African clawed frog
The African clawed frog (Xenopus laevis, also known as platanna) is a species of South African aquatic frog of the genus Xenopus. It can grow up to a length of 5 in (12 cm) with a flattened head and body, but no external ear or tongue. Its name is derived from the three short claws on each of its hind feet, which it probably uses to stir up mud to hide itself from predators.
The species is found throughout most of Africa, and in isolated, introduced populations in North America, South America, and Europe.
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 more commonly sold as pets. The ACF pictured can also be found available for sale but is quite often incorrectly labeled as a Congo Frog or African dwarf frog because of similar colourings. They reproduce by laying eggs (see frog reproduction).
The average life-span of these frogs ranges from 5 to 15 years with some individuals having been recorded to live for nearly 20 years. 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. Most unusually, females also answer vocally, signaling either acceptance (a rapping sound) or rejection (slow ticking) of the male.
Use in research
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. X. laevis takes 1 to 2 years to reach sexual maturity and, like most of its genus, it is tetraploid. However, it does have a large and easily manipulable embryo. 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).
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.
X. laevis is also notable for its use as 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 breed these frogs in captivity at any time of the year.
As a pest
When African clawed frogs are imported into non-native countries, they have the capacity to wreck entire ecosystems by eating native wildlife that have no natural defense against these creatures.
In 2003, these frogs were discovered in a pond at San Francisco's Golden Gate Park, where much debate exists on how to terminate these creatures and keep them from spreading. It is unknown if these frogs entered the San Francisco ecosystem through intentional release or escape into the wild.
Because these frogs are immune to the fungi Batrachochytrium dendrobatidis (a chytridomycota) and B. dendrobatidis has been traced back to the habitat of Xenopus laevis in Africa, many scholars believe it is the source of the worldwide frog population crash. Due to its extensive use in obstetrics and research, it appears Xenopus laevis has carried B. dendrobatidis with it out of Africa to all over the world, causing chytridomycosis and eventually death in native frogs naïve to the fungi.
- ^ Tinsley et al. (2004). Xenopus laevis. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 12 May 2006. Database entry includes a range map and justification for why this species is of least concern
- ^ http://animaldiversity.ummz.umich.edu/site/accounts/information/Xenopus_laevis.html
- ^ ADW: Xenopus Laevis: Information
- ^ Comparison of morpholino based translational inhib...[Genesis. 2001] - PubMed Result
- ^ Blow JJ, Laskey RA (1986 month = November). "Initiation of DNA replication in nuclei and purified DNA by a cell-free extract of Xenopus eggs..". Cell 47 (4): 577-87. PMID 3779837.
- ^ "Killer Meat-Eating Frogs Terrorize San Francisco". FoxNews. http://www.foxnews.com/story/0,2933,258519,00.html.
- ^ "The Killer Frogs of Lily Pond:San Francisco poised to checkmate amphibious African predators of Golden Gate Park". San Francisco Chronicle. http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/03/12/BAGLUOJLDE1.dlkhgidofyugDTL.