Xenopus

Xenopus (Gk., xeno=strange, pous=foot) is a genus of highly aquatic frogs native to Sub-Saharan Africa. There are 19 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 organism.

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 line sensory organ that has 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 claws.

The frog's eyes are positioned on top of the head, looking upwards. The pupils are circular. They have no moveable eyelids, tongues (rather it is completely attached to the floor of the mouth ) or eardrums (similarly to the Surinam toad ).

Internally, they are somewhat unique in that, unlike most amphibians, they have no haptoglobin in their blood.

Behaviour
Xenopus species are entirely aquatic, though they have been witnessed to migrate on land to nearby bodies of water during times of drought. They 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 sac, they make clicking sounds underwater (again similarly to the Surinam toad). The Xenopus species are also active during the twilight hours (or crepuscular).

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 language the genus name literally means "Medical frog".

Xenopus research models
Xenopus are a popular model system for gene and protein expression and knockdown studies. At 1 mm diameter, Xenopus oocytes are very large cells which are easy for scientists to culture and use in experiments. RNA from other organisms can be injected into the large oocytes and the resulting expression studied via molecular biology techniques or through electrophysiology experimentation. Gene expression can be knocked down or splicing modified using Morpholino antisense oligos injected into Xenopus oocytes (for whole-body effects) or early embryos (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 envelope formation, DNA replication and spindle assembly.

While Xenopus laevis is the most commonly used species for developmental biology studies, genetic studies can be complicated by their pseudotetraploid genome. Xenopus tropicalis provides a simpler model for genetic studies, having a diploid genome. Morpholino knockdown 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 of X. tropicalis genes published in 2006.

X. laevis is often used as a low-tech pregnancy test; for details, see the species account.