Teeth discovered in mutant chickens

Teeth discovered in mutant chickens

MILWAUKEE - Matthew Harris didn't flinch at the crocodilian-like teeth flashing six inches in front of his face. He didn't scream or whimper, either.

Instead, he sat back, shook his head and leaned in for a better look.

That's because Harris, a researcher at the University of Wisconsin-Madison last year, wasn't looking into the mouth of a giant, dentition-ridden reptile.

He was looking into the mouth of a chicken.

Harris, who is now at a research institute in Germany, and a team of researchers from the University of Wisconsin-Madison and the University of Manchester in England have discovered teeth in a mutant line of chickens.

And their discovery supports the premise that ancient genetic signals can be resurrected - remaining dormant or not expressed in an organism's DNA - for millions of years, waiting for the right conditions to spark their return.

This is a phenomenon known to developmental biologists and geneticists as atavism, popularly called "throwbacks."

The discovery also sheds light on the molecular and developmental mechanisms behind tooth loss and beak growth in birds, giving paleontologists a bird's-eye view of the biological processes that took place millions of years ago, as one line of a raptor-like dinosaur crossed the taxonomic boundary from reptile to bird.

The paper appears in this week's issue of Current Biology.

"I find the paper to be an impressive demonstration of the molecular basis of an atavism," said Richard Prum, an evolutionary biologist at Yale University who was not involved in the study.

Tooth growth in birds has been discussed, debated and ridiculed for nearly 200 years. In 1882, noted French biologist Geoffroy St. Hilaire claimed to have seen something that looked like rudimentary reptilian teeth on the jaw of a parrot - teeth that later disappeared when the beak grew.

Excited and perplexed, he discussed his publication with Georges Cuvier, a well-known French anatomist. He told Cuvier the teeth resembled those of reptiles.

Cuvier dismissed his findings and told St. Hilaire he was mistaken. The subject fell silent, despite a couple of peeps by other scientists who had observed "hen's teeth" later that century.

Then, in the 1970s, researchers discovered that if they introduced mouse oral tissue into the mouth of a developing chick, the chicken could be coaxed to grow teeth. Apparently, the molecular signaling in mouse tissue spurs the chicken to respond with tooth-bearing instructions.

The discovery provoked further investigations into chicken teeth and a revitalized interest in the concept of atavism - sparking scientists to look into the role of embryonic development as a way of understanding the evolution of birds.

But the chickens in these experiments didn't grow just any old teeth. They grew teeth that looked like molars. And this had some scientists scratching their heads.

If, indeed, the teeth they were seeing in these birds were "throwbacks" to their dinosaur-like ancestors, then why were they growing mammal teeth instead of reptile teeth?

It is believed that birds descended from raptor-like dinosaurs around 70 million to 80 million years ago. (The link between mammals and birds goes back 300 million years.)

So, given that chicken jaws could be prompted to develop teeth - even if they were the wrong kind - researchers wondered, was it possible the signal still resided in living birds? And if it did, what could it tell them about the evolutionary history of tooth loss in birds?

Harris, who is now at the Max Planck Institute in Germany, and John Fallon, an anatomist at the University of Wisconsin, seem to have stumbled on the answers, although that wasn't their initial intention.

Harris and Fallon had been looking at a particular strain of mutant chickens to see what they could uncover about the development of feathers and scales. The chickens have several developmental abnormalities and do not generally survive to hatch.

It was while Harris was looking at these chickens that he made his startling discovery. Even though he and other researchers had looked at the embryos hundreds if not thousands of times, no one had noticed the little tooth buds growing in the jaws.

"It was like finding gold in the bathroom," said Harris.

Harris showed Fallon, and the two consulted Mark Ferguson, an expert on alligator teeth at the University of Manchester. He agreed that they looked like first-stage, or "baby," teeth in alligators, the closest living relatives to birds. And they appeared to develop in a similar fashion.

The observation vindicates St. Hilaire's publication.

The question then became: Why is this happening in this mutant strain?

The authors propose that these chickens express a genetic mutation code that brings together two layers of the chicken's embryonic mouth, which are separated in normal chickens.

"It's like having two handset radios that don't work," said Fallon. "With a little tweaking and adjusting, you can get them to work again" to receive and transmit signals to one another.

In an additional test, the researchers found that teeth formation can be initiated in normal chickens, too, with the right molecular rejiggering.

Julia Clarke, a paleontologist at North Carolina State University who specializes in raptor-like dinosaurs, said the findings will shed light on the dinosaur fossil record, indicating potential mechanisms for different patterns of tooth loss observed in various lines of dinosaurs.

"Some have no teeth present in the beak tip," while others have none in the back of the mouth, she said. This work provides a mechanism by which such variations can be explained.

It also points to the "incredible conservation of genetic mechanisms" that control not only tooth formation, but other characteristics as well.

"I wonder what is the longest instance of a character being retained" throughout evolution, Clarke asked. Tens of millions of years? Hundreds of millions of years?

"This is exactly what we'd predict"-the genetic linking of a living animal and its ancestors-by using an evolutionary approach to biology, she said.