Xenopus (L., strange foot
) is a genus of highly aquatic frogs native to Sub-Saharan AfricaAfrica is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people in 61 territories, it accounts for about 14.8% of the...
. There are 18 species in the Xenopus
genus. They are known collectively as African Clawed Frogs or Platanna.
The best-known species belonging to this genus is Xenopus laevis
, which is commonly studied as a model organismA model organism is a species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms...
.
All species of Xenopus have flattened, somewhat egg-shaped and streamlined bodies, as well as very slippery skin (because of a protective mucous covering).
Xenopus (L., strange foot
) is a genus of highly aquatic frogs native to Sub-Saharan AfricaAfrica is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people in 61 territories, it accounts for about 14.8% of the...
. There are 18 species in the Xenopus
genus. They are known collectively as African Clawed Frogs or Platanna.
The best-known species belonging to this genus is Xenopus laevis
, which is commonly studied as a model organismA model organism is a species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms...
.
Description
All species of Xenopus have flattened, somewhat egg-shaped and streamlined bodies, as well as very slippery skin (because of a protective mucous covering). The frog's skin is smooth albeit with a
lateral lineIn aquatic organisms , the lateral line is a sense organ used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...
sensory organ that have a stitch-like appearance. The frogs are all excellent swimmers and have powerful, fully-webbed toes though the fingers lack webbing. Three of the toes on each foot have conspicuous black
clawsA claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...
.
The frog's eyes are positioned on top of the head, looking upwards. The pupils are circular. They have no moveable eyelids,
tonguesThe tongue is a muscle on the floor of the mouth that manipulates food for chewing and swallowing . It is the primary organ of taste, as much of the upper surface of the tongue is covered in papillae and taste buds. A secondary function of the tongue is speech, in which the organ assists...
(rather it is completely attached to the floor of the mouth) or
eardrumsThe tympanic membrane , is a thin membrane that separates the external ear from the middle ear. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
(similarly to the
Surinam toadSurinam toads, also called star-fingered toads, are members of the frog genus Pipa, within the family Pipidae. They are native to northern South America. Like other pipids, these frogs are almost exclusively aquatic.-Description:...
).
Internally, they are somewhat unique in that, unlike most amphibians, they have no
haptoglobinHaptoglobin is a protein that in humans is encoded by the HP gene. In blood plasma, haptoglobin binds free hemoglobin released from erythrocytes with high affinity and thereby inhibits its oxidative activity. The haptoglobin-hemoglobin complex will then be removed by the reticuloendothelial system...
in their
bloodBlood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and oxygen — and transports waste products away from those same cells....
.
Behaviour
Xenopus species are entirely
aquaticAquatic means relating to water; living in or near water or taking place in water.Aquatic may also refer to:* Aquatic animal, either vertebrate or invertebrate, which lives in water for most or all of its life...
, though they have been witnessed to migrate on land to nearby bodies of water during times of
droughtA drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
. The are usually found in lakes, rivers, swamps and man-made reservoirs.
Adult frogs are usually both predators and scavengers and since their tongue is unusable, the frogs use their small forelimbs to aid in the feeding process. Since they also lack a
vocal sacThe vocal sac is the flexible membrane of skin possessed by most male frogs. The purpose of the vocal sac is usually as an amplification of their mating or advertisement call...
, they make clicking sounds underwater (again similarly to the Surinam toad). The Xenopus species are also active during the twilight hours (or
crepuscularCrepuscular is a term used to describe some animals that are primarily active during twilight, that is at dawn and at dusk. The word is derived from the Latin word crepusculum, meaning "twilight." Crepuscular is thus in contrast with diurnal and nocturnal behavior. Crepuscular animals may also be...
).
During breeding season, the males have ridge-like nuptial pads (black in color) on the fingers to aid in grasping the female. The mating embrace the frogs use is inguinal, which means that the male grasps the female around her waist.
Like many other anurans, they are often used in laboratory as research subjects (see section below). In the
Hebrew languageHebrew is a Semitic language of the Afro-Asiatic language family. Culturally, it is considered a Jewish language. Hebrew in its modern form is spoken by more than seven million people in Israel while Classical Hebrew has been used for prayer or study in Jewish communities around the world for over...
the genus name literally means "Medical frog".
Xenopus research models
Xenopus
are a popular model system for geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
and proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
expression and knockdownGene knockdown refers to techniques by which the expression of one or more of an organism's genes is reduced, either through genetic modification or by treatment with a reagent such as a short DNA or RNA oligonucleotide with a sequence complementary to either an mRNA transcript or a gene...
studies. At 1 mm diameter, Xenopus
oocyteAn oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is part of the ovary development. The germ cells produce a primordial germ cell which becomes an oogonium which marks the start of mitosis...
s are very large cells which are easy for scientists to culture and use in experiments.
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
from other organisms can be injected into the large oocytes and the resulting
expressionGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA...
studied via
molecular biologyMolecular biology is the study of biology at a molecular level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
techniques or through
electrophysiologyElectrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
experimentation. Gene expression can be knocked down or
splicingIn molecular biology, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation...
modified using
MorpholinoIn molecular biology, a Morpholino is a molecule used to modify gene expression. Morpholino oligomers are an antisense technology used to block access of other molecules to specific sequences within nucleic acid...
antisense oligos injected into Xenopus
oocytes (for whole-body effects) or early embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s (for effects in the daughter cells descended from the injected cell). Cytoplasmic extracts made by centrifuging Xenopus
eggs or embryos (allowing direct addition or depletion of proteins) can recapitulate a wide range of complex cellular processes including nuclear envelopeThe nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...
formation, DNA replicationDNA replication, the basis for biological inheritance, is a fundamental process occurring in all living organisms to copy their DNA. This process is "semiconservative" in that each strand of the original double-stranded DNA molecule serves as template for the reproduction of the complementary strand...
and spindle assembly.
While Xenopus laevis
is the most commonly used species for developmental biologyDevelopmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis," which is the process that gives rise to tissues, organs and anatomy.Developmental biology is that...
studies, genetic studies can be complicated by their pseudotetraploidPloidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that compose the body are diploid , but sex cells are haploid...
genomeIn modern molecular biology the genome refers to all of its hereditary information encoded in DNA .The genome includes both the genes and the non-coding sequences of the DNA. The term was adapted in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany...
. Xenopus tropicalis
provides a simpler model for genetic studies, having a diploidPloidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that compose the body are diploid , but sex cells are haploid...
genome. Morpholino knockdownGene knockdown refers to techniques by which the expression of one or more of an organism's genes is reduced, either through genetic modification or by treatment with a reagent such as a short DNA or RNA oligonucleotide with a sequence complementary to either an mRNA transcript or a gene...
techniques are also used in X. tropicalis
to probe the function of a protein by observing the results of eliminating the protein's activity, as was done in a screen http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=17112317 http://www.gurdon.cam.ac.uk/~smithlab/screens/Xenopus-morpholino-pilot/ of X. tropicalis
genes published in 2006.
X. laevis
is often used as a low-tech pregnancy testA pregnancy test attempts to determine whether or not a woman is pregnant. Records of attempts at pregnancy testing have been found as far back as the ancient Greek and ancient Egyptian cultures. Modern pregnancy tests look for chemical markers associated with pregnancy. These markers are found...
; for details, see the species account.
External links
Xenopus laevis
and tropicalis Web Resource