Birds that make teeth

Gone does not necessarily mean forgotten, especially in biology. A recent finding by researchers at the University of Wisconsin, Madison, and colleagues from the University of Manchester have found new evidence that the ability to form previously lost organs--in this case, teeth--can be maintained millions of years after the last known ancestor possessed them.

Birds do not have teeth. However, their ancestors did--about 70–80 million years ago. The evolutionary loss of teeth corresponded to the formation of the beak that is present in all living birds. Nonetheless, it has been known that if mouse tooth-forming tissue is in contact with bird jaw tissue, the bird tissue is able to follow the instructions given by the mouse tissue and participate in making teeth, and that these teeth look very much like those of mammals. However, Drs. Matthew Harris and John F. Fallon and colleagues have found that modern birds retain the ability to make teeth even without instruction from their tooth-bearing cousins.

In the new work, the researchers show that the talpid2 strain of chicken harbors a genetic change that permits tooth formation in both the upper and lower jaw of embryonic birds. These teeth show similar developmental position as mammalian teeth and are associated with similar molecular instructions. Furthermore, when comparing the initial development of the structures, the researchers realized that the teeth forming in the chicken did not look like mammalian teeth, but resembled those of the alligator, the closest living relative of modern birds.

The findings strongly suggest that the birds were initiating developmental programs similar to those of their reptilian ancestors. In addition, the authors found that the capacity to form teeth still resides in normal chickens and can be triggered experimentally by molecular signals. Taken together, the new findings indicate that even though modern birds lost teeth millions of years ago, the potential to form them persists.

The researchers include Matthew P. Harris of the University of Wisconsin in Madison, Wisconsin and Max Planck Institute fur Entwicklungsbiologie in Tubingen, Germany; Sean M. Hasso and John F. Fallon of the University of Wisconsin in Madison, Wisconsin; Mark W.J. Ferguson of the University of Manchester in Manchester, United Kingdom. This work was supported by National Institutes of Health grant #32551 to J.F.F.

Harris et al.: "The Development of Archosaurian First-Generation Teeth in a Chicken Mutant." Publishing in Current Biology 16, 371–377, February 21, 2006. DOI 10.1016/j.cub.2005.12.047 www.current-biology.com

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