Armfield and her co-workers studied FGF8 and BMP4 in pig embryos, relatives of whales and dolphins. Pigs have the four typical types of teeth, and, sure enough, the two proteins are distributed in the same way as they are in other mammals, showing that whales' ancestors likely had this distribution of gene expression too. Next, the authors moved on to examine dolphin embryos. Here, the pattern was different: FGF8 is present in the back part of the jaw, but BMP4 is present along the entire length of the jaw, including where FGF8 is found. The authors hypothesize that the overlapping presence of BMP4 in these new areas causes the teeth all along the jaw to be simple in shape, like incisors, and to be similar to each other. Interestingly, other researchers had shown that in mouse embryos in which BMP4 is experimentally introduced in the back of the jaw, the back teeth also take on this simplified appearance.
"It is exciting to identify a molecular change that occurred in nature and that so dramatically influenced the way in which a mammal can thrive in the ocean and to then trace the evolution of that change in the fossil record," says Armfield. "The simple shift in the location of proteins that influence tooth shape found in whales may help us to better understand how mammals evolved their complex tooth in the first place."
Says co-author Thewissen: "This shows that major changes to the design of an animal can result from small changes in early development, by simply shifting the region where an already existing protein occurs. It's a beautiful, detailed exampl
|Contact: Brooke Armfield|