the moths, the dark coloration developed because they were trying to hide, but the butterflies use bright colors to advertise their toxicity to predators," Jiggins explained in a statement. "It raises the question that given the diversity in butterflies and moths, and the hundreds of genes involved in making a wing, why is it this one every time?"
There are about 160,000 types of moths and 17,000 butterflies, and nearly all of them have different patterns on their wings. Their colors and markings can serve a variety of functions: attract mates, warn would-be predators, camouflage, or temperature regulation. The patterns on butterfly wings are made of tiny, overlapping colored scales. As a cell cycle regulator, cortex controls when different scales develop within the wing.
"The butterfly and moth polymorphisms appear very different to the eye, and the species are separated by over 100 million years," Saccheri said in another statement. "What this suggests is that the cortex gene is central to generating pattern diversity across the Lepidoptera, and more generally, that adaptive evolution often relies on a conserved toolkit of developmental switches."
Images in the text: patterns on a Heliconius melpomene wing by Nicola Nadeau (top) and Heliconius erato demophoon by Melanie Brien (bottom).
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