Mutation appears to have arisen once and then spread through breeding
A change in a single gene ruffles the feathers of all pigeons with collars and crests, a new study shows.
Many breeds of rock pigeons have these crests, even though they come from different branches of the pigeon family tree. So it was a surprise that all the birds owe their fancy plumage to the same mutation in a gene called EphB2, Michael Shapiro of the University of Utah and colleagues report online January 31 in Science. The researchers found that the mutation arose once and spread to many different types of pigeons through breeding programs. They don’t yet know whether the mutation arose in a wild ancestor of domesticated pigeons or if it sprang up only after domestication.
As different species evolve, some traits show up again and again. Scientists debate whether each occurrence comes from the same genetic mechanism, as in the pigeons, or from different mutations with the same result, says James Hanken, an evolutionary biologist at Harvard University.
The finding in pigeons may help answer the question in other species. Pinpointing the genes behind feather colors, ornamentation or other characteristics in bred species might give researchers clues about the causes of similar features in wild birds, says Scott Edwards, a Harvard University evolutionary biologist who specializes in birds. Cardinals and blue jays also have crests, but no one knows yet whether those feathered cowlicks arise thanks to changes in the same gene as in pigeons. Alternatively, tweaks in multiple genes could give the wild birds their distinctive styles.
“We still don’t know whether the genetics of domestic species are simpler than those of wild species,” Edwards says.
To make the discovery, Shapiro’s colleagues in China and Denmark deciphered the genome of a male Danish tumbler pigeon. That bird’s blueprint served as reference for putting together the genomes of 40 other rock pigeons, all members of the species Columba livia, including individuals from 36 breeds and two feral pigeons.
Rock pigeons, which account for more than 350 breeds including most familiar, garden-variety pigeons, are not native to North America. All the wild pigeons there owe at least some of their heritage to escaped domesticated birds, especially to racing homers, which are homing pigeons bred to race, the researchers discovered from comparing the DNA of feral pigeons to the domesticated breeds.
Shapiro, who is interested in genes that give rise to physical characteristics, decided to investigate feather crests because earlier studies had indicated that just one gene might be involved. But no one had delved deeply into pigeon genetics, so the researchers had no idea where to look for the crest gene.
“It must have been a very nerve-wracking study,” Edwards says. The researchers may have come up empty handed.
To track down the gene, Shapiro’s group pooled genetic data from pigeons that have crests and compared that information with data from breeds with smooth heads and necks. Just one part of the genome stood out as differing between the two groups. Closer examination revealed that all the birds with crests carried a single change in the EphB2 gene. That mutation causes feathers on the neck and head to grow in the wrong direction--upward toward the face, rather than downward toward the torso.
“This mutation that we found appears to be the on/off switch for crest development,” Shapiro says. He doesn’t yet know why the mutation affects feathers only on the head and neck.Crests come in many shapes and sizes, and Shapiro thinks other genes play a role in determining how big and elaborate the ornaments grow. The researchers are trying to track down those modifying genes and to determine whether EphB2 is involved in crest development in other bird species.