A while back I wrote about research by University of Melbourne biologist Brian Fry demonstrating that many lizard species, from the cuddly bearded dragon to the not-at-all cuddly Komodo dragon, were venomous. The orthodoxy used to be that there were only two venomous lizard species, the Gila monster and the beaded lizard. Now there were dozens, at least. Venom glands seem to be a shared characteristic of most lizard lineages. It may help to remember that snakes, among which venom is common, are basically just highly specialized lizards.
Since then, Fry has collaborated with scientists from Australia, the Netherlands, Israel, and Walla Walla on another study, just been published in Nature, on the development of fangs in venomous snakes. (The lead author is Freek J. Vonk of Leiden University.) It’s a nice piece of evo-devo—evolutionary developmental biology—that uses snake embryology to illuminate the evolution of the venom delivery system. My thanks to science blogger P. Z. Myers, proprietor of Pharyngula, for showcasing the article.
There are three basic kinds of fangs among venomous snakes (and yes, I insist on “venomous” not “poisonous”; rattlesnakes, for instance, are safe to eat, and reputedly tasty.) In the colubrid family, a huge group that includes rat snakes, kingsnakes, and gopher snakes, some species have enlarged teeth in the back of the mouth, with or without grooves that channel the venom from the Duvernoy’s gland. Some rearfanged colubrids are basically harmless; others, like the African boomslang and the Asian yamakagashi, have deadly bites.
More “advanced” venomous snakes have hollow fangs with internal venom channels, like hypodermic needles. The elapids—cobras, kraits, mambas, coral snakes, sea snakes, and all those notorious Australians—have short front fangs that are fixed in place. The viperids—Old World vipers and adders, rattlesnakes, and other pit vipers—have longer, movable fangs that swing forward and down. A third family, the oddball stiletto snakes of Africa, includes rearfangers, frontfangers with fixed fangs, and frontfangers whose fangs swing sideways.
The question is whether the fangs of cobras, vipers, and stiletto snakes were derived from the less-sophisticated delivery system of a common rear-fanged ancestor, or whether each group invented the hypodermic fang on its own. Either route seems plausible. Venom-injection systems have evolved independently in wildly diverse groups of animals, including jellyfish, cone snails, and the duck-billed platypus.
Snake genetics tell us a lot about evolutionary relationships, but not about how the fangs changed as they evolved. For that, Vonk, Fry, and colleagues turned to evo-devo. One key idea among its practitioners (see Sean B. Carroll’s Endless Forms Most Beautiful for a good introduction) is that almost all animals share a common toolkit of genes that govern development. To oversimplify, the genes make proteins that turn developmental processes on or off. The Hox genes control the sequencing of body parts; the Pax-6 gene is responsible for eye development. Similar genes do the same kind of work in humans as in fruit flies.
There there’s Sonic Hedgehog. The original hedgehog gene was discovered by German fruit fly researchers and named because a mutant version gave the fly larvae a spiky look. (Fruit fly people have a tradition of bestowing whimsical names on genes; see home.earthlink.net/~misaak/taxonomy/taxGene.html.) Equivalent genes found in vertebrates were given hedgehog-related names. The Sonic Hedgehog protein is a jack of all trades, involved in the organization of the vertebrate brain, the growth of fingers and toes, and, as it happens, the development of snake jaws.
So the team looked at the expression of Sonic Hedgehog in the jaws of embryos of eight species of snakes, including colubrid rearfangers, cobras, and vipers. All the embryonic snakes had an odontogenic band, a thin strip of tissue in their mouths from which the teeth developed, with front and rear subdivisions. In rearfangers, the fangs, as well as the venom gland, developed from the rear portion; no surprise there. The front part formed the regular-sized teeth that the snakes use to hold their prey.
In frontfanged snakes like cobras and rattlers, only the rear portion of the tooth-forming tissue is switched on by Sonic Hedgehog.
The fangs form back there and then shift forward as the jaw of the developing embryo expands, while the venom gland remains in the back of the skull. It appears, then, that the fang-venom gland complex is the common heritage of all “advanced” snakes, and may be one of the secrets of their remarkable diversification. Cobras, vipers, and stiletto snakes independently evolved front-mounted hypodermic fangs, but not the whole apparatus.
Fry has pointed out that venom is expensive to manufacture, and some snake lineages that no longer need it to subdue active prey—such as a sea snake that specializes in eating fish eggs, and others that feed on snails—have reduced venom glands. It’s a matter of use it or lose it.