Working late in the developmental biology lab one night, Matthew Harris of the University of Wisconsin noticed that the beak of a mutant chicken embryo he was examining had fallen off. Upon closer examination of the snubbed beak, he found tiny bumps and protuberances along its edge that looked like teeth--alligator teeth to be specific. The accidental discovery revealed that chickens retain the ability to grow teeth, even though birds lost this feature long ago. The finding also resurrected the controversial theory of one of the founders of comparative anatomy, Etienne Geoffroy Saint-Hillaire.
In the early 19th century, Saint-Hillaire observed that developing parrots have tiny bumps on their beaks that resemble teeth, something he ascribed to modern animals deriving from more basic primitive forms. But due to his developing battles with Georges Cuvier over evolution, the finding was forgotten until Harris, a graduate student, rediscovered it nearly 200 years later.
The mutant chickens Harris studied bear a recessive trait dubbed talpid2. This trait is lethal, meaning that such mutants are never born, but some incubate in eggs as long as 18 days. During that time, the same two tissues from which teeth develop in mammals come together in the jaw of the mutant embryo--and this leads to nascent teeth, a structure birds have lacked for at least 70 million years. "They don't make a molar," explains development biologist John Fallon, who oversaw Harris's work. "What they make is this conical, saber-shaped structure that is clearly a tooth. The other animal that has a tooth like that is an alligator."
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Previous efforts to produce teeth in chickens had relied on introducing genetic information from mice, resulting in chickens growing mammalian molars. But a chicken's underlying ability to grow teeth derives from a common ancestor with alligators--archosaurs--that is more recent than the one linking birds and mammals. Nevertheless, the underlying genetic mechanism that produces teeth in mice, alligators and mutant chickens remains the same.
Exactly how the mutation causes the chickens to sprout teeth is unknown, Fallon notes, but a similar effect can be produced in normal chickens. Harris proved this by engineering a virus to mimic the molecular signals of the mutation and caused normal chickens to briefly develop teeth that were then reabsorbed into the beak. The finding of such an atavism--presented in yesterday's issue of Current Biology--opens a new avenue of exploration in the quest to understand how particular structures like teeth are lost in different evolutionary lineages. It also vindicates the long ago observations of one of the early fathers of comparative anatomy.
