African clawed frog

African clawed frog
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Anura
Family: Pipidae
Genus: Xenopus
Species: X. laevis
Binomial name
Xenopus laevis
Daudin, 1802

The African clawed frog (Xenopus laevis, also known as the xenopus, African clawed toad, African claw-toed frog or the platanna) is a species of African aquatic frog of the Pipidae family. Its name is derived from the three short claws on each hind foot, which it uses to tear apart its food. The word Xenopus means "strange foot" and laevis means "smooth".

The species is found throughout much of Sub-Saharan Africa (Nigeria and Sudan to South Africa),[2] and in isolated, introduced populations in North America, South America, and Europe.[1] All species of the Pipidae family are tongueless, toothless and completely aquatic. They use their hands to shove food in their mouths and down their throats and a hyobranchial pump to draw or suck things in their mouth. Pipidae have powerful legs for swimming and lunging after food. They also use the claws on their feet to tear pieces of large food. They lack true ears, but have lateral lines running down the length of the body and underside, which is how they can sense movements and vibrations in the water. They use their sensitive fingers, sense of smell, and lateral line system to find food. Pipidae are scavengers and will eat almost anything living, dying, or dead and any type of organic waste.

Description

These frogs are plentiful in ponds and rivers within the south-eastern portion of Sub-Saharan Africa. They are aquatic and are often greenish-grey in color. Albino varieties are commonly sold as pets. "Wild type" African clawed frogs are also frequently sold as pets, and often incorrectly labeled as a Congo frog or African dwarf frog because of similar colorings. They are easily distinguished from African dwarf frogs because African clawed frogs have webbing only on their hind feet while African dwarf frogs have webbing on all four feet.

They reproduce by fertilizing eggs outside of the female's body (see frog reproduction). Of the seven amplexus modes (positions in which frogs mate), these frogs are found breeding in inguinal amplexus, where the male clasps the female in front of the female's back legs and squeezes until eggs come out. The eggs are then fertilized.

The clawed frogs are the only amphibians to have actual claws used to climb and shred foods like fish or tadpoles. They lay their eggs from winter till spring. During wet rainy seasons they will travel to other ponds or puddles of water to search for food.[3]

The average life-span of these frogs ranges from 5–15 years with some individuals recorded to have lived for 20–25 years.[4] They shed their skin every season, and eat their own shed skin.

Although lacking a vocal sac, the males make a mating call of alternating long and short trills, by contracting the intrinsic laryngeal muscles. Females also answer vocally, signaling either acceptance (a rapping sound) or rejection (slow ticking) of the male.[5][6] This frog has smooth slippery skin which is multicolored on its back with blotches of olive gray or brown. The underside is creamy white with a yellow tinge.

Male and female frogs can be easily distinguished through the following differences. Male frogs are usually about 20% smaller than females, with slim bodies and legs. Males make mating calls to attract females, sounding very much like a cricket calling underwater. Females are larger than the males, appearing far more plump with hip-like bulges above their rear legs (where their eggs are internally located).

Both males and females have a cloaca, which is a chamber through which digestive and urinary wastes pass and through which the reproductive systems also empty. The cloaca empties by way of the vent which in reptiles and amphibians is a single opening for all three systems.[7]

In the wild

The monogenean Protopolystoma xenopodis,[8] a parasite of the urinary bladder of Xenopus laevis

In the wild, Xenopus laevis are native to wetlands, ponds, and lakes across arid/semiarid regions of Sub-Saharan Africa.[2][9] Xenopus laevis and Xenopus muelleri occur along the western boundary of the Great African Rift. The people of the sub-Saharan are generally very familiar with this frog, and some cultures use it as a source of protein, an aphrodisiac, or as fertility medicine. Two historic outbreaks of priapism have been linked to consumption of frog legs from frogs that ate insects containing cantharidin.[10] Wild Xenopus are much larger than their captive bred counterparts.
Xenopus laevis in the wild are commonly infected by various parasites,[8] including monogeneans in the urinary bladder.

Use in research

Xenopus embryos and eggs are a popular model system for a wide variety of biological studies.[11][12] This animal is widely used because of its powerful combination of experimental tractability and close evolutionary relationship with humans, at least compared to many model organisms.[11][12] For a more comprehensive discussion of the use of these frogs in biomedical research, see Xenopus.

Xenopus has long been an important tool for in vivo studies in molecular, cell, and developmental biology of vertebrate animals. However, the wide breadth of Xenopus research stems from the additional fact that cell-free extracts made from Xenopus are a premier in vitro system for studies of fundamental aspects of cell and molecular biology. Thus, Xenopus is the only vertebrate model system that allows for high-throughput in vivo analyses of gene function and high-throughput biochemistry. Finally, Xenopus oocytes are a leading system for studies of ion transport and channel physiology.[11]

Although X. laevis does not have the short generation time and genetic simplicity generally desired in genetic model organisms, it is an important model organism in developmental biology, cell biology, toxicology and neurobiology. X. laevis takes 1 to 2 years to reach sexual maturity and, like most of its genus, it is tetraploid. It does have a large and easily manipulated embryo, however. The ease of manipulation in amphibian embryos has given them an important place in historical and modern developmental biology. A related species, Xenopus tropicalis, is now being promoted as a more viable model for genetics.

Roger Wolcott Sperry used X. laevis for his famous experiments describing the development of the visual system. These experiments led to the formulation of the Chemoaffinity hypothesis.

Xenopus oocytes provide an important expression system for molecular biology. By injecting DNA or mRNA into the oocyte or developing embryo, scientists can study the protein products in a controlled system. This allows rapid functional expression of manipulated DNAs (or mRNA). This is particularly useful in electrophysiology, where the ease of recording from the oocyte makes expression of membrane channels attractive. One challenge of oocyte work is eliminating native proteins that might confound results, such as membrane channels native to the oocyte. Translation of proteins can be blocked or splicing of pre-mRNA can be modified by injection of Morpholino antisense oligos into the oocyte (for distribution throughout the embryo) or early embryo (for distribution only into daughter cells of the injected cell).[13]

Extracts from the eggs of X. laevis frogs are also commonly used for biochemical studies of DNA replication and repair, as these extracts fully support DNA replication and other related processes in a cell-free environment which allows easier manipulation.[14]

The first vertebrate ever to be cloned was an African clawed frog, an experiment for which Sir John Gurdon was awarded the Nobel Prize in Physiology or Medicine 2012 "for the discovery that mature cells can be reprogrammed to become pluripotent".[15]

Amphibian metamorphosis

Additionally, several African clawed frogs were present on the Space Shuttle Endeavour (which was launched into space on September 12, 1992) so that scientists could test whether reproduction and development could occur normally in zero gravity.[16][17]

X. laevis is also notable for its use in the first well-documented method of pregnancy testing when it was discovered that the urine from pregnant women induced X. laevis oocyte production. Human chorionic gonadotropin (HCG) is a hormone found in substantial quantities in the urine of pregnant women. Today, commercially available HCG is injected into Xenopus males and females to induce mating behavior and to breed these frogs in captivity at any time of the year.[18]

Amphibian frog Xenopus laevis also serves as an ideal model system for the study of the mechanisms of apoptosis. In fact, iodine and thyroxine stimulate the spectacular apoptosis of the cells of the larval gills, tail and fins in amphibians metamorphosis, and stimulate the evolution of their nervous system transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog.[19][20][21][22]

Genome sequencing

Early work on sequencing of the X. laevis genome was started when the Wallingford and Marcotte labs obtained funding from the Texas Institute for Drug and Diagnostic Development (TI3D), in conjunction with projects funded by the National Institutes of Health. The work rapidly expanded to include de novo reconstruction of X. laevis transcripts, in collaboration with groups around the world donating Illumina Hi-Seq RNA sequencing datasets. Genome sequencing by the Rokhsar and Harland groups (UC Berkeley) and by Taira and collaborators (University of Tokyo, Japan) gave a major boost to the project, which, with additional contributions from investigators in the Netherlands, Korea, Canada and Australia, led to publication of the genome sequence and its characterization in 2016.[23]

Xenbase is the Model Organism Database (MOD) with the full details and release information regarding the current Xenopus laevis genome (9.1).

As pets

Xenopus laevis have been kept as pets and research subjects since as early as the 1950s. They are extremely hardy and long lived, having been known to live up to 20 or even 30 years in captivity.[24]

African clawed frogs are frequently mislabeled as African dwarf frogs in pet stores. The astute pet owner will recognize the difference, however, because of the following characteristics:

As a pest

African clawed frogs are voracious predators and easily adapt to many habitats.[25] For this reason, they can easily become a harmful invasive species. They can travel short distances to other bodies of water, and some have even been documented to survive mild freezes. They have been shown to devastate native populations of frogs and other creatures by eating their young.

In 2003, Xenopus laevis frogs were discovered in a pond at San Francisco's Golden Gate Park. Much debate now exists in the area on how to exterminate these creatures and keep them from spreading.[26][27] It is unknown if these frogs entered the San Francisco ecosystem through intentional release or escape into the wild. San Francisco officials drained Lily Pond and fenced off the area to prevent the frogs from escaping to other ponds in the hopes they starve to death. Currently now, the government is starting a new program where they will use pythons to combat the spread of this invasive species.

Due to incidents in which these frogs were released and allowed to escape into the wild, African clawed frogs are illegal to own, transport or sell without a permit in the following US states: Arizona, California, Kentucky, Louisiana, New Jersey, North Carolina, Oregon, Virginia, Hawaii,[28] Nevada, and Washington state. However, it is legal to own Xenopus laevis in New Brunswick (Canada) and Ohio.[29][30]

Feral colonies of Xenopus laevis exist in South Wales, United Kingdom.[31]

The African clawed frog may be an important vector and the initial source of Batrachochytrium dendrobatidis, a chytrid fungus that has been implicated in the drastic decline in amphibian populations in many parts of the world.[2] Unlike in many other amphibian species (including the closely related western clawed frog) where this chytrid fungus causes the disease Chytridiomycosis, it does not appear to affect the African clawed frog, making it an effective carrier.[2]

References

  1. 1 2 Tinsley, R.; Minter, L.; Measey, J.; Howell, K.; Veloso, A.; Núñez, H. & Romano, A. (2009). "Xenopus laevis". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 4 May 2014.
  2. 1 2 3 4 Weldon; du Preez; Hyatt; Muller; and Speare (2004). Origin of the Amphibian Chytrid Fungus. Emerging Infectious Disease 10(12).
  3. ↑ Maddin HC, Eckhart L, Jaeger K, Russell AP, Ghannadan M (April 2009). "The anatomy and development of the claws of Xenopus laevis (Lissamphibia: Anura) reveal alternate pathways of structural evolution in the integument of tetrapods". Journal of Anatomy. 214 (4): 607–19. doi:10.1111/j.1469-7580.2009.01052.x. PMC 2736125Freely accessible. PMID 19422431.
  4. ↑ Garvey, Nathan. "ADW: Xenopus laevis: INFORMATION". Animaldiversity.ummz.umich.edu. Retrieved 2013-06-08.
  5. ↑ Garvey, Nathan. "ADW: Xenopus Laevis: Information". Animaldiversity.ummz.umich.edu. Retrieved 2013-06-08.
  6. ↑ Talk of the Nation. "ADW: NPR: Listening To Love Songs of African Clawed Frogs". NPR. Retrieved 2013-06-08.
  7. ↑ Reference: National Audubon Society. Field Guide To Reptiles & Amphibians, pp: 701 & 704; Alfred A. Knopf, 24th Printing 2008.
  8. 1 2 Theunissen, M., Tiedt, L. & Du Preez, L. H. 2014: The morphology and attachment of Protopolystoma xenopodis (Monogenea: Polystomatidae) infecting the African clawed frog Xenopus laevis. Parasite, 21, 20. doi:10.1051/parasite/2014020
  9. ↑ John Measey. "Ecology of Xenopus Laevis". Bcb.uwc.ac.za. Retrieved 2013-06-08.
  10. ↑ "Historic priapism pegged to frog legs. - Free Online Library". www.thefreelibrary.com. Retrieved 2016-06-20.
  11. 1 2 3 Wallingford, J., Liu, K., and Zheng, Y. 2010. Current Biology v. 20, p. R263-4
  12. 1 2 Harland, R.M. and Grainger, R.M. 2011. Trends in Genetics v. 27, p 507-15
  13. ↑ Nutt, S. L.; Bronchain, O. J.; Hartley, K. O.; Amaya, E. (2001). "Comparison of morpholino based translational inhibition during the development ofXenopus laevis andXenopus tropicalis". Genesis. 30 (3): 110–113. doi:10.1002/gene.1042. PMID 11477685.
  14. ↑ Blow JJ, Laskey RA (November 1986). "Initiation of DNA replication in nuclei and purified DNA by a cell-free extract of Xenopus eggs". Cell. 47 (4): 577–87. doi:10.1016/0092-8674(86)90622-7. PMID 3779837.
  15. ↑ "The Nobel Prize in Physiology or Medicine 2012". www.nobelprize.org. Retrieved 2016-06-20.
  16. ↑ "Ludington Daily News - Sep 14, 1992, p. 7". News.google.com. 1992-09-14. Retrieved 2013-06-08.
  17. ↑ "Reading Eagle - Sep 11, 1992, p. A8". News.google.com. 1992-09-11. Retrieved 2013-06-08.
  18. ↑ Green, SL. The Laboratory Xenopus sp: The Laboratory Animal Pocket Reference Series. Editor: M. Suckow. Taylor and Francis Group, LLC, Boca Raton, Fla., 2010
  19. ↑ Jewhurst K, Levin M, McLaughlin KA (2014). "Optogenetic Control of Apoptosis in Targeted Tissues of Xenopus laevis Embryos.". J Cell Death. 7: 25–31. doi:10.4137/JCD.S18368. PMC 4213186Freely accessible. PMID 25374461.
  20. ↑ Venturi, Sebastiano (2011). "Evolutionary Significance of Iodine". Current Chemical Biology-. 5 (3): 155–162. doi:10.2174/187231311796765012. ISSN 1872-3136.
  21. ↑ Venturi, Sebastiano (2014). "Iodine, PUFAs and Iodolipids in Health and Disease: An Evolutionary Perspective". Human Evolution-. 29 (1-3): 185–205. ISSN 0393-9375.
  22. ↑ Tamura K, Takayama S, Ishii T, Mawaribuchi S, Takamatsu N, Ito M (2015). "Apoptosis and differentiation of Xenopus tail-derived myoblasts by thyroid hormone.". J Mol Endocrinol. 54 (3): 185–92. doi:10.1530/JME-14-0327.
  23. ↑ Session, Adam; et al. (October 19, 2016). "Genome evolution in the allotetraploid frog Xenopus laevis.". Nature. 538 (7625): 336–343. doi:10.1038/nature19840. PMID 27762356.
  24. ↑ "NPR December 22, 2007". Npr.org. 2007-12-22. Retrieved 2013-06-08.
  25. ↑ James A. Danoff-Burg. "ADW: Columbia: Introduced Species Summary Project". Columbia.edu. Retrieved 2013-06-08.
  26. ↑ "Killer Meat-Eating Frogs Terrorize San Francisco". FoxNews. 2007-03-14.
  27. ↑ "The Killer Frogs of Lily Pond:San Francisco poised to checkmate amphibious African predators of Golden Gate Park". San Francisco Chronicle. Archived from the original on 2013-06-06.
  28. ↑ "ADW: Honolulu Star-Bulletin Wednesday, July 3, 2002". Archives.starbulletin.com. 2002-07-03. Retrieved 2013-06-08.
  29. ↑ ADW: New Brunswick Regulation 92-74 Archived August 19, 2011, at the Wayback Machine.
  30. ↑ "ADW: New Brunswick Acts and regulations". Gnb.ca. Retrieved 2013-06-08.
  31. ↑ John Measey. "Feral Xenopus laevis in South Wales, UK". Bcb.uwc.ac.za. Retrieved 2013-06-08.

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