Xenopus () (from Greek ξενος, xenos 'strange' + πους, pous 'foot', commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known species of this genus are Xenopus laevis and Xenopus tropicalis, which are commonly studied as model organisms for developmental biology, cell biology, toxicology, neuroscience and for modelling human disease and birth defects. The genus is also known for its allopolyploidy, with some species having up to 12 sets of chromosomes (all have undergone diploidization; the highest is 2n = 12x = 108).
Characteristics Xenopus laevis is a rather inactive creature. It is incredibly hearty and can live up to 15 years. At times the ponds that Xenopus laevis is found in dry up, compelling it, in the dry season, to burrow into the mud, leaving a tunnel for air. It may lie dormant for up to a year. If the pond dries up in the rainy season, Xenopus laevis may migrate long distances to another pond, maintaining hydration by the rains. It is an adept swimmer, swimming in all directions with ease. It is barely able to hop, but it is able to crawl. It spends most of its time underwater and comes to surface to breathe. Respiration is predominantly through its well-developed lungs; there is little cutaneous respiration.
Description All species of Xenopus have flattened, somewhat egg-shaped and streamlined bodies, and very slippery skin (because of a protective mucus covering). The frog's skin is smooth, but 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 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 Pipa pipa, the common Suriname toad). Unlike most amphibians, they have no haptoglobin in their blood.
Behaviour Xenopus species are entirely aquatic, though they have been observed migrating on land to nearby bodies of water during times of drought or in heavy rain. They are usually found in lakes, rivers, swamps, potholes in streams, and man-made reservoirs. Adult frogs are usually both predators and scavengers, and since their tongues are unusable, the frogs use their small fore limbs to aid in the feeding process. Since they also lack vocal sacs, they make clicks (brief pulses of sound) underwater (again similar to Pipa pipa). Males establish a hierarchy of social dominance in which primarily one male has the right to make the advertisement call. The females of many species produce a release call, and Xenopus laevis females produce an additional call when sexually receptive and soon to lay eggs. The Xenopus species are also active during the twilight (or crepuscular) hours. During breeding season, the males develop ridge-like nuptial pads (black in color) on their fingers to aid in grasping the female. The frogs' mating embrace is inguinal, meaning the male grasps the female around her waist.
Species
Extant species
Fossil species The following fossil species have been described:
†Xenopus arabiensis Henrici and Báez 2001 - Oligocene Yemen Volcanic Group, Yemen †Xenopus hasaunus Spinar 1980 †Xenopus romeri Estes 1975 - Itaboraian Itaboraí Formation, Brazil †Xenopus stromeri Ahl 1926 cf. Xenopus sp. - Campanian - Los Alamitos Formation, Argentina Xenopus (Xenopus) sp. - Late Oligocene Nsungwe Formation, Tanzania Xenopus sp. - Miocene Morocco Xenopus sp. - Early Pleistocene Olduvai Formation, Tanzania There are also numerous "molecular fossil" species known today only as extinct parents of living allopolyploid species.
Model organism for biological research Like many other frogs, they are often used in laboratory as research subjects. Xenopus embryos and eggs are a popular model system for a wide variety of biological studies. This animal is used because of its powerful combination of experimental tractability and close evolutionary relationship with humans, at least compared to many model organisms. 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 a vertebrate model system that allows for high-throughput in vivo analyses of gene function and high-throughput biochemistry. Furthermore, Xenopus oocytes are a leading system for studies of ion transport and channel physiology. Xenopus is also a unique system for analyses of genome evolution (espcially allopolyploidization and diploidization) in vertebrates, as different Xenopus species form a allo-ploidy series (2x, 4x, 8x, 12x) formed by interspecific hybridization. (For an overview of Xenopus phylogeny taking into account past hybridization events, see Figure 3 of Evans et al., 2015.) In 1931, Lancelot Hogben noted that Xenopus laevis females ovulated when injected with the urine of pregnant women. This led to a pregnancy test that was later refined by South African researchers Hillel Abbe Shapiro and Harry Zwarenstein. A female Xenopus frog injected with a woman's urine was put in a jar with a little water. If eggs were in the water a day later it meant the woman was pregnant. Four years after the first Xenopus test, Zwarenstein's colleague, Dr Louis Bosman, reported that the test was accurate in more than 99% of cases. From the 1930s to the 1950s, thousands of frogs were exported across the world for use in these pregnancy tests. The National Xenopus Resource of the Marine Biological Laboratory is an in vivo repository for transgenic and mutant strains and a training center.
Online Model Organism Database Xenbase is the Model Organism Database (MOD) for both Xenopus laevis and Xenopus tropicalis.
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