Toxicofera
Encyclopedia
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
which represents about 4600 species (nearly 60%) of extant squamates (scaled lizards.) It encompasses all venomous reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, as well as numerous related non-venomous species.
Cladistics
Toxicofera would combine the following groups from traditional classification:- Suborder Serpentes (snakes)
- Suborder IguaniaIguaniaIguania is the suborder of Squamata that contains the iguanas, chameleons, agamids, and "New World lizards" such as anoles and Phrynosomatidae...
(iguanaIguanaIguana is a herbivorous genus of lizard native to tropical areas of Central America and the Caribbean. The genus was first described in 1768 by Austrian naturalist Josephus Nicolaus Laurenti in his book Specimen Medicum, Exhibens Synopsin Reptilium Emendatam cum Experimentis circa Venena...
s, agamid lizards, chameleonChameleonChameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...
s, etc.) - Infraorder AnguimorphaAnguimorphaAnguimorphs of the infraorder Anguimorpha include the anguids , monitor lizards, the extinct mosasaurs, and helodermatids . The infraorder was named by Fürbringer in 1900 to include all autarchoglossans closer to Varanus and Anguis than Scincus...
, consisting of:- Family VaranidaeVaranidaeVaranidae is a group of lizards of the superfamily Varanoidea. The family is a group of carnivorous lizards which includes the largest living lizard, the Komodo dragon, and the crocodile monitor. Varanidae contains the living genus Varanus and a number of extinct taxa...
(monitor lizardMonitor lizardMonitor lizards are usually large reptiles, although some can be as small as in length. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semiaquatic monitors are also known...
s) - Family AnguidaeAnguidaeThe Anguidae is a large and diverse family of lizards native to the northern hemisphere. The group includes the slowworms, glass lizards, and alligator lizards, among others. Anguidae is divided into three subfamilies and contains 94 species in eight genera. Their closest living relatives are the...
(alligator lizardAlligator lizardAlligator Lizard may refer to one of the following:Species of the genus Elgaria:*Northern Alligator Lizard*Southern Alligator LizardSpecies of the genus Gerrhonotus.Species of the genus Abronia ...
s, glass lizardGlass lizardThe glass lizards or glass snakes, genus Ophisaurus, are a group of reptiles that resemble snakes, but are actually lizards. Although most species have no legs, their head shape and the fact that they have movable eyelids and external ear openings identify them as lizards. A few species have very...
s, etc.) - Family Helodermatidae (Gila monsterGila monsterThe Gila monster is a species of venomous lizard native to the southwestern United States and northwestern Mexican state of Sonora...
and Mexican beaded lizard)
- Family Varanidae
Research
Venom in squamates has historically been considered a rarity; while it has been known in Serpentes since ancient timesAsp (reptile)
Asp is the modern Anglicisation of the word aspis, which in antiquity referred to any one of several venomous snake species found in the Nile region. It is believed that the aspis referred to in Egyptian mythology is the modern Egyptian cobra....
, the actual percentage of snake species considered venomous
Venomous snake
"Poisonous snake" redirects here. For true poisonous snakes, see Rhabdophis.Venomous snakes are snakes which have venom glands and specialized teeth for the injection of venom...
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 evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary terms and the result of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
among the seemingly-polyphyletic venomous snake families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
.
In 2003 however, a study was published that described venom in snake subfamilies previously thought to lack it. 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. As a practical matter, Dr. Bryan Fry cautioned:
Some non-venomous snakes have been previously thought to have only mild 'toxic salivaSalivaSaliva , referred to in various contexts as spit, spittle, drivel, drool, or slobber, is the watery substance produced in the mouths of humans and most other animals. Saliva is a component of oral fluid. In mammals, saliva is produced in and secreted from the three pairs of major salivary glands,...
'. But these results suggest that they actually possess true venoms. We even isolated from a rat snakeRat snakeRat snakes are medium to large constrictors that can be found through a great portion of the northern hemisphere. They feed primarily on rodents and birds and, with some species exceeding 3 m , they can occupy top levels of some food chains. Many species make attractive and docile pets and one,...
[Coelognathus radiatus (formerly known as Elaphe radiataElaphe radiataThe Radiated Ratsnake or Copperhead Rat Snake, Coelognathus radiata, is a species of Colubrid snake.-Distribution:*Indonesia ,*Malaysia and Brunei ; Borneo,*Singapore Island,...
)], a snake common in pet stores, a typical cobraCobraCobra is a venomous snake belonging to the family Elapidae. However, not all snakes commonly referred to as cobras are of the same genus, or even of the same family. The name is short for cobra capo or capa Snake, which is Portuguese for "snake with hood", or "hood-snake"...
-style neurotoxinNeurotoxinA neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...
, 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
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
) in species from groups which were not previously known to produce it, e.g. in Iguania (specifically Pogona barbata from the Family Agamidae
Agamidae
Agamids, lizards of the family Agamidae, include more than 300 species in Africa, Asia, Australia, and a few in Southern Europe. Many species are commonly called dragons or dragon lizards. Phylogenetically they may be sister to the Iguanidae, and have a similar appearance. Agamids usually have...
) 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
Scientific community
The scientific community consists of the total body of scientists, its relationships and interactions. It is normally divided into "sub-communities" each working on a particular field within science. Objectivity is expected to be achieved by the scientific method...
. The venom clade included Anguidae for phylogenetic reasons and adopted a previously suggested clade name: Toxicofera.
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
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
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
Parallel evolution
Parallel evolution is the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades.-Parallel vs...
; 'non-venomous' snake lineages have either lost the ability to produce venom (but may still have lingering venom pseudogene
Pseudogene
Pseudogenes are dysfunctional relatives of known genes that have lost their protein-coding ability or are otherwise no longer expressed in the cell...
s) 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
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
, for example. Previously known venomous squamates have already provided the basis for medication
Medication
A pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease.- Classification :...
s such as Ancrod
Ancrod
Ancrod is a defibrinogenating agent derived from the venom of the Malayan pit viper. Defibrinogenating blood produces an anticoagulant effect. Ancrod is not approved or marketed in any country, but is being investigated as a stroke treatment in worldwide clinical trials. In January 2005, the U.S...
, Captopril
Captopril
Captopril is an angiotensin-converting enzyme inhibitor used for the treatment of hypertension and some types of congestive heart failure. Captopril was the first ACE inhibitor developed and was considered a breakthrough both because of its novel mechanism of action and also because of the...
, Eptifibatide
Eptifibatide
Eptifibatide , is an antiplatelet drug of the glycoprotein IIb/IIIa inhibitor class. Eptifibatide is a cyclic heptapeptide derived from a protein found in the venom of the southeastern pygmy rattlesnake...
, Exenatide
Exenatide
Exenatide is a medication approved in April 2005 for the treatment of diabetes mellitus type 2. It belongs to the group of incretin mimetics and is manufactured by Amylin Pharmaceuticals and Eli Lilly and Company....
and Tirofiban
Tirofiban
Tirofiban is an antiplatelet drug. It belongs to a class of antiplatelet named glycoprotein IIb/IIIa inhibitors...
.
External links
- Lizards' poisonous secret is revealed November 16, 2005
- The Surprising Origin of Venom Revealed November 17, 2005
- Which Came First, the Snake or the Venom? November 21, 2005
- Genealogy of scaly reptiles rewritten by new research November 22, 2005
- Venomdoc Homepage, Downloads
- The Venom Cure: The Power of Poison
- Nature Podcast November 17, 2005 (segment on the venom clade begins approximately 22 minutes into the program)