Warning: Mutant chickens may bite. Researchers have identified a genetic mutation that creates incipient teeth in bird embryos. The discovery provides a modern day glimpse of a feature that hasn't been seen in avians for millions of years.
Birds lost their choppers 70 million to 80 million years ago. That's what made an experiment in 1980 so surprising: After scientists grafted oral tissue from mice onto a chicken's gums, the birds grew round, mouselike teeth. But because avians and mammals are not closely related, scientists doubted whether the experiment proved that birds had truly retained a genetic vestige of their forbearers' bite.
Now a group of developmental biologists has found a strain of birds that don't need outside help to grow teeth. While investigating a gene mutation known to affect organ development in chickens, Matthew Harris of the Max Planck Institute in Tübingen, Germany, noticed sharp protrusions on the jaw of a 16-day-old embryo. Scientists had never suspected a connection between tooth formation and the gene--known as talpid2--because embryos with the mutation rarely survive past 12 days. Further investigation by Harris and colleague John Fallon at the University of Wisconsin, Madison, indicated that the teeth were conical and saber-shaped, resembling those of an alligator or crocodile.
To see how tooth formation in these chickens compares to that of other animals, the team looked at the expression pattern of a gene called sonic hedgehog (shh), which is essential for tooth production in vertebrates. In normal chicks, shh was expressed in a region analogous to the sides of the gums, but in alligators and talpid2 mutants, shh appeared in the center of the gums. The mutant version of talpid2 thus appears to turn shh on in the right place for growing teeth. Over time, changes in the gene may have disrupted this ability, resulting in tooth loss, the researchers report 21 February in Current Biology.
The finding is a great example of how altering the location of gene expression can cause changes in body types over time, says biologist Scott Gilbert of Swarthmore College near Philadelphia, Pennsylvania. "The real wow here," adds Paul Sharpe, a biochemist at the Department of Craniofacial Development at Kings College London, "is that these guys essentially show a glimpse of what the teeth probably looked like in the first birds."