Pigeon genome unlocks clues to evolution

in 1855, Charles Darwin took up a new hobby. He started raising pigeons.

"The diversity of the breeds is something astonishing," he wrote in "On the Origin of Species." Pigeon breeding, he argued, was an analogy for what happened in the wild. Nature played the part of the fancier, selecting which individuals would be able to reproduce. Natural selection might work more slowly than human breeders, but it had far more time to produce the diversity of life around us.

Now, a team of scientists are following Darwin's example by using the birds to find clues to the way evolution works in general. To do so, they delved into a source of information Darwin didn't even know about: the genome.

So far, they have sequenced the DNA of 40 breeds, seeking to pinpoint the mutations that produced their different forms. The scientists are particularly interested in the mutations that produce radically new kinds of anatomy. "Pigeons are an ideal way to look at these things," said Michael Shapiro, a biologist at the University of Utah and the leader of the study published online in the journal Science.

The work supports Darwin's claim that all pigeon breeds descend from the rock pigeon, whose range stretched from Europe to North Africa and east into Asia. "It's a brilliant bit of investigative science, the type of research that hopefully will come to define the genomic era," said Beth Shapiro (no relation to Michael), an evolutionary molecular biologist at the University of California, Santa Cruz.

Archaeologists have speculated that rock pigeons flocked to the first farms in the Fertile Crescent, where they pecked at loose grain. Farmers then domesticated them for food.

Later, humans bred the birds to carry messages. By the eighth century B.C., Greeks were using pigeons to send the results of Olympic Games from town to town. Genghis Khan used pigeons to create a communication network across his empire in the 12th century A.D.

Michael Shapiro and his colleagues have been able to work out the genealogy of these breeds. They found, for example, that fantail pigeons, one of Akbar's favorite breeds, are closely related to breeds from Iran. European colonists brought their domesticated pigeons to the New World, where some escaped and evolved into a new population of feral pigeons -- the ones that thrive in U.S. cities.

Once the team worked out the genealogy of pigeons, they could then investigate how they had evolved into so many different forms. To begin this stage of the project, he picked out a particularly extravagant trait to study: head crests.

"There are many different kinds of crests," he said. "Some birds just have just a little peak, some have what looks like an inverted shell, some have a mane, and some have their entire head engulfed in feathers."

The scientists compared the genomes of the crested pigeons with one another, as well as with other pigeons and with chickens, turkeys and other species. They hunted for mutated genes unique to the crested breeds, and found that all of them shared the same mutation in the same gene, EphB2. In most pigeons, the gene instructs the feathers to grow down the neck; but the mutation changes the location where EphB2 switches on, effectively turning the feathers upside down and producing a crest.

He found that it takes two copies of the mutant gene to reverse the feathers. When the mutation arose, it was passed down invisibly. Only when two carriers mated did they suddenly produce a crested chick. "It's an interesting example of how a single gene change can have a profound effect," said Cliff Tabin, a developmental geneticist at Harvard.