Western clawed frog

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Western clawed frog
Xenopus tropicalis02.jpeg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Anura
Family: Pipidae
Genus: Xenopus
Species:
X. tropicalis
Binomial name
Xenopus tropicalis
(Gray, 1864)
Synonyms
  • Silurana tropicalis
    Gray, 1864

The western clawed frog (Xenopus tropicalis) is a species of frog in the family Pipidae, also known as tropical clawed frog. [2] It is the only species in the genus Xenopus to have a diploid genome. [3] [4] Its genome has been sequenced, [5] [6] making it a significant model organism for genetics that complements the related species Xenopus laevis (the African clawed frog), [7] a widely used vertebrate model for developmental biology. X. tropicalis also has a number of advantages over X. laevis in research, such as a much shorter generation time (<5 months), smaller size (4–6 cm (1.6–2.4 in) body length), and a larger number of eggs per spawn. [8]

Contents

It is found in Benin, Burkina Faso, Cameroon, Ivory Coast, Equatorial Guinea, Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Nigeria, Senegal, Sierra Leone, Togo, and possibly Mali. Its natural habitats are subtropical or tropical moist lowland forests, moist savanna, rivers, intermittent rivers, swamps, freshwater lakes, intermittent freshwater lakes, freshwater marshes, intermittent freshwater marshes, rural gardens, heavily degraded former forests, water storage areas, ponds, aquaculture ponds, and canals and ditches.

Description

The western clawed frog is a medium-sized species with a somewhat flattened body and a snout-vent length of 28 to 55 mm (1.1 to 2.2 in), females being larger than males. The eyes are bulging and situated high on the head and there is a short tentacle just below each eye. A row of unpigmented dermal tubercles runs along the flank from just behind the eye, and are thought to represent a lateral line organ. The limbs are short and plump, and the fully webbed feet have horny claws. The skin is finely granular. The dorsal surface varies from pale to dark brown and has small grey and black spots. The ventral surface is dull white or yellowish with some dark mottling. [9]

Distribution and habitat

The western clawed frog is an aquatic species and is found in the West African rainforest belt with a range stretching from Senegal to Cameroon and eastern Zaire. It is generally considered a forest-dwelling species and inhabits slow-moving streams, but it is also found in pools and temporary ponds in the northern Guinea and Sudan savannas. [9]

Biology

In the dry season, this frog lives in shallow streams and hides under tree roots, under flat stones, or in holes in the riverbank. It feeds primarily on earthworms, insect larvae and tadpoles. When the rainy season starts it migrates across the forest floor at night to find temporary pools. Spawning may take place in large pools with much vegetation, but tadpoles are also sometimes found in muddy pools with no vegetation. Single eggs may be attached to plants or they may float. The tadpoles have broad mouths and no jaws, but have long tentacles on their upper lips. The ventral fins of their tails are broader than the dorsal ones. Their body colour is generally orange and the tail transparent but in darker locations the tail may be blackish. The tadpoles feed by filtering zooplankton from the water. In large water bodies, they may form dense swarms. Metamorphosis takes place when the tadpoles measure about 5 cm (2 in) in length. [9]

Sex determination

Sex determination in the vast majority of amphibians is controlled by homomorphic (morphologically indistinguishable) sex chromosomes. [10] As a result of this difficulty in sex chromosome identification, only a relatively small proportion of anuran species that have been karyotyped have also had their sex chromosomes identified. [11] Of the species in the genus Xenopus, all have homomorphic sex chromosomes. [11] Additionally, the DM-W gene on the W chromosome in some Xenopus species is the only sex-determining gene that has been identified in amphibians. [11] This DM-W gene was first identified in X. laevis, however it is not found in X. tropicalis. [11] Experimentation involving sex-reversed individuals, gynogenesis, triploids, and conventional crosses, has determined that X. tropicalis has three sex chromosomes: Y, W, and Z. [11] These three sex chromosomes produce three different male genotypes, YW, YZ, and ZZ (all are phenotypically identical) and two different female genotypes, ZW, and WW (all are phenotypically identical). [11] As a result, offspring of X. tropicalis can have sex ratios that differ from the commonly known 1:1 usually found in species with only two different sex chromosomes. For example, offspring resulting from a ZW female and a YZ male will have a sex ratio of 1:3 females to males and offspring resulting from a WW female and a ZZ male will be all female. [11] As a result of this sex determination system, both male and female X. tropicalis can be either heterogametic or homogametic which is extremely rare in nature. [11] The exact genetic mechanism and the exact alleles underlying this system is not yet known. [12] One possible explanation is that the W chromosome contains a female-determining allele that has function that is not found on the Z chromosome while the Y chromosome contains an allele that acts a negative regulator that is dominant over the female-determining allele on the W chromosome. [12]

Although X. tropicalis does have these three sex chromosomes, the frequency of these three sex chromosomes is not evenly distributed among this species' populations throughout its natural range. The Y chromosome has been identified from two localities in Ghana and in a laboratory strain that originated in Nigeria and the Z chromosome has been confirmed to exist in individuals from western and eastern Ghana. [12] Additionally, all three sex chromosomes have been found to exist together in X. tropicalis populations in Ghana and potentially elsewhere in its range as well. [12] Additionally, having irregular sex ratios in offspring is generally thought to be disadvantageous so whether or not the existence of three sex chromosomes in X. tropicalis is evolutionarily stable or an indication that the species is going through a sex chromosome transition (turnover), is still a question. [12] It seems likely that the emergence of the Y chromosome is the most recent event in the evolution of this species' sex chromosomes. [12] It is possible that in the future extinction of the Z chromosome would make it so that the W chromosome transitions into a X chromosome making this species with sex determined by an XY system. [12] It is also possible that if the Y chromosome were to go extinct, this species will have reverted to using an ancestral ZW system. [12]

Status

The IUCN lists the western clawed frog as "Least Concern" because it has a wide distribution and is an adaptable species living in a range of habitats, and the population trend seems to be steady. [1]

Use as a genetic model system

See also Xenopus#Model organism for biological research

Xenopus embryos and eggs are a popular model system for a wide range of biomedical research. [3] [13] This animal is widely used because of its powerful combination of experimental tractability and close evolutionary relationship with humans, at least compared to many model organisms. [13]

Unlike its sister species X. laevis, X. tropicalis is diploid and has a short generation time, facilitating genetic studies. [3] The complete genome of X. tropicalis has been sequenced. [5] This species has n=10 chromosomes. [14]

X. tropicalis has three transferrin genes, all of which are close orthologs of other vertebrates. They are relatively far from non-vertebrate chordates, and widely divergent from protostome orthologs. [15]

Online Model Organism Database

Xenbase [16] is the Model Organism Database (MOD) for both Xenopus laevis and Xenopus tropicalis. [17]

Related Research Articles

<span class="mw-page-title-main">Genetics</span> Science of genes, heredity, and variation in living organisms

Genetics is the study of genes, genetic variation, and heredity in organisms. It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar working in the 19th century in Brno, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.

<span class="mw-page-title-main">Ploidy</span> Number of sets of chromosomes in a cell

Ploidy is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair, which chromosomes naturally exist as. Somatic cells, tissues, and individual organisms can be described according to the number of sets of chromosomes present : monoploid, diploid, triploid, tetraploid, pentaploid, hexaploid, heptaploid or septaploid, etc. The generic term polyploid is often used to describe cells with three or more sets of chromosomes.

<span class="mw-page-title-main">African clawed frog</span> Species of amphibian

The African clawed frog, 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'.

Selfish genetic elements are genetic segments that can enhance their own transmission at the expense of other genes in the genome, even if this has no positive or a net negative effect on organismal fitness. Genomes have traditionally been viewed as cohesive units, with genes acting together to improve the fitness of the organism. However, when genes have some control over their own transmission, the rules can change, and so just like all social groups, genomes are vulnerable to selfish behaviour by their parts.

<span class="mw-page-title-main">Sex-determination system</span> Biological system that determines the development of an organisms sex

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two common sexes and a few less common intersex variations.

<i>Xenopus</i> Genus of amphibians

Xenopus 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.

<span class="mw-page-title-main">Dominance (genetics)</span> One gene variant masking the effect of another in the other copy of the gene

In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second is called recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance, such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes and co-dominance, in which different variants on each chromosome both show their associated traits.

<span class="mw-page-title-main">Y chromosome</span> Sex chromosome in the XY sex-determination system

The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining because it is the presence or absence of Y chromosome that determines the male or female sex of offspring produced in sexual reproduction. In mammals, the Y chromosome contains the SRY gene, which triggers development of male gonads. The Y chromosome is passed only from male parents to male offspring.

<span class="mw-page-title-main">Haldane's rule</span> Observation in evolutionary biology

Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J. B. S. Haldane, that states that if — in a species hybrid — only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex. The heterogametic sex is the one with two different sex chromosomes; in therian mammals, for example, this is the male.

<span class="mw-page-title-main">Sex-chromosome dosage compensation</span>

Dosage compensation is the process by which organisms equalize the expression of genes between members of different biological sexes. Across species, different sexes are often characterized by different types and numbers of sex chromosomes. In order to neutralize the large difference in gene dosage produced by differing numbers of sex chromosomes among the sexes, various evolutionary branches have acquired various methods to equalize gene expression among the sexes. Because sex chromosomes contain different numbers of genes, different species of organisms have developed different mechanisms to cope with this inequality. Replicating the actual gene is impossible; thus organisms instead equalize the expression from each gene. For example, in humans, female (XX) cells randomly silence the transcription of one X chromosome, and transcribe all information from the other, expressed X chromosome. Thus, human females have the same number of expressed X-linked genes per cell as do human males (XY), both sexes having essentially one X chromosome per cell, from which to transcribe and express genes.

<span class="mw-page-title-main">X-inactivation</span> Inactivation of copies of X chromosome

X-inactivation is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome.

<span class="mw-page-title-main">Male</span> Sex of an organism which produces sperm

Male is the sex of an organism that produces the gamete known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilisation. A male organism cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually and asexually. Most male mammals, including male humans, have a Y chromosome, which codes for the production of larger amounts of testosterone to develop male reproductive organs.

Intragenomic conflict refers to the evolutionary phenomenon where genes have phenotypic effects that promote their own transmission in detriment of the transmission of other genes that reside in the same genome. The selfish gene theory postulates that natural selection will increase the frequency of those genes whose phenotypic effects cause their transmission to new organisms, and most genes achieve this by cooperating with other genes in the same genome to build an organism capable of reproducing and/or helping kin to reproduce. The assumption of the prevalence of intragenomic cooperation underlies the organism-centered concept of inclusive fitness. However, conflict among genes in the same genome may arise both in events related to reproduction and altruism.

Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will affect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenotypic expression. More simply, meiotic drive is when one copy of a gene is passed on to offspring more than the expected 50% of the time. According to Buckler et al., "Meiotic drive is the subversion of meiosis so that particular genes are preferentially transmitted to the progeny. Meiotic drive generally causes the preferential segregation of small regions of the genome".

<span class="mw-page-title-main">Animal testing on frogs</span> Overview article

Frogs have been used in animal tests throughout the history of biomedical science.

<span class="mw-page-title-main">ZW sex-determination system</span> Chromosomal system

The ZW sex-determination system is a chromosomal system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects, the schistosome family of flatworms, and some reptiles, e.g. majority of snakes, lacertid lizards and monitors, including Komodo dragons. It is also present in some plants, where it has probably evolved independently on several occasions. The letters Z and W are used to distinguish this system from the XY sex-determination system. In the ZW system, females have a pair of dissimilar ZW chromosomes, and males have two similar ZZ chromosomes.

<span class="mw-page-title-main">Sex chromosome</span> Chromosome that differs from an ordinary autosome in form, size, and behavior

Sex chromosomes are chromosomes that carry the genes that determine the sex of an individual. The human sex chromosomes are a typical pair of mammal allosomes. They differ from autosomes in form, size, and behavior. Whereas autosomes occur in homologous pairs whose members have the same form in a diploid cell, members of an allosome pair may differ from one another.

Xenbase is a Model Organism Database (MOD), providing informatics resources, as well as genomic and biological data on Xenopus frogs. Xenbase has been available since 1999, and covers both X. laevis and X. tropicalis Xenopus varieties. As of 2013 all of its services are running on virtual machines in a private cloud environment, making it one of the first MODs to do so. Other than hosting genomics data and tools, Xenbase supports the Xenopus research community though profiles for researchers and laboratories, and job and events postings.

Sex determination in <i>Silene</i> Sex determination in the flower genus Silene

Silene is a flowering plant genus that has evolved a dioecious reproductive system. This is made possible through heteromorphic sex chromosomes expressed as XY. Silene recently evolved sex chromosomes 5-10 million years ago and are widely used by geneticists and biologists to study the mechanisms of sex determination since they are one of only 39 species across 14 families of angiosperm that possess sex-determining genes. Silene are studied because of their ability to produce offspring with a plethora of reproductive systems. The common inference drawn from such studies is that the sex of the offspring is determined by the Y chromosome.

References

  1. 1 2 IUCN SSC Amphibian Specialist Group (2019). "Xenopus tropicalis". IUCN Red List of Threatened Species . 2019: e.T89256756A107607050. doi: 10.2305/IUCN.UK.2019-2.RLTS.T89256756A107607050.en . Retrieved 11 November 2021.
  2. ↑ Frost, Darrel R. (2014). "Xenopus tropicalis (Gray, 1864)". Amphibian Species of the World: an Online Reference. Version 6.0. American Museum of Natural History. Retrieved 3 March 2015.
  3. 1 2 3 Harland RM, Grainger RM (December 2011). "Xenopus research: metamorphosed by genetics and genomics". Trends in Genetics. 27 (12): 507–15. doi:10.1016/j.tig.2011.08.003. PMC   3601910 . PMID   21963197.
  4. ↑ Amaya E, Offield MF, Grainger RM (July 1998). "Frog genetics: Xenopus tropicalis jumps into the future". Trends in Genetics. 14 (7): 253–5. doi:10.1016/s0168-9525(98)01506-6. PMID   9676522.
  5. 1 2 Hellsten U, Harland RM, Gilchrist MJ, Hendrix D, Jurka J, Kapitonov V, et al. (2010-04-30). "The genome of the Western clawed frog Xenopus tropicalis". Science . 328 (5978): 633–636. Bibcode:2010Sci...328..633H. doi:10.1126/science.1183670. PMC   2994648 . PMID   20431018.
  6. ↑ JGI X. tropicalis v4.1
  7. ↑ Bowes JB, Snyder KA, Segerdell E, Gibb R, Jarabek C, Noumen E, et al. (January 2008). "Xenbase: a Xenopus biology and genomics resource". Nucleic Acids Research. 36 (Database issue): D761-7. doi: 10.1093/nar/gkm826 . PMC   2238855 . PMID   17984085.
  8. ↑ "Bringing Genetics To Xenopus: Half The Genome, Twice As Fast". University of Virginia. Retrieved 2009-10-24.
  9. 1 2 3 M. O. Roedel; Vance Vredenburg; M. J. Mahoney; Tate Tunstall; Kellie Whittaker (2010-05-01). "Xenopus tropicalis". AmphibiaWeb. Retrieved 2013-12-06.
  10. ↑ Bachtrog, Doris; Mank, Judith E.; Peichel, Catherine L.; Kirkpatrick, Mark; Otto, Sarah P.; Ashman, Tia-Lynn; Hahn, Matthew W.; Kitano, Jun; Mayrose, Itay; Ming, Ray; Perrin, Nicolas (July 2014). "Sex determination: why so many ways of doing it?". PLOS Biology. 12 (7): e1001899. doi: 10.1371/journal.pbio.1001899 . ISSN   1545-7885. PMC   4077654 . PMID   24983465.
  11. 1 2 3 4 5 6 7 8 Roco, Álvaro S.; Olmstead, Allen W.; Degitz, Sigmund J.; Amano, Tosikazu; Zimmerman, Lyle B.; Bullejos, Mónica (August 2015). "Coexistence of Y, W, and Z sex chromosomes in Xenopus tropicalis". Proceedings of the National Academy of Sciences of the United States of America. 112 (34): E4752-61. Bibcode:2015PNAS..112E4752R. doi: 10.1073/pnas.1505291112 . ISSN   0027-8424. PMC   4553762 . PMID   26216983.
  12. 1 2 3 4 5 6 7 8 Furman, Benjamin L. S.; Cauret, Caroline M. S.; Knytl, Martin; Song, Xue-Ying; Premachandra, Tharindu; Ofori-Boateng, Caleb; Jordan, Danielle C.; Horb, Marko E.; Evans, Ben J. (2020-11-09). Peichel, Catherine L. (ed.). "A frog with three sex chromosomes that co-mingle together in nature: Xenopus tropicalis has a degenerate W and a Y that evolved from a Z chromosome". PLOS Genetics. 16 (11): e1009121. doi: 10.1371/journal.pgen.1009121 . ISSN   1553-7404. PMC   7652241 . PMID   33166278.
  13. 1 2 Wallingford, J., Liu, K., and Zheng, Y. 2010.Current Biology vol. 20, p. R263–4
  14. ↑ "Xenopus tropicalis (ID 80) - Genome - NCBI". www.ncbi.nlm.nih.gov. Retrieved 2020-12-01.
  15. ↑ Gabaldón, Toni; Koonin, Eugene V. (2013-04-04). "Functional and evolutionary implications of gene orthology". Nature Reviews Genetics . Nature Portfolio. 14 (5): 360–366. doi:10.1038/nrg3456. ISSN   1471-0056. PMC   5877793 . PMID   23552219.
  16. ↑ Karimi K, Fortriede JD, Lotay VS, Burns KA, Wang DZ, Fisher ME, et al. (January 2018). "Xenbase: a genomic, epigenomic and transcriptomic model organism database". Nucleic Acids Research. 46 (D1): D861–D868. doi:10.1093/nar/gkx936. PMC   5753396 . PMID   29059324.
  17. ↑ "Xenopus model organism database". Xenbase.org.
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