Evolutionary secrets of Snake fangs deciphered
Michael K. Richardson’s group published an article which appeared on cover of Nature recently deals with fundamental question in snake evolution about evolutionary and developmental origin of front and rear fangs.
Fangs are specialized teeth associated with a venom gland used by snakes to introduce venom into prey.Snakes like Cobra and Vipers are front fanged groups as they have fangs positioned in anterior upper jaw,unlike Grass snakes where it is situated in posterior side (back of mouth).
Background about fangs in Snakes:
Basal or primitive snakes like Boas lack venom and hence these specialised fangs as well ,whereas Cobras (Elapid snakes) have very prominent fangs situated at anterior(front) end of jaws. Viperidae and pit vipers also have these poison delivering front fangs,but when one look at phylogenetic tree Vipers doesn’t fall in same lineage as Cobras.
if you want to understand the origin of novel morphological features in multicellular organisms, you have to look at their development-PZ Myers
Here lies the evolutionary puzzle related to origin of fangs: Did these lineages have a common origin for fangs or they developed them independently???The Grass snakes which are rear fanged ones lies somewhere between Cobra and recently evolved Vipers indicating fangs evolved multiple times during evolution to form front fanged snakes.The problem with multiple origin in this case for evolutionary biologists is that specialised structures like fangs just don’t appear and vanish or else other vertebrates with fangs would have popped up and which is not the case.
“This will become a textbook example in evolutionary biology identifying how development produces diversity in the natural world that natural selection can then act on.”-Ken Kordong
By looking at the phylogenetic tree of Snakes where Grass Snake (with rear fangs) are placed between Two lineages with front fangs.There could be two possibilities for these 1) Independent origin of fangs in all these groups and 2) front fangs are primitive, and Grass Snakes secondarily lost them.
Vonk et al took this task of solving evolutionary mystery of fangs by looking into the early development of embryos using sonic hedgehog as marker,to visualize the dental primordia in snake embryos.By studying tooth-forming epithelium in the upper jaw of 96 snake embryos, covering eight species they saw initial plans for fang formation consistently began at the back of the mouth, even in front fang snakes.This finding supports the idea that there’s an evolutionarily common origin for the fang and that it hasn’t evolved totally independently.
In front-fanged snakes(which also begin to form at back of the jaw), the anterior part of the upper jaw lacks sonic hedgehog expression, and ontogenetic allometry during embryonic growth displaces the fang from its posterior developmental origin to its adult front position.
“Nature took the easiest path,” “and the easiest path is not to evolve a whole new structure but to just move this wonderful biological weapon around in the skull to suit different lifestyles.” -Richardson
Authors say that boas (basal snakes) bear just one section of dental tissue, while all fanged snakes have two. So, they are of the opinion that in millions of years between the fangless and fanged snakes, dental tissue split and in doing so, allowed fangs to form,an event that underlies the massive radiation of advanced snakes during the Cenozoic era.
Reference:
Vonk FJ, Admiraal JF, Jackson K, Reshef R, de Bakker MAG, Vanderschoot K, van den Berge I, van Atten M, Burgerhout E, Beck A, Mirtschin PJ, Kochva E, Witte F, Fry BG, Woods AE, Richardson MK (2008)
Evolutionary origin and development of snake fangs. Nature 454:630-633.
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