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Caenorhabditis elegans

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Caenorhabditis elegans
Adult Caenorhabditis elegans.jpg
An adult hermaphrodite C. elegans worm
Scientific classification
Kingdom: Animalia
Phylum: Nematoda
Class: Chromadorea
Order: Rhabditida
Family: Rhabditidae
Genus: Caenorhabditis
Species: C. elegans
Binomial name
Caenorhabditis elegans
(Maupas, 1900)[1]

Caenorhabditis elegans var. bergerac[2]

Caenorhabditis elegans (/ˌsnræbˈdtəs ˈɛləɡænz/[3]) is a free-living (not parasitic), transparent nematode (roundworm), about 1 mm in length,[4] that lives in temperate soil environments. The name is a blend of the Greek caeno- (recent), rhabditis (rod-like)[5] and Latin elegans (elegant). In 1900, Maupas initially named it Rhabditides elegans, Osche placed it in the subgenus Caenorhabditis in 1952, and in 1955, Dougherty raised it to the status of genus.[6]

C. elegans is an unsegmented pseudocoelomate, and lacks a respiratory and a circulatory system. It possesses gut granules which emit a brilliant blue fluorescence, a wave of which is seen at death in a 'death fluorescence'.[7] The majority of these nematodes are hermaphrodites. Males have specialised tails for mating that include spicules.


C. elegans was the first multicellular organism to have its whole genome sequenced, and as of 2012, the only organism to have its connectome (neuronal "wiring diagram") completed.[9][10][11]


Movement of wild-type C. elegans


The pharynx is a muscular food pump in the head of C. elegans, which is triangular in cross-section. This grinds food and transports it directly to the intestine. A set of "valve cells" connects the pharynx to the intestine, but how this valve operates is not understood. After digestion, the contents of the intestine are released via the rectum, as is the case with all other nematodes. No direct connection exists between the pharynx and the excretory canal, which functions in the release of liquid urine.




Reproduction and development[edit]

Anatomical diagram of a male C. elegans
Once he recognizes a hermaphrodite worm, the male nematode begins tracing the hermaphrodite with his tail until he reaches the vulval region. The male then probes the region with his spicules to locate the vulva, inserts them, and releases sperm.[14]







Wild isolates of Caenorhabditis elegans are regularly found with infections by Microsporidia fungi. One such species, Nematocida parisii, replicates in the intestines of C. elegans.[25]

Research use[edit]


In 1963, Sydney Brenner proposed using C. elegans as a model organism for the investigation primarily of neural development in animals. It is one of the simplest organisms with a nervous system. In the hermaphrodite, this system comprises 302 neurons[26] the pattern of which has been comprehensively mapped, in what is known as a connectome, and shown to be a small-world network.[27] Research has explored the neural and molecular mechanisms that control several behaviors of C. elegans, including chemotaxis, thermotaxis, mechanotransduction, learning, memory, and mating behaviour.[28] Brenner also chose it as it is easy to grow in bulk populations, and convenient for genetic analysis.[29] It is a multicellular eukaryotic organism, yet is simple enough to be studied in great detail. The transparency of C. elegans facilitates the study of cellular differentiation and other developmental processes in the intact organism. The spicules in the male clearly distinguish males from females. Strains are cheap to breed and can be frozen. When subsequently thawed, they remain viable, allowing long-term storage.[30]

Notable findings[edit]

The developmental fate of every single somatic cell (959 in the adult hermaphrodite; 1031 in the adult male) has been mapped.[31][32] These patterns of cell lineage are largely invariant between individuals, whereas in mammals, cell development is more dependent on cellular cues from the embryo. The first cell divisions of early embryogenesis in C. elegans are among the best understood examples of asymmetric cell divisions.[33]




Environmental RNAi uptake is much worse in other species of worms in the Caenorhabditis genus. Although injecting RNA into the body cavity of the animal induces gene silencing in most species, only C. elegans and a few other distantly related nematodes can take up RNA from the bacteria they eat for RNAi.[37] This ability has been mapped down to a single gene, sid-2, which, when inserted as a transgene in other species, allows them to take up RNA for RNAi as C. elegans does.[38]

Research into meiosis has been considerably simplified since every germ cell nucleus is at the same given position as it moves down the gonad, so is at the same stage in meiosis. In an early phase of meiosis, the oocytes become extremely resistant to radiation and this resistance depends on expression of genes rad51 and atm that have key roles in recombinational repair.[39][40] Gene mre-11 also plays a crucial role in recombinational repair of DNA damage during meiosis.[41] A study of the frequency of outcrossing in natural populations showed that selfing is the predominant mode of reproduction in C. elegans, but that infrequent outcrossing events occur at a rate around 1%.[42] Meioses that result in selfing are unlikely to contribute significantly to beneficial genetic variability, but these meioses may provide the adaptive benefit of recombinational repair of DNA damages that arise, especially under stressful conditions.[43]


As for most model organisms, scientists that work in the field curate a dedicated online database and the WormBase is that for C. elegans. The WormBase attempts to collate all published information on C. elegans and other related nematodes. Their website has advertised a reward of $4000 for the finder of a new species of closely related nematode.[45] Such a discovery would broaden research opportunities with the worm.[46]

C. elegans has been a model organism for research into ageing; for example, the inhibition of an insulin-like growth factor signaling pathway has been shown to increase adult lifespan threefold.[47] Moreover, extensive research on C. elegans has identified RNA-binding proteins as essential factors during germline and early embryonic development.[48]


Spaceflight research[edit]

C. elegans made news when specimens were discovered to have survived the Space Shuttle Columbia disaster in February 2003.[50] Later, in January 2009, live samples of C. elegans from the University of Nottingham were announced to be spending two weeks on the International Space Station that October, in a space research project to explore the effects of zero gravity on muscle development and physiology. The research was primarily about genetic basis of muscle atrophy, which relates to spaceflight or being bed-ridden, geriatric, or diabetic.[51] Descendants of the worms aboard Columbia in 2003 were launched into space on Endeavour for the STS-134 mission.[52]


C. elegans hermaphrodite

C. elegans was the first multicellular organism to have its whole genome sequenced. The sequence was published in 1998,[53] although some small gaps were present; the last gap was finished by October 2002.

Size and gene content. The C. elegans genome is about 100 million base pairs long and consists of six chromosomes and a mitochondrial genome. Its gene density is about one gene per five kilo-base pairs. Introns make up 26% and intergenic regions 47% of the genome. Many genes are arranged in clusters and how many of these are operons is unclear.[54] C. elegans and other nematodes are among the few eukaryotes currently known to have operons; these include trypanosomes, flatworms (notably the trematode Schistosoma mansoni), and a primitive chordate tunicate Oikopleura dioica. Many more organisms are likely to be shown to have these operons.[55]

Protein-coding genes. The genome contains an estimated 20,470 protein-coding genes.[56] About 35% of C. elegans genes have human homologs. Remarkably, human genes have been shown repeatedly to replace their C. elegans homologs when introduced into C. elegans. Conversely, many C. elegans genes can function similarly to mammalian genes.[17] The number of known RNA genes in the genome has increased greatly due to the 2006 discovery of a new class of 21U-RNA genes,[57] and the genome is now believed to contain more than 16,000 RNA genes, up from as few as 1,300 in 2005.[58] Scientific curators continue to appraise the set of known genes; new gene models continue to be added and incorrect ones modified or removed.

Related genomes. In 2003, the genome sequence of the related nematode C. briggsae was also determined, allowing researchers to study the comparative genomics of these two organisms.[59] The genome sequences of more nematodes from the same genus e.g., C. remanei,[60] C. japonica[61] and C. brenneri (named after Brenner), have also been studied using the shotgun sequencing technique.[62] These sequences have now been completed.[63][64]

The reference C. elegans genome sequence continues to change as new evidence reveals errors in the original sequencing. Most changes are minor, adding or removing only a few base pairs of DNA. For example, the WS202 release of WormBase (April 2009) added two base pairs to the genome sequence.[65] Sometimes, more extensive changes are made as noted in the WS197 release of December 2008, which added a region of over 4,300 bp to the sequence.[66][67]

Scientific community[edit]

In 2002, the Nobel Prize in Physiology or Medicine was awarded to Sydney Brenner, H. Robert Horvitz, and John Sulston for their work on the genetics of organ development and programmed cell death in C. elegans. The 2006 Nobel Prize in Physiology or Medicine was awarded to Andrew Fire and Craig C. Mello for their discovery of RNA interference in C. elegans.[68] In 2008, Martin Chalfie shared a Nobel Prize in Chemistry for his work on green fluorescent protein; some of the research involved the use of C. elegans.


See also[edit]


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Further reading[edit]

External links[edit]