Some bird species are permanently grounded. New research shows they may have evolved this way due to tweaks in DNA that bosses genes around.
Emus, ostriches, kiwis, rheas, cassowaries and tinamous all belong to a group of birds called ratites. (So do the extinct moa and elephant birds.) Of these, only tinamous can fly. Scientists studied the regulatory DNA of these birds to learn why most of them can’t fly. The researchers found that mutations in regulatory DNA caused ratites to lose flight. That happened in up to five separate branches of the birds’ family tree. The researchers reported their results April 5 in Science.
Regulatory DNA is more mysterious than the DNA that makes up genes. Studying how this bossy DNA drives evolution could shed light on how closely related species can evolve such different traits.
Genes are pieces of DNA that hold instructions for making proteins. In turn, the proteins do tasks inside your body. But regulatory DNA doesn’t carry protein-making instructions. Instead, it controls when and where genes turn on and off.
Researchers have long debated how big evolutionary changes happen, such as gaining or losing flight. Is it because of mutations — changes — to protein-making genes that are tied to the trait? Or is it mainly because of tweaks to the more mysterious regulatory DNA?
Scientists often had stressed the importance in evolution of changes in the genes that code for (or make) proteins. Examples are relatively easy to find. For instance, an earlier study suggested that mutations in a single gene shrank the wings of flightless birds known as Galápagos cormorants.
In general, mutations that change proteins are likely to do more damage than changes to regulatory DNA, says Camille Berthelot. That makes those changes easier to spot. Berthelot is an evolutionary geneticist in Paris at the French national medical research institute, INSERM. One protein may have many jobs throughout the body. “So everywhere this protein is [made], there’s going to be consequences,” she says.
By contrast, many pieces of DNA may help regulate a gene’s activity. Each piece of bossy DNA might work in only one or a few types of tissue. That means a mutation in one regulatory piece won’t do as much damage. So changes can add up in those bits of DNA as animals evolve.
But that also means it’s much harder to tell when regulatory DNA is involved in big evolutionary changes, says Megan Phifer-Rixey. She’s an evolutionary geneticist who works at Monmouth University in West Long Branch, N.J. Those pieces of DNA don’t all look alike. And they may have changed a lot from species to species.