Toxicofera

Taxobox
name = Toxicofera

---

image_width = 220px
image_caption = Venomous snakes, such as the rattlesnake shown above, are the most well-known venomous squamates.
regnum = Animalia
phylum = Chordata
subphylum = Vertebrata
classis = Sauropsida
subclassis = Diapsida
infraclassis = Lepidosauromorpha
superordo = Lepidosauria
ordo = Squamata
unranked_familia = Toxicofera

Toxicofera (Greek for "those who bear toxins"), is a hypothetical clade which represents about 4600 species (nearly 60%) of extant squamates (scaled lizards.)cite journal
author = Fry, B. et al
year= 2006 |month= February
title= Early evolution of the venom system in lizards and snakes
journal= Nature
volume= 439
pages= 584–588
url= http://www.nature.com/nature/journal/v439/n7076/abs/nature04328.html
doi = 10.1038/nature04328
format = [http://www.venomdoc.com/downloads/2005_BGF_Nature_squamate_venom.pdf PDF] ] It encompasses all venomous reptile species, as well as numerous related non-venomous species.

Cladistics

Toxicofera would combine the following groups from traditional classification: ]

* suborder Serpentes (snakes)
* suborder Iguania (iguanas, agamid lizards, chameleons, etc.)
* infraorder Anguimorpha, consisting of:
** family Varanidae (monitor lizards)
** family Anguidae (alligator lizards, glass lizards, etc.)
** family Helodermatidae (Gila monster and Mexican beaded lizard)

Research

Venom in squamates has historically been considered a rarity; while it has been known in Serpentes since ancient times, the actual percentage of snake species considered venomous was relatively small (around 25%). Following the classification of Helodermatidae in the 19th century, their venom was thought to have developed independently. The origin of venom in squamates was thus considered relatively recent in evolutionary terms and the result of convergent evolution among the seemingly-polyphyletic venomous snake families.

In 2003 however, a study was published that described venom in snake subfamilies previously thought to lack it.cite journal
author = Fry, B. et al
year= 2003 |month= July
title= Molecular Evolution and Phylogeny of Elapid Snake Venom Three-Finger Toxins
journal= Journal of Molecular Evolution
volume= 57
issue= 1
pages= 110–129
doi = 10.1007/s00239-003-2461-2
format= [http://www.venomdoc.com/downloads/2003_BGF_Elapid_3FTx_phylog.pdf PDF] ] Further study claimed nearly all "non-venomous" snakes produce venom to a certain extent, suggesting a single, and thus far more ancient origin for venom in Serpentes than had been considered until then.cite journal
author = Fry, B. et al
year= 2003 |month= October
title= Isolation of a Neurotoxin (α-colubritoxin) from a Nonvenomous Colubrid: Evidence for Early Origin of Venom in Snakes
journal= Journal of Molecular Evolution
volume= 57
issue= 4
pages= 446–452
doi = 10.1007/s00239-003-2497-3
format= [http://www.venomdoc.com/downloads/2003_BGF_alpha-colubritoxin.pdf PDF] ] cite journal
author = Fry, B. and Wüster, W.
year= 2004 |month= May
title= Assembling an Arsenal: Origin and Evolution of the Snake Venom Proteome Inferred from Phylogenetic Analysis of Toxin Sequences
journal= Molecular Biology and Evolution
volume= 21
issue= 5
pages= 870–883
doi = 10.1093/molbev/msh091
format = [http://www.venomdoc.com/downloads/2005_BGF_Nature_squamate_venom.pdf PDF] ] As a practical matter, Dr. Bryan Fry cautioned: [ [http://www.sciencedaily.com/releases/2003/12/031216075937.htm Venom Hunt Finds 'Harmless' Snakes A Potential Danger] December 16, 2003]

"Some non-venomous snakes have been previously thought to have only mild 'toxic saliva'. But these results suggest that they actually possess true venoms. We even isolated from a rat snake" ["Coelognathus radiatus" (formerly known as "Elaphe radiata") ] ] ", a snake common in pet stores, a typical cobra-style neurotoxin, one that is as potent as comparative toxins found in close relatives of the cobra. These snakes typically have smaller quantities of venom and lack fangs, but they can still deliver their venom via their numerous sharp teeth. But not all of these snakes are dangerous. It does mean, however, that we need to re-evaluate the relative danger of non-venomous snakes."

This prompted still further research, which led to the discovery of venom (and venom genes) in species from groups which were not previously known to produce it, e.g. in Iguania (specifically "Pogona barbata" from the Family Agamidae) and Varanidae (from "Varanus varius"). ] It is thought that this was the result of descent from a common venom-producing squamate ancestor; the hypothesis was described simply as the "venom clade" when first proposed to the scientific community. ] The venom clade included Anguidae for phylogenetic reasons and adopted a previously suggested clade name: Toxicofera.cite journal
author = Vidal, N. and Hedges, S.
year= 2005 |month= October-November
title= The phylogeny of squamate reptiles (lizards, snakes, and amphisbaenians) inferred from nine nuclear protein-coding genes
journal= Comptes Rendus Biologies
volume= 328
issue= 10-11
pages= 1000–1008
doi = 10.1016/j.crvi.2005.10.001
format= [http://evo.bio.psu.edu/hedgeslab/Publications/PDF-files/170.pdf PDF] ]

It was estimated that the common ancestral species that first developed venom in the venom clade lived on the order of 200 million years ago. ] The venoms are thought to have resulted after genes normally active in various parts of the body duplicated and the copies found new use in the salivary glands. ]

Among snake families traditionally classified as venomous, the capacity seems to have evolved to extremes more than once by parallel evolution; 'non-venomous' snake lineages have either lost the ability to produce venom (but may still have lingering venom pseudogenes) or actually do produce venom in small quantities (e.g. 'toxic saliva'), likely sufficient to assist in small prey capture, but not normally cause harm to humans if bitten.

The newly discovered diversity of squamate species producing venoms is a treasure trove for those seeking to develop new pharmaceutical drugs; many of these venoms lower blood pressure, for example. ] Previously known venomous squamates have already provided the basis for medications such as Ancrod, Captopril, Eptifibatide, Exenatide and Tirofiban.

References

External links

* [http://www.newscientist.com/article.ns?id=dn8331 Lizards' poisonous secret is revealed] November 16, 2005
* [http://www.livescience.com/animals/051117_lizard_venom.html The Surprising Origin of Venom Revealed] November 17, 2005
* [http://www.corante.com/loom/archives/2005/11/21/which_came_first_the_snake_or_the_venom.php Which Came First, the Snake or the Venom?] November 21, 2005
* [http://www.sciencedaily.com/releases/2005/11/051122183250.htm Genealogy of scaly reptiles rewritten by new research] November 22, 2005
* [http://venomdoc.com/ Venomdoc Homepage] , [http://www.venomdoc.com/downloads/ Downloads]
* [http://www.pbs.org/wnet/nature/venomcure/venom.html The Venom Cure: The Power of Poison]
* [http://www.nature.com/nature/podcast/v438/n7066/nature-2005-11-17.mp3 Nature Podcast November 17, 2005] (segment on the venom clade begins approximately 22 minutes into the program)