Jumping gene turned peppered moths the color of soot

Peppered moths and copycat butterflies owe their wing color-changing abilities to a single gene, two independent studies suggest.

A genetic tweak in a portion of the cortex gene that doesn’t make protein painted the speckled gray wings of peppered moths black, researchers report online June 1 in Nature. Genetic variants in DNA interspersed with and surrounding the cortex gene also help some tasty species of Heliconius butterflies mimic unpalatable species and avoid getting eaten by predators, a second team of scientists reports, also June 1 in Nature.

In the often-told evolutionary tale, the color shift in moths began as factories in Britain started to darken the skies with coal smoke during the Industrial Revolution in the 1800s. Victorian naturalists took note as a newly discovered, all-black carbonaria form of peppered moths (Biston betularia) blended into soot-covered backgrounds; the light-colored typica moths, which lacked the mutation, were easily picked off by birds. By 1970, nearly 99 percent of peppered moths were black in some localities. As air pollution decreased in the late 20^th century, black moths became more visible to birds. As a result, carbonaria moths are now rare.

“This begins to unravel exactly what the original mutation was that produced the black … moths that were favored by natural selection” during much of the last century, says evolutionary biologist Paul Brakefield of the University of Cambridge in England. “It adds a new and exciting element to the story.” 

Wing pattern changes in butterflies and peppered moths are textbook examples of natural selection, but the molecular details behind the adaptation have eluded scientists for decades. In 2011, researchers tracked the traits to a region of a chromosome all the species have in common (SN: 5/7/11, p. 11; SN: 9/24/11, p. 16). Which of the many genes in that region might be responsible remained a mystery.

In peppered moths, the region of interest stretches over about 400,000 DNA bases and contains 13 genes and two microRNAs. “There aren’t really any genes that scream out to you, ‘I’m involved in wing patterning,’” says evolutionary geneticist Ilik Saccheri at the University of Liverpool in England.

Saccheri and colleagues compared that region in one black moth and three typical moths. The researchers found 87 places where the black moth differed from the light-colored moths. Most of the differences were changes in single DNA bases — the information-carrying chemicals in DNA. Such genetic variants are known as SNPs for single nucleotide polymorphisms. One difference was the insertion of a 21,925-base-long stretch of DNA into the region. This big chunk of DNA contained multiple copies of a transposable element, or jumping gene. Transposable elements are viruslike pieces of DNA that copy and insert themselves into a host’s DNA.

By examining the DNA of hundreds more typica moths and ruling out mutations one by one, the team ended up with one candidate: the large transposable element that had landed in the cortex gene. But the jumping gene didn’t land in the DNA that encodes the protein. Instead it landed in an intron — a stretch of DNA that gets chopped out after the gene is copied into RNA and before a protein is made.

The jumping gene first landed in the cortex intron in about 1819, the researchers calculated from historical measurements of how common the trait was throughout history. That timing gave the mutation about 20 to 30 moth generations to spread through the population before people first reported sightings of the black moths in 1848. Saccheri and colleagues found the transposable element in 105 of 110 wild-caught carbonaria moths and none of the 283 typica moths tested. The remaining five moths are black because of another, unknown, genetic variation.

Similarly, Nicola Nadeau, an evolutionary geneticist at the University of Sheffield in England, and colleagues combed through more than 1 million DNA bases in each of five species of Heliconius butterflies. The researchers were looking for genetic variants associated with the presence or absence of yellow bands on the wings.

Nadeau’s team found 108 SNPs in all H. erato favorinus butterflies that have a yellow band on their hind wings. Most of those SNPs were in introns of the cortex gene or outside of the gene. Butterflies that lack the yellow band don’t have those SNPs.

Other DNA changes were found to draw yellow bars on the wings of different species of Heliconius butterflies, suggesting that evolution acted multiple times on the cortex gene with similar results.

The finding that the same gene influences wing patterns in butterflies and moths supports an idea that some genes are hot spots of natural selection, says Robert Reed, an evolutionary biologist at Cornell University.

None of the genetic differences in the butterflies or peppered moths change the cortex gene itself. That leaves open the possibility that the transposable element and SNPs aren’t doing anything to cortex, but may be regulating a different gene. But the evidence that cortex really is the gene upon which natural selection has acted is strong, says Reed. “I’d be surprised if they were wrong.”  

Still, it’s not obvious how cortex changes wing patterns, says Saccheri. “We’re both equally puzzled about how it is doing what it appears to be doing.” The teams have evidence that cortex helps determine when certain wing scales grow. In butterflies and moths, the timing of wing scale development affects the color of the wings, says Reed. “You see colors popping up almost like a paint-by-numbers.”

Yellow, white and red scales develop first. Black scales come later. Cortex is known to be involved in cell growth. So varying levels of the protein may speed up development of wing scales, causing them to become colored, or slow their growth, allowing them to turn black, the researchers speculate.