New Insights into the Evolution of Snake Fangs
A rare examination of snake embryos suggests that an ancient change in the development of the upper jaw in some snakes may have paved the way for the evolution of fangs and associated venom glands. The findings, published in the July 31 issue of the journal Nature, offer new clues to the origin of an extraordinary adaptation that allowed snakes to flourish in nearly every corner of the globe.
The work by a multinational team of scientists, including Dr. Ram Reshef of the Faculty of Biology, Technion-Israel Institute of Technology, reveals that fanged snakes have two distinct embryonic developmental centers that form the tissue of their upper jaw teeth. Snakes without fangs have only one developmental center that runs the full length of the upper jaw.
Reshef said the Nature paper has implications for biologists beyond the history of snakes. "The whole thing is about understanding the big question of evolution and development, the way in which genes underlying developmental changes can cause changes in form and function," he noted.
The two developmental centers in fanged snakes-one toward the front of the jaw and one toward the back-may have allowed the back teeth to evolve in tandem with a venom gland, the researchers say.
Rear-fanged snakes such as grass snakes maintain this distinct dual pattern. But front-fanged snakes such as vipers and cobras appear to have taken the change even further, doing away with the front developmental center entirely. In these snakes, fangs that develop in the back of the jaw migrate to the front as the snake's head changes in shape as it grows, Reshef said.
The findings are "an extraordinarily important study that addresses the underlying force of variation upon which the venomous system of snakes was built," said Kenneth Kardong, a professor of zoology at Washington State University and a pioneering researcher in snake evolution.
The embryonic clues gleaned by Reshef and colleagues could help answer some long-standing questions about the snake family tree. For instance, did venomous fangs evolve more than once in snakes? In some family trees based on genetic information, rear-fanged and front-fanged snakes don't appear to share a recent common ancestor, suggesting that fangs may have evolved independently in the two groups.
On the other hand, the new embryonic evidence indicates that rear- and front-fanged snakes share a developmental pathway in the upper jaw that is distinct from the pathway in non-fanged snakes. They may have inherited this pathway from a common ancestor, making them more closely related than some researchers have suggested.
The current study does not overturn any particular snake family tree, said Reshef. Rather, "it gives a nice explanation of how developmentally new structures appear and how they have helped the wonderful success of snakes," he said.
Fanged snakes are members of the "advanced snakes," which differ from more ancient snake groups such as boas and other constrictors that wrap their body around the prey and swallow it whole. Advanced snakes began sweeping through the world 50 to 60 million years, often at the expense of constrictor snakes, as the climate shifted to a cool and dry pattern that led to more open, less forested landscapes.
In the new environment, the slow-moving constrictors were less successful predators than the quicker advanced snakes, which were replacing the primitive jaw with a plethora of new adaptations, including venomous fangs and glands.
Researchers from Leiden University in The Netherlands, Whitman College in Walla Walla, Washington, USA, the University of South Australia in Adelaide, Venom Supplies Pty. Ltd. in Australia, Tel Aviv University in Israel and the University of Melbourne in Australia also contributed to the Nature study.