Африка тырнақты бақасы: сипаттамасы мен ерекшеліктері

Species of amphibian

The African clawed frog (Xenopus laevis), also known as simply Xenopus, African clawed toad, African claw toed frog or the Platanna) is a species of African aquatic frog of the family Pipidae. Its name is derived from the short black claws on its feet. 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), and in isolated, introduced populations in North America, South America, Europe, and Asia. They use their sensitive fingers and sense of smell to find food. Pipidae are scavengers and will eat almost anything living, dying, or dead and any type of organic waste. It is considered an invasive species in several countries, including across Europe.

These frogs are plentiful in ponds and rivers within the south eastern portion of Sub Saharan Africa. They are aquatic and are often a mottled greenish grey brown in color, sometimes with yellowish botches, and with a pale white cream belly. African clawed frogs have been frequently sold as pets, and are sometimes misidentified as African dwarf frogs. Albino clawed frogs are common and sold as animals for laboratories. Amphibians 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 until eggs are laid, and the male fertilizes the egg mass with the release of sperm. African clawed frogs are highly adaptable and will lay their eggs whenever conditions allow it. During wet rainy seasons they will travel to other ponds or puddles of water to search for food and new ponds. During times of drought, the clawed frogs can burrow themselves into the mud, becoming dormant for up to a year. Xenopus laevis have been known to survive 15 or more years in the wild and 25–30 years in captivity. 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. 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 small and slim, while females are larger and more rotund. Males have black patches on their hands and arms which aid in grabbing onto females during amplexus. Females have a more pronounced cloaca and have 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.

African clawed frogs are fully aquatic and will rarely leave the water except to migrate to new water bodies during droughts or other disturbances. Clawed frogs have powerful legs that help them move quickly both underwater and on land. Feral clawed frogs in South Wales have been found to travel up to between locations. The feet of Xenopus species have three black claws on the last three digits. These claws are used to rip apart food and scratch predators. Clawed frogs are carnivores and will eat both living and dead prey including fish, tadpoles, crustaceans, annelids, arthropods, and more. Clawed frogs will try to consume anything that is able to fit into their mouths. Being aquatic, clawed frogs use their sense of smell and their lateral line to detect prey rather than eyesight like other frogs. However, clawed frogs can still see using their eyes and will stalk prey or watch predators by sticking their heads out of the water. Clawed frogs will dig through substrate to unearth worms and other food. Unlike other frogs, they have no tongue to extend to catch food, so clawed frogs use their hands to grab food and shovel it into their mouths. These frogs are particularly cannibalistic; the stomach contents of feral clawed frogs in California have revealed large amounts of the frog's larvae. Clawed frog larvae are filter feeders and collect nutrients from plankton, allowing adult frogs that consume the tadpoles to have access to these nutrients. This allows clawed frogs to survive in areas that have little to no other food sources. Clawed frogs are nocturnal and most reproductive activity and feeding occurs after dark. Male clawed frogs are very promiscuous and will grab onto other males and even other species of frogs. Male frogs that are grasped will make release calls and attempt to break free. If not feeding, clawed frogs will just sit motionless on top of the substrate or floating, legs splayed below, at the waters surface with their nostrils and eyes sticking out.

The clawed frog liver responds to low temperatures by increasing production of type II iodothyronine deiodinase This in turn spurs the thyroid to increase T to increase body temperature. (This T increase also induces germ cell apoptosis, mediated through genes left over from tadpole metamorphosis.)

The lipidomics of Xenopus oocytes have been studied by Tian et al 2014 and Phan et al 2015.

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.

X. laevis oocytes are often used as an easy model for the artificially induced expression of transgenes. For example, they are commonly used when studying chloroquine resistance produced by specialized transporter mutants. Even so the foreign expression tissue may itself confer some alterations to the expression, and so findings may or may not be entirely identical to native expression: For example, iron has been found by Bakouh et al 2017 to be an important substrate for one such transporter in X. l. oocytes, but as of 2020 iron is merely presumptively involved in native expression of the same gene. is the Model Organism Database (MOD) for both Xenopus laevis and Xenopus tropicalis. Xenbase hosts the full details and release information regarding the current v10 Xenopus laevis genome released in 2022.

The clawed frog 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. African clawed frogs are frequently mislabeled as African dwarf frogs in pet stores. Identifiable differences are:
Dwarf frogs have four webbed feet. African clawed frogs have webbed hind feet while their front feet have autonomous digits. African dwarf frogs have eyes positioned on the side of their head, while African clawed frogs have eyes on the top of their heads. African clawed frogs have curved, flat snouts. The snout of an African dwarf frog is pointed.

African clawed frogs are voracious predators and easily adapt to many habitats. 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. 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. 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, Vermont, Virginia, Hawaii, Nevada, and Washington state. However, it is legal to own Xenopus laevis in New Brunswick (Canada) and Ohio. Feral colonies of Xenopus laevis exist in South Wales, United Kingdom. In Yunnan, China there is a population of albino clawed frogs in Lake Kunming, along with another invasive: the American bullfrog. Because this population is albino, it suggests that the clawed frogs originated from the pet trade or a laboratory. 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. 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.