February 20, 2013
The Evolutionary Origins Of Whale Teeth
April Flowers for redOrbit.com - Your Universe Online
Although whales are mammals, they do not resemble any of the mammals surrounding us on land and have a unique evolutionary history. Whales have a triangular fluke for a tail, and they have no (visible) hind legs or body hair. One of the strangest differences between whales and their terrestrial cousins, however, is in the whale's mouth. Whale teeth are simpler and more "peg like" than those of other mammals. A new study led by Dr. Brooke Armfield of the University of Florida, Gainesville, has brought together the fossil record and the embryonic development process of whales to investigate how this majestic marine mammal got its curious teeth.
The majority of mammals have four types of teeth, each shaped for a specific task. The four types are wedge-shaped incisors, a pointy canine, premolars and molars that have bumps and valleys shaped to fit together like a mortar and pestle. Not every species of whale, however, has teeth. The ones that do, like killer whales, have rows of simple peg-like teeth that all look alike. Whales do not chew with their mouths but instead use this spiked row of teeth to grab prey.
In the new study, published in the open access journal PeerJ, an international research group investigated the developmental processes that shape the teeth of dolphins, whales´ smaller cousins. The team also tracked the evolutionary progression of the dolphins' unique dentition across the fossil record.
Because whales evolved from land mammals, the team first examined the fossil record to trace back when and how whales evolved their simple teeth. This record revealed that roughly 48 million years ago, whales had four types of teeth like other mammals. Whale teeth became simpler over time, however, gradually acquiring their characteristic peg-like appearance somewhere around 30 million years ago. This was long after the time that whales had acquired an array of adaptations for living in the water.
The scientists explored how teeth are shaped during development next. Developing teeth grow into certain shapes because of specific proteins that are activated when the whale is in the embryonic stage. Two particular proteins caught the attention of Armfield and her team — BMP4 (Bone Morphogenetic Protein 4) and FGF8 (Fibroblast Growth Factor 8). The expression of BMP4 leads to the development of teeth into simple prongs, occurring near the tip of the jaws where incisors form. Prior to the formation of teeth in the embryo, the expression of FGF8 leads to the development of molar teeth in the back of the jaw of mice and other mammals.
The team used pig embryos to study FGF8 and BMP4. They found that pigs have the four typical tooth types and that the two proteins are distributed in the same way as in other mammals, indicating that whales' ancestors likely had this same distribution of gene expression as well. An examination of dolphin embryos, however, revealed a different pattern. As with other mammals, FGF8 is found in the back part of the jaw in dolphins. However, BMP4 is present along the entire length of the jaw, including where FGF8 is found. The team believes that the overlapping presence of BMP4 and FGF8 in these new areas forces the teeth all along the jaw to be simple in shape and similar appearance to each other. The team introduced BMP4 in the back of the jaw of mouse embryos, which caused the back teeth to take on this same 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."
Dr. J.G.M. 'Hans' Thewissen, Professor of Anatomy at Northeast Ohio Medical University explains: "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 example of a small developmental change having a big effect in evolution."