Even though they don’t look like one, whales are mammals. They have a tail fluke, no hind limb and no body hair. Most whales don’t have teeth and those that do, like killer whales, have only a row of simple peg-like teeth, which looks the same. On the other hand, most mammals have four types of teeth, each shaped for specific task. A multidisciplinary team of researchers investigated how whales got its teeth using fossil records and the embryonic development process.
In this study, Brooke Armfield and her colleagues investigated the developmental processes of dolphins, which are whales' smaller cousins, and tracked the evolutionary progression of their dentition throughout the fossil records. Fossil records show that, 48 million years ago, whales had the same four types of teeth just like other mammals. Eventually, as they started adapting to life in the water, 30 million years ago, their teeth became simpler and transformed into their peg-like appearance.
The next step was to search how teeth are shaped during development. There are two proteins – FGF8 and BMP4 – in the embryo, which cause the developing teeth to grow into a certain shape. FGF8 expression occurs in the back of the jaw leading to development of molar teeth in mammals. BMP4 expression occurs near the tip of the jaws. Studies in pig embryos – relatives of whales and dolphins – have shown that these two proteins are distributed in the same way as they are in other mammals. This proves that whales' ancestors might have this distribution of gene expression as well.
However, studies on dolphin embryos show a different pattern. FGF8 is still present in the back part of the jaw, but BMP4 is present along the entire length of the jaw, overlapping with the presence of FGF8. The new hypothesis was that the overlapping presence of BMP4 in these new areas causes the teeth all along the jaw to be simple in shape and similar to each other.
It is interesting to see only a simple shift in the location of proteins would influence the shape of teeth in whales. This demonstrates 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.
Posted by Setareh Sepasi (3)