Molecular Biology and Genetics
Statistics of barcoding coverage
Specimen Records: 15
Specimens with Sequences: 14
Specimens with Barcodes: 13
Species With Barcodes: 4
Public Records: 11
Public Species: 3
Public BINs: 3
|This article may be too technical for most readers to understand. (August 2012)|
Agkistrodon is a genus of venomous pit vipers found in North America from the United States south to northern Costa Rica. Three species are currently recognized, all of them polytypic and closely related. Common names include: moccasins, copperheads, cantils.
- 1 Name origin
- 2 Description
- 3 Geographic range
- 4 Behavior
- 5 Reproduction
- 6 Venom
- 7 Species
- 8 Taxonomy
- 9 See also
- 10 References
- 11 Further reading
- 12 External links
The name Agkistrodon comes from the Greek words agkistron (ἄγκιστρον, 'fishhook', with the irregular transliteration gk rather than the usual nk) and odous (ὁδοὐς, 'tooth,' from the stem ὁδόντ-) and is likely a reference to the fangs.
Members of this genus have a number of features in common. All species have a relatively broad head with short fangs. A loreal scale is present, except in A. piscivorus. There are usually nine large symmetrical platelike scales on the crown of the head, but in all species these are often irregularly fragmented or have sutures, especially in A. bilineatus. All have a sharply defined canthus rostralis and a vertically elliptical pupil. There are 6-10 (usually 8) supralabial scales and 8-13 (usually 10-11) sublabials. The dorsal scales are mostly keeled and at midbody number 21-25 (usually 23), while A. piscivorus has 23-27 (usually 25). There are 127-157 ventral scales and 36-71 subcaudals. Of the latter, some may be divided. The anal scale is single. All have a color pattern of 10-20 dark crossbands on a lighter ground color, although sometimes the crossbands are staggered as half bands on either side of the body.
The phylogeny of the three species has long been controversial. Studies based on morphological (Gloyd & Conant, 1990) and venom characteristics (Jones, 1976) support the idea that A. bilineatus and A. contortrix are more closely related. However, an analysis of mitochondrial DNA was conducted by Knight et al. (1992), as well as more recent molecular studies (Parkinson et al., 1997, 1999) have concluded that A. bilineatus and A. piscivorus are sister taxa, with A. contortrix being a sister species to them both.
Found in North America from the northeastern and central USA southward through peninsular Florida and southwestern Texas. In Central America on the Atlantic versant from Tamaulipas and Nuevo León southward to the Yucatan Peninsula, Belize and Guatemala. Along the Pacific coastal plane and lower foothills from Sonora south through Guatemala, El Salvador, Honduras and Nicaragua to northwestern Costa Rica.
Pit vipers of the genus Agkistrodon rely on a potent venom they produce for their survival. Used to immobilize prey and fend off predators, one bite can inject enough venom into a human to cause severe pain, swelling, weakness, difficulty breathing, hemorrhaging, gangrene, fever, vomiting, and in rare instances, even death. When concentrations are controlled, however, traditional Chinese medicine has discovered a clinical use for certain species’ venom. Today, this correlation has spread to modern research on the effects of snake venom in cancer patients and victims of strokes.
It is assumed that the venom of all three species is not unlike that of A. contortrix, which contains thrombin-like enzymes that act upon the coagulant activity of the blood. A study of electrophoretic patterns of proteins in venoms among and within populations of A. contortrix and A. piscivorus showed that substantial variation exists (Jones, 1976), and there is no reason to believe that these differences do not correspond with variations in toxicity.
Agkistrodon piscivorus Venom
The largest species of Agkistrodon, Agkistrodon piscivorus, relies on venom to quickly catch prey. In a study conducted at the College of Medicine at the University of Florida, their venom was injected into the lymph fluid of a frog. The frog immediately suffocated because of the collapse of its lung sacs. The venom even resulted in lung constriction when directly applied to the surface of the frog's lungs. To test this, trace amounts of venom were dropped onto a single pulmonary sac in a frog's lung after it was anesthetized and its chest cavity dissected open. A drop of solution containing a venom concentration of 1 mg/mL was enough to cause contraction of the pulmonary artery adventitia after 5-8sec in a frog weighing 40g. The study found, however, that this toxic effect is simply a tool the snake can choose to employ from an accessory venom gland it has. In most instances, the viper injects a venom that tends to immobilize, not kill its prey before ingestion. In this case, the main venom glands secrete a toxin that inhibits the prey’s sympathetic response to flee or fend off its predator. This essentially stuns the animal so that the predator can easily attack.
Deinagkistrodon acutus Venom
One species formerly of Agkistrodon, Deinagkistrodon acutus or the “100 pace snake,” is commonly used for research purposes. Researchers have found that their venom contains protease activity, meaning it attacks and degrades intra- and extracellular proteins. If injected into mice, within 2 hours the venom begins a process known as mesangiolysis (the degeneration and death of cells that line the inner layer of the glomerulus and regulate glomerular filtration in the kidney). Eventually, the kidneys no longer function and the mouse dies.
Venom and Cancer Treatment
Although Agkistrodon venom is lethal to both prey and humans in vivo, when controlled, research shows that the venom has some clinical application. Deinagkistrodon acutus snake venom contains a protein called ACTX-6. This protein was shown to induce apoptosis (cell death) in isolated cancer cells through Fas pathway activation. Fas is a protein that becomes a death receptor in the cellular membrane. When activated, Fas turns on what is called a “Caspase Cascade.” This pathway is made up of a series of proteins called initiator and executioner caspases. Initiator caspases help form an apoptosis initiation factor that eventually activates executioner caspases (see figure 3). Executioner caspases go on to “digest” the cell from the inside out. They cleave cytoskeleton filaments and DNA until the cell completely implodes.
If this pathway can be activated in tumor cells using Agkistrodon venom, then theoretically, the proteins in the venom could be used to target and kill cancerous cells.
Venom and Thrombosis Treatment
Currently, the venom of a different species of pit viper, Agkistrodon rhodostoma, is used to isolate a thrombin-like enzyme called ancrod. This enzyme is used clinically to break down and dissolve thrombi (blood clots) in patients and lower blood viscosity to help prevent heart attack and stroke.
Venom and Traditional Chinese Medicine
Deinagkistrodon acutus venom has been used in traditional Chinese medicine for centuries to extract antivenin that is successfully used to treat snakebites. Different parts of the snake are also prescribed to help alleviate ailments known as “wind diseases.” Because these snakes move so quickly, it is theorized that substances from their bodies can easily treat these fast moving “wind” syndromes. Agkistrodon acutus is currently used in patients with arthritis, leprosy, tetanus, boils, and, as previously mentioned, tumors. It is believed that the same qualities that make snakes flexible, capable of regenerating skin, and able to inflict paralysis can be transferred to human conditions if applied medicinally. The vipers are prepared by cooking the flesh of the headless body, grinding a paste of snake ash and mixing it with honey, drying the snake and compacting it into a powder, or even injecting their venom intravenously. Although these practices are common in Chinese medicine, there are no current studies that have affirmed the effectiveness of these treatments. It is unknown whether or not these "cures" simply have a placebo effect or actually heal the patients.
|Species||Taxon author||Subsp.*||Common name||Geographic range|
|A. bilineatus||(Günther, 1863)||3||Cantil/Mexican Moccasin||Mexico and Central America. On the Atlantic side it is found in Mexico in Tamaulipas, Nuevo León, possibly northern Veracruz and Chiapas (in the Middle Grijalva Valley). On the Yucatan Peninsula it occurs in Campeche, Yucatán, Quintana Roo and northern Belize. On the Pacific side it is found from southern Sonora in Mexico south through Guatemala, El Salvador, Honduras and Nicaragua to northwestern Costa Rica. On the Pacific side the distribution is almost continuous, while on the Atlantic side it is disjunct.|
|A. contortrixT||(Linnaeus, 1766)||4||Copperhead||The United States (Texas, Oklahoma, Kansas, Missouri, Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida, South Carolina, North Carolina, Tennessee, Kentucky, Virginia, West Virginia, Illinois, Indiana, Ohio, Iowa, Pennsylvania, Maryland, New Jersey, Delaware, New York, Connecticut, Massachusetts), Mexico (Chihuahua, Coahuila).|
|A. piscivorus||(Lacépède, 1789)||2||Cottonmouth|
|The eastern United States from extreme southeastern Virginia, south through peninsular Florida and west to Arkansas, southeastern Kansas, eastern and southern Oklahoma and eastern and central Texas. A few records exist from along the Rio Grande in Texas, but these are thought to represent isolated populations that possibly no longer exist.|
This genus was previously much larger and also included the following genera:
- Calloselasma - The Malayan pit viper found in Southeast Asia.
- Deinagkistrodon - The Hundred-pace viper found mostly in southern China.
- Gloydius - Moccasins found in Asia.
- Hypnale - Hump-nosed vipers found in India and Sri Lanka.
- List of crotaline species and subspecies
- Crotalinae by common name
- Crotalinae by taxonomic synonyms
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