Toxicofera

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Toxicoferans
Temporal range: Late Jurassic–present
Mexican ridged nosed rattlesnake head.jpg
Venomous snakes, such as the rattlesnake shown above, are the best-known venomous squamates
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Clade: Toxicofera
Vidal & Hedges, 2005
Subgroups

Toxicofera (Greek for "those who bear toxins") is a proposed clade of scaled reptiles (squamates) that includes the Serpentes (snakes), Anguimorpha (monitor lizards, gila monster, and alligator lizards) and Iguania (iguanas, agamas, and chameleons). Toxicofera contains about 4,600 species, (nearly 60%) of extant Squamata. [1] It encompasses all venomous reptile species, as well as numerous related non-venomous species. There is little morphological evidence to support this grouping; however, it has been recovered by all molecular analyses as of 2012. [2] [3] [4] [ needs update ]

Cladistics

Toxicofera combines the following groups from traditional classification: [1]

The relationship between these extant groups and a couple of extinct taxa are shown in the following cladogram, which is based on Reeder et al. (2015; Fig. 1). [5]

Toxicofera

Alongside these groups, Mosasauria, an extinct group including large marine reptiles primarily known from the Late Cretaceous, has been placed as part of the group. It has often been supposed that mosasaurs are most closely related to snakes, with the group containing the two dubbed Pythonomorpha, however, other studies have quesitioned this, finding that the closest relatives of mosasaurs are members of Varanoidea. [6] Polyglyphanodontia, a group of extinct herbivorous lizards known from the Cretaceous, have also been placed as part of this group in some studies as the sister group to Iguania, though other studies have instead suggested that they are most closely related to Teiioidea and thus placed outside Toxicofera. [7]

Venom

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%). [8] Of the approximately 2,650 species of advanced snakes (Caenophidia), only the front-fanged species (~650) were considered venomous by the anthropocentric definition. Following the classification of Helodermatidae in the 19th century, their venom was thought to have developed independently. [1] In snakes, the venom gland is in the upper jaw, but in helodermatids, it is found in the lower jaw. [1] 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.[ citation needed ]

In 2003 a study was published that described venom in snake subfamilies previously thought to lack it. [9] 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. [10] [11] As a practical matter, Fry cautioned: [12]

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 ) [10] ], 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 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 ). [1] 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. [1] The venom clade included Anguidae for phylogenetic reasons and adopted a previously suggested clade name: Toxicofera. [13]

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. [1] The venoms are thought to have evolved after genes normally active in various parts of the body duplicated and the copies found new use in the salivary glands. [9]

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 causing harm to humans if bitten.[ citation needed ]

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. [1] Previously known venomous squamates have already provided the basis for medications such as Ancrod, Captopril, Eptifibatide, Exenatide and Tirofiban.[ citation needed ]

The world's largest venomous lizard and the largest species of venomous land animal is the Komodo dragon. [14]

Criticism

Other scientists such as Washington State University biologist Kenneth V. Kardong and toxicologists Scott A. Weinstein and Tamara L. Smith, have stated that the allegation of venom glands found in many of these animals "has had the effect of underestimating the variety of complex roles played by oral secretions in the biology of reptiles, produced a very narrow view of oral secretions and resulted in misinterpretation of reptilian evolution". According to these scientists "reptilian oral secretions contribute to many biological roles other than to quickly dispatch prey". These researchers concluded that, "Calling all in this clade venomous implies an overall potential danger that does not exist, misleads in the assessment of medical risks, and confuses the biological assessment of squamate biochemical systems". [15] More recently, it has been suggested that many of the shared toxins that underlie the Toxicofera hypothesis are in fact not toxins at all. [16]

Related Research Articles

<span class="mw-page-title-main">Lizard</span> Informal group of reptiles

Lizard is the common name used for all squamate reptiles other than snakes, encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The grouping is paraphyletic as some lizards are more closely related to snakes than they are to other lizards. Lizards range in size from chameleons and geckos a few centimeters long to the 3-meter-long Komodo dragon.

<span class="mw-page-title-main">Venom</span> Toxin secreted by an animal

Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved venom apparatus, such as fangs or a stinger, in a process called envenomation. Venom is often distinguished from poison, which is a toxin that is passively delivered by being ingested, inhaled, or absorbed through the skin, and toxungen, which is actively transferred to the external surface of another animal via a physical delivery mechanism.

<span class="mw-page-title-main">Colubridae</span> Family of snakes

Colubridae is a family of snakes. With 249 genera, it is the largest snake family. The earliest fossil species of the family date back to the Late Eocene epoch, with earlier origins suspected. Colubrid snakes are found on every continent except Antarctica.

<span class="mw-page-title-main">Lepidosauria</span> Superorder of reptiles

The Lepidosauria is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes lizards and snakes. Squamata contains over 9,000 species, making it by far the most species-rich and diverse order of non-avian reptiles in the present day. Rhynchocephalia was a formerly widespread and diverse group of reptiles in the Mesozoic Era. However, it is represented by only one living species: the tuatara, a superficially lizard-like reptile native to New Zealand.

<span class="mw-page-title-main">Squamata</span> Order of reptiles

Squamata is the largest order of reptiles, comprising lizards and snakes. With over 11,500 species, it is also the second-largest order of extant (living) vertebrates, after the perciform fish. Members of the order are distinguished by their skins, which bear horny scales or shields, and must periodically engage in molting. They also possess movable quadrate bones, making possible movement of the upper jaw relative to the neurocranium. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. Squamates are the most variably sized living reptiles, ranging from the 16 mm (0.63 in) dwarf gecko to the 6.5 m (21 ft) reticulated python. The now-extinct mosasaurs reached lengths over 14 m (46 ft).

<span class="mw-page-title-main">Crotaphytidae</span> Family of lizards

The Crotaphytidae, or collared lizards, are a family of desert-dwelling reptiles native to the Southwestern United States and northern Mexico. Alternatively they are recognized as a subfamily, Crotaphytinae, within the clade Pleurodonta. They are very fast-moving animals, with long limbs and tails; some species are capable of achieving bipedal running at top speed. This species is carnivorous, feeding mainly on insects and smaller lizards. The two genera contain 12 species.

<span class="mw-page-title-main">Elapidae</span> Family of venomous snakes

Elapidae is a family of snakes characterized by their permanently erect fangs at the front of the mouth. Most elapids are venomous, with the exception of the genus Emydocephalus. Many members of this family exhibit a threat display of rearing upwards while spreading out a neck flap. Elapids are endemic to tropical and subtropical regions around the world, with terrestrial forms in Asia, Australia, Africa, and the Americas and marine forms in the Pacific and Indian Oceans. Members of the family have a wide range of sizes, from the 18 cm (7.1 in) white-lipped snake to the 5.85 m king cobra. Most species have neurotoxic venom that is channeled by their hollow fangs, and some may contain other toxic components in various proportions. The family includes 55 genera with around 360 species and over 170 subspecies.

<span class="mw-page-title-main">Iguanomorpha</span> Infraorder of lizards

Iguania is an infraorder of squamate reptiles that includes iguanas, chameleons, agamids, and New World lizards like anoles and phrynosomatids. Using morphological features as a guide to evolutionary relationships, the Iguania are believed to form the sister group to the remainder of the Squamata, which comprise nearly 11,000 named species, roughly 2000 of which are iguanians. However, molecular information has placed Iguania well within the Squamata as sister taxa to the Anguimorpha and closely related to snakes. The order has been under debate and revisions after being classified by Charles Lewis Camp in 1923 due to difficulties finding adequate synapomorphic morphological characteristics. Most iguanians are arboreal but there are several terrestrial groups. They usually have primitive fleshy, non-prehensile tongues, although the tongue is highly modified in chameleons. Today they are scattered occurring in Madagascar, the Fiji and Friendly Islands and Western Hemisphere.

<span class="mw-page-title-main">Autarchoglossa</span> Clade of lizards

Autarchoglossa is a clade of squamates that includes skinks, anguimorphs, snakes, and relatives. Autarchoglossa is supported as a monophyletic grouping by morphological features in living and extinct lizards and snakes. Some phylogenetic analyses based on molecular features such as DNA sequences in living squamates do not support Autarchoglossa.

<i>Naja</i> Genus of snakes

Naja is a genus of venomous elapid snakes commonly known as cobras. Members of the genus Naja are the most widespread and the most widely recognized as "true" cobras. Various species occur in regions throughout Africa, Southwest Asia, South Asia, and Southeast Asia. Several other elapid species are also called "cobras", such as the king cobra and the rinkhals, but neither is a true cobra, in that they do not belong to the genus Naja, but instead each belong to monotypic genera Hemachatus and Ophiophagus.

<span class="mw-page-title-main">Anguimorpha</span> Order of lizards

The Anguimorpha is a suborder of squamates. The group was named by Fürbringer in 1900 to include all autarchoglossans closer to Varanus and Anguis than Scincus. These lizards, along with iguanians and snakes, constitute the proposed "venom clade" Toxicofera of all venomous reptiles.

<span class="mw-page-title-main">Scleroglossa</span> Evolutionary group of lizards

Scleroglossa is a group of lizards that includes geckos, autarchoglossans, and amphisbaenians. Scleroglossa is supported by phylogenetic analyses that use morphological features. According to most morphological analyses, Scleroglossa is the sister group of the clade Iguania, which includes iguanas, chameleons, agamids, and New World lizards. Together, Scleroglossa and Iguania make up the crown group Squamata, the smallest evolutionary grouping to include all living lizards and snakes.

<span class="mw-page-title-main">Lacertoidea</span> Superfamily of lizards

The Lacertoidea is a group of squamate reptiles that includes the Lacertidae, Teiidae, Gymnophthalmidae, and Amphisbaenia. The finding from molecular phylogenetic studies that the burrowing Amphisbaenia were nested in a clade with the lizard forms led Vidal & Hedges (2005) to propose a new name for the group based on shared morphogical characters, Laterata, "referring to the presence of tile-like scales that form the rings in Amphisbaenia, and are also present ventrally in Lacertiformata and Teiformata".

<span class="mw-page-title-main">Evolution of snake venom</span> Origin and diversification of snake venom through geologic time

Venom in snakes and some lizards is a form of saliva that has been modified into venom over its evolutionary history. In snakes, venom has evolved to kill or subdue prey, as well as to perform other diet-related functions. While snakes occasionally use their venom in self defense, this is not believed to have had a strong effect on venom evolution. The evolution of venom is thought to be responsible for the enormous expansion of snakes across the globe.

<span class="mw-page-title-main">Three-finger toxin</span> Toxin protein

Three-finger toxins are a protein superfamily of small toxin proteins found in the venom of snakes. Three-finger toxins are in turn members of a larger superfamily of three-finger protein domains which includes non-toxic proteins that share a similar protein fold. The group is named for its common structure consisting of three beta strand loops connected to a central core containing four conserved disulfide bonds. The 3FP protein domain has no enzymatic activity and is typically between 60-74 amino acid residues long. Despite their conserved structure, three-finger toxin proteins have a wide range of pharmacological effects. Most members of the family are neurotoxins that act on cholinergic intercellular signaling; the alpha-neurotoxin family interacts with muscle nicotinic acetylcholine receptors (nAChRs), the kappa-bungarotoxin family with neuronal nAChRs, and muscarinic toxins with muscarinic acetylcholine receptors (mAChRs).

<span class="mw-page-title-main">Three-finger protein</span> Protein superfamily

Three-finger proteins or three-finger protein domains are a protein superfamily consisting of small, roughly 60-80 amino acid residue protein domains with a common tertiary structure: three beta strand loops extended from a hydrophobic core stabilized by disulfide bonds. The family is named for the outstretched "fingers" of the three loops. Members of the family have no enzymatic activity, but are capable of forming protein-protein interactions with high specificity and affinity. The founding members of the family, also the best characterized by structure, are the three-finger toxins found in snake venom, which have a variety of pharmacological effects, most typically by disruption of cholinergic signaling. The family is also represented in non-toxic proteins, which have a wide taxonomic distribution; 3FP domains occur in the extracellular domains of some cell-surface receptors as well as in GPI-anchored and secreted globular proteins, usually involved in signaling.

<span class="mw-page-title-main">Neoanguimorpha</span> Clade of lizards

Neoanguimorpha is a clade of anguimorphs comprising Monstersauria and Diploglossa. Morphological studies in the past had classified helodermatids with the varanoids in the clade Platynota, while the Chinese crocodile lizard was classified as a xenosaurid. However molecular work found no support in these groupings and instead has found the helodermatids more related to Diploglossa, while the Chinese crocodile lizard and varanoids to form the clade Paleoanguimorpha.

<span class="mw-page-title-main">Paleoanguimorpha</span> Clade of lizards

Paleoanguimorpha is a clade of anguimorphs comprising Shinisauria and Goannasauria. Morphological studies in the past also classified helodermatids and pythonomorphs with the varanoids in the clade Platynota, while the Chinese crocodile lizard was classified as a xenosaurid. Current molecular work finds no support in these groupings and instead has found the helodermatids more related to Diploglossa in the sister clade Neoanguimorpha, while the Chinese crocodile lizard is the closet living relative to varanoids. Pythonomorphs represented by snakes today are not closely related to varanoids and are instead a sister lineage to Anguimorpha and Iguania in the clade Toxicofera.

<span class="mw-page-title-main">Colubroides</span> Clade of snakes

The Colubroides are a clade in the suborder Serpentes (snakes). It contains over 85% of all the extant species of snakes. The largest family is Colubridae, but it also includes at least six other families, at least four of which were once classified as "Colubridae" before molecular phylogenetics helped in understanding their relationships. It has been found to be monophyletic.

<span class="mw-page-title-main">Afrophidia</span> Clade of snakes comprising large and venomous species

Afrophidia is a clade of alethinophidian snakes comprising the groups Henophidia and Caenophidia, essentially making up the snakes people commonly associate with. The name refers to the deep split between Afrophidia and their sister taxon, Amerophidia, which originated in South American origin, and the afrophidians was recently hypothesized to represent a vicariant event of the breakup of Gondwanan South America and Africa.

References

  1. 1 2 3 4 5 6 7 8 Fry, Bryan G.; Vidal, Nicolas; Norman, Janette A.; Vonk, Freek J.; Scheib, Holger; Ramjan, S. F. Ryan; Kuruppu, Sanjaya; Fung, Kim; Blair Hedges, S.; Richardson, Michael K.; Hodgson, Wayne. C.; Ignjatovic, Vera; Summerhayes, Robyn; Kochva, Elazar (2005). "Early evolution of the venom system in lizards and snakes". Nature . 439 (7076): 584–8. Bibcode:2006Natur.439..584F. doi:10.1038/nature04328. PMID   16292255. S2CID   4386245.
  2. ↑ Vidal, Nicolas; Hedges, S. Blair (2009). "The molecular evolutionary tree of lizards, snakes, and amphisbaenians". Comptes Rendus Biologies. 332 (2–3): 129–39. doi:10.1016/j.crvi.2008.07.010. PMID   19281946. S2CID   23137302.
  3. ↑ Pyron, R.; Burbrink, Frank T.; Wiens, John J. (2013). "A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes". BMC Evolutionary Biology . 13: 93. doi: 10.1186/1471-2148-13-93 . PMC   3682911 . PMID   23627680.
  4. ↑ Wiens, J. J.; Hutter, C. R.; Mulcahy, D. G.; Noonan, B. P.; Townsend, T. M.; Sites, J. W.; Reeder, T. W. (2012). "Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species". Biology Letters . 8 (6): 1043–6. doi:10.1098/rsbl.2012.0703. PMC   3497141 . PMID   22993238.
  5. ↑ Reeder, Tod W.; Townsend, Ted M.; Mulcahy, Daniel G.; Noonan, Brice P.; Wood, Perry L.; Sites, Jack W.; Wiens, John J. (2015). "Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa". PLOS One . 10 (3): e0118199. Bibcode:2015PLoSO..1018199R. doi: 10.1371/journal.pone.0118199 . PMC   4372529 . PMID   25803280.
  6. ↑ Polcyn, Michael J.; Augusta, Bruno G.; Zaher, Hussam (2022-08-11), Gower, David J.; Zaher, Hussam (eds.), "Reassessing the Morphological Foundations of the Pythonomorph Hypothesis", The Origin and Early Evolutionary History of Snakes (1 ed.), Cambridge University Press, pp. 125–156, doi:10.1017/9781108938891.010, ISBN   978-1-108-93889-1 , retrieved 2024-01-20
  7. ↑ Xing, Lida; Niu, Kecheng; Evans, Susan E. (January 2023). "A new polyglyphanodontian lizard with a complete lower temporal bar from the Upper Cretaceous of southern China". Journal of Systematic Palaeontology. 21 (1). doi: 10.1080/14772019.2023.2281494 . ISSN   1477-2019.
  8. ↑ Fry, Bryan G.; Vidal, Nicolas; Van Der Weerd, Louise; Kochva, Elazar; Renjifo, Camila (2009). "Evolution and diversification of the Toxicofera reptile venom system". Journal of Proteomics . 72 (2): 127–36. doi:10.1016/j.jprot.2009.01.009. PMID   19457354.
  9. 1 2 Fry, B. G.; Wüster, W.; Kini, R. M.; Brusic, V.; Khan, A.; Venkataraman, D.; Rooney, A. P. (2003). "Molecular Evolution and Phylogeny of Elapid Snake Venom Three-Finger Toxins". Journal of Molecular Evolution . 57 (1): 110–29. Bibcode:2003JMolE..57..110F. CiteSeerX   10.1.1.539.324 . doi:10.1007/s00239-003-2461-2. PMID   12962311. S2CID   12358977.
  10. 1 2 Fry, Bryan G.; Lumsden, Natalie G.; Wüster, Wolfgang; Wickramaratna, Janith C.; Hodgson, Wayne C.; Manjunatha Kini, R. (2003). "Isolation of a Neurotoxin (α-colubritoxin) from a Nonvenomous Colubrid: Evidence for Early Origin of Venom in Snakes". Journal of Molecular Evolution . 57 (4): 446–52. Bibcode:2003JMolE..57..446F. doi:10.1007/s00239-003-2497-3. PMID   14708577. S2CID   21055188.
  11. ↑ Fry, B. G.; Wüster, W (2004). "Assembling an Arsenal: Origin and Evolution of the Snake Venom Proteome Inferred from Phylogenetic Analysis of Toxin Sequences". Molecular Biology and Evolution . 21 (5): 870–83. doi: 10.1093/molbev/msh091 . PMID   15014162.
  12. ↑ "Venom Hunt Finds 'Harmless' Snakes A Potential Danger". ScienceDaily . December 16, 2003.
  13. ↑ Vidal, Nicolas; Hedges, S. Blair (2005). "The phylogeny of squamate reptiles (lizards, snakes, and amphisbaenians) inferred from nine nuclear protein-coding genes". Comptes Rendus Biologies. 328 (10–11): 1000–8. doi:10.1016/j.crvi.2005.10.001. PMID   16286089.
  14. ↑ Glenday, Craig (2013). Guinness World Records 2014 . The Jim Pattison Group. ISBN   9781908843159.[ page needed ]
  15. ↑ Weinstein, Scott A.; Smith, Tamara L.; Kardong, Kenneth V. (14 July 2009). "Reptile Venom Glands Form, Function, and Future". In Stephen P. Mackessy (ed.). Handbook of Venoms and Toxins of Reptiles. Taylor & Francis. pp. 76–84. ISBN   978-1-4200-0866-1 . Retrieved 18 July 2013– via Google Books.
  16. ↑ Hargreaves, Adam D.; Swain, Martin T.; Logan, Darren W.; Mulley, John F. (2014). "Testing the Toxicofera: Comparative transcriptomics casts doubt on the single, early evolution of the reptile venom system" (PDF). Toxicon. 92: 140–56. doi:10.1016/j.toxicon.2014.10.004. PMID   25449103.
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