HOW did snakes evolve one of nature鈥檚 most complex bioweapons, the snake fang and its associated venom gland? The origin of the fang has been a puzzle, but now it seems this weapon may have evolved as a result of a simple embryological change.
Vipers and cobras both have large, hollow fangs at the front of their mouth, while other snakes have fangs at the back, and some have no fangs at all. Vipers and cobras are only distantly related, suggesting their fangs evolved independently, yet it strains credulity that such a sophisticated system should have evolved twice. A huge controversy in snake evolution has been whether these groups鈥� front fangs, and indeed front and rear fangs generally, share the same origin.
To try to resolve the mystery, Freek Vonk of Leiden University in the Netherlands and colleagues studied the embryological development of the teeth on the upper jaw, or maxilla, of eight species of snake. In both front and rear-fanged snakes, they found the fangs arose from the same tooth-forming area at the back of the upper jaw, but in front-fanged snakes these moved forward during development.
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Vonk suggests that during snake evolution, the teeth that formed fangs became uncoupled from the other teeth during embryonic development to become associated with the venom gland.
The team found that the most primitive snakes, the non-venomous boas and pythons, have a single strip of tooth-forming tissue on the maxilla. In contrast, 鈥渁dvanced鈥� snakes have a separate area of tooth-forming tissue behind the first, and this gives rise to fangs. 鈥淚t is as if our wisdom teeth developed from their own specialised tissue,鈥� says Vonk.
The pattern of jaw growth and development suggests that the earliest venomous snakes were rear-fanged. In the front-fanged vipers and cobras, these rear fangs end up at the front of the mouth because the front of the jaw, including the front tooth-forming tissue, fails to grow. Both groups halt that growth by inactivating a key developmental gene called sonic hedgehog at the front of the maxilla, Vonk鈥檚 team says (Nature, ).
Because the change needed to bring rear fangs forward involves inactivating just one gene, this evolutionary step could plausibly have happened independently in vipers and cobras, says Vonk. But he believes the complex venom-delivery system 鈥� fang plus venom gland and all the associated muscles needed to release venom 鈥� is most likely to have evolved just once, with later changes merely shuffling the location of the fang.
However, Kenneth Kardong, an evolutionary biologist at Washington State University in Pullman, is sceptical. He notes that some snakes that split off the evolutionary tree only after the common ancestor of vipers and cobras show no trace of fangs or venom glands, where you would expect at least some remnant.
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