Caenorhabditis elegans is a small (~1mm long) nematode worm (roundworm) in the family Rhabditidae. It is cosmopolitan in distribution, with reproductive stages found most reliably in rotting fruits and surrounding soil. When food is plentiful, a fertilized egg completes embryogenesis, passes through four larval stages, and attains reproductive maturity in only three days at room temperature. As with most terrestrial nematodes, under stressful conditions an alternative third larval stage specialized for dispersal, the dauer larva, may be formed. In soil, C. elegans is often found in the dauer form. The reproductive mode of C. elegans involves a mixture of self-fertile hermaphrodites and males (this system was derived relatively recently from an ancestral male/female system). The transparency, anatomical simplicity, rapid development, and mix of outcrossing and selfing in C. elegans led American nematologist Ellsworth Dougherty and British molecular geneticist Sydney Brenner to champion this species as a model organism for basic biological research beginning in the 1970s. By the early 1980's, Brenner and colleagues had carried out pioneering studies investigating the invariant cell lineages, neuroanatomy, and aspects of the genome of C. elegans. This rich body of work garnered the attention of many more researchers and quickly led to C. elegans becoming one of the most widely studied laboratory organisms in the fields of genetics, cell biology, development, aging, evolution, and neuroscience. In 1998, C. elegans became the first animal to have its entire genome sequence determined and it remains at the forefront of functional genomics.
Caenorhabditis elegans have elongated cylindrical bodies, tapered at both ends, with smooth skin, no segmentation, and no appendages. Adults grow to approximately 1mm in length. Exactly 959 cells compose Caenorhabditis elegans, and their bodies are transparent; therefore, individual cells are easily observed with a microscope (Edgley 1999).
Caenorhabditis elegans are terrestrial organisms. They live primarily in soil (Lee & Atkinson 1977). The soil must have a constant level of moisture, so that the worm can move in the film of water and draw water from the soil. The soil must also have a moderate oxygen content. Worms may not be able to penetrate soils with high clay content. For ideal movement, the worm should be about three times as long as the diameter of the soil particles ( Nicholas 1975). Worms are also found in or on rotting vegetation above ground (Edgley 1999).
Life History and Behavior
Status: captivity: 0.16 years.
Lifespan, longevity, and ageing
Caenorhabditis elegans have two naturally occurring sexes, a male and a self-fertilizing hermaphrodite; females do not naturally occur. The majority of individuals are hermaphrodites; males usually comprise no more than 0.20% of the natural population. The number of males can be increased, however, by raising the temperature at the onset of sexual maturity (Nicholas 1975). Hermaphrodites are protandrous; the individuals produce sperm first and then produce eggs (Blaxter 1999). Most commonly, worms will simply fertilize their own eggs (Bird 1991). However, the males that do exist copulate with hermaphrodites, thus mixing up the gene pool in the population (Nicholas 1975). Eggs are laid within two to three hours of fertilization and hatch approximately twelve hours later. The worms develop into adults in four larval stages; this generally takes about three days when the temperature ranges from 20 to 25 degrees Celsius (Blaxter 1999). Temperature plays a major role in the development of Caenorhabditis elegans. The worms' average lifespan is two to three weeks (Edgley 1999).
Average age at sexual or reproductive maturity (male)
Sex: male: 3 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 3 days.
Molecular Biology and Genetics
Barcode data: Caenorhabditis elegans
There are 18 barcode sequences available from BOLD and GenBank. Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species. See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
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Download FASTA File
Statistics of barcoding coverage: Caenorhabditis elegans
Public Records: 18
Specimens with Barcodes: 18
Species With Barcodes: 1
Relevance to Humans and Ecosystems
Economic Importance for Humans: Negative
This species has no known negative impact on humans.
Caenorhabditis elegans (pron.: / /) is a free-living, transparent nematode (roundworm), about 1 mm in length, which lives in temperate soil environments. The name is a blend of Greek (Caeno = recent + rhabditis = rod-like) and Latin (elegans = elegant). It was initially named Rhabditis elegans by Maupas (1900). It was then placed in the subgenus Caenorhabditis by Osche (1952) and then raised to generic status by Dougherty (1955). Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model organism.
C. elegans is unsegmented, vermiform, and bilaterally symmetrical, with a cuticle integument, four main epidermal cords and a fluid-filled pseudocoelomate cavity. Members of the species have many of the same organ systems as other animals. In the wild, they feed on bacteria that develop on decaying vegetable matter. C. elegans has two sexes: hermaphrodites and males. Individuals are almost all hermaphrodite, with males comprising just 0.05% of the total population on average. The basic anatomy of C. elegans includes a mouth, pharynx, intestine, gonad, and collagenous cuticle. Males have a single-lobed gonad, vas deferens, and a tail specialized for mating. Hermaphrodites have two ovaries, oviducts, spermatheca, and a single uterus.
C. elegans eggs are laid by the hermaphrodite. After hatching, they pass through four juvenile stages (L1–L4). When crowded or in the absence of food, C. elegans can enter an alternative third larval stage called the dauer state. Dauer larvae are stress-resistant and do not age. Hermaphrodites produce all their sperm in the L4 stage (150 sperm per gonadal arm) and then switch over to producing oocytes. The sperm are stored in the same area of the gonad as the oocytes until the first oocyte pushes the sperm into the spermatheca (a chamber where the oocytes become fertilized by the sperm). The male can inseminate the hermaphrodite, which will use male sperm preferentially (both types of sperm are stored in the spermatheca). When self-inseminated, the wild-type worm will lay approximately 300 eggs. When inseminated by a male, the number of progeny can exceed 1,000. At 20 °C, the laboratory strain of C. elegans has an average life span of approximately 2–3 weeks and a generation time of approximately 4 days.
C. elegans has been a model organism for aging research; for instance, inhibition of an insulin/insulin-like growth factor (IGF) signaling pathway has been shown to increase adult lifespan 3-fold. Moreover, extensive research on the nematode C. elegans has identified RNA-binding protein as essential factors during germline and early embryonic development.
C. elegans has five pairs of autosomes and one pair of sex chromosomes. Sex in C. elegans is based on an X0 sex-determination system. Hermaphrodite C. elegans have a matched pair of sex chromosomes (XX); the rare males have only one sex chromosome (X0). The sperm of C. elegans is ameboid, lacking flagella and acrosomes.
C. elegans is notable in animal sleep studies as the most primitive organism in which sleep-like states have been observed. In C. elegans, a lethargus phase occurs in short periods preceding each moult.
The different Caenorhabditis species occupy various nutrient- and bacteria-rich environments. They do not form self-sustaining populations in soil, as it lacks enough organic matter. C. elegans can survive on a diet of a variety of kinds of bacteria (not all bacteria, though), but its wild ecology is largely unknown. Most laboratory strains were found in human-made environments such as gardens and compost piles. Recently, however, C. elegans has been found to be abundant in rotting organic matter, particularly rotting fruit. Dauer larvae can be transported by invertebrates including millipedes, insects, isopods, and gastropods. When they reach a desirable location they then get off, and at least in the lab they will also feed on the host if it dies.
In 1963, Sydney Brenner proposed using C. elegans as a model organism for the investigation of animal development including neural development. Brenner chose it mainly because it is simple, easy to grow in bulk populations, and convenient for genetic analysis. It is a multicellular eukaryotic organism that is simple enough to be studied in great detail. Strains are cheap to breed and can be frozen. When subsequently thawed, they remain viable, allowing long-term storage. Other desirable properties are:
C. elegans is transparent, facilitating the study of cellular differentiation and other developmental processes in the intact organism. The males can be easily distinguished from hermaphrodites based on the morphology of the tail region.
The developmental fate of every single somatic cell (959 in the adult hermaphrodite; 1031 in the adult male) has been mapped. These patterns of cell lineage are largely invariant between individuals, in contrast to mammals, where cell development from the embryo is more largely dependent on cellular cues.
In both sexes, a large number of additional cells (131 in the hermaphrodite, most of which would otherwise become neurons), are eliminated by programmed cell death (apoptosis). Because of this "apoptotic predictability", it has contributed to the elucidation of some apoptotic genes, mainly through observation of abnormal, apoptosis-surviving nematodes.
It is one of the simplest organisms with a nervous system. In the hermaphrodite, this comprises 302 neurons whose pattern of connectivity, or "connectome", has been completely mapped and shown to be a small-world network. Research has explored the neural mechanisms responsible for several of the more interesting behaviors shown by C. elegans, including chemotaxis, thermotaxis, mechanotransduction, and male mating behavior.
A useful feature is a relatively straightforward method to disrupt the function of specific genes by RNA interference (RNAi). Silencing the function of a gene in this way can sometimes allow a researcher to infer what the function of that gene may be. The nematode can either be soaked in or injected with a solution of double-stranded RNA, the sequence of which is complementary to the sequence of the gene the researcher wishes to disable. Alternatively, worms can be fed genetically transformed bacteria that express the double-stranded RNA of interest.
The study of meiosis is simplified considerably with this organism. As sperm and egg nuclei move down the length of the gonad, they undergo a temporal progression through meiotic events. This progression means every nucleus at a given position in the gonad will be at roughly the same step in meiosis, eliminating the difficulties of heterogeneous populations of cells.
It can also be used in the study of nicotine dependence, as it has been found to exhibit behavioral responses to nicotine that parallel those observed in mammals, including acute response, tolerance, withdrawal, and sensitization.
As for most model organisms, a dedicated online database for the species is actively curated by scientists working in this field. The WormBase database attempts to collate all published information on C. elegans and other related nematodes. A reward of $4000 has been advertised on their website, for the finder of a new species of closely related nematode. Such a discovery would broaden research opportunities with the worm.
A 15% glycerol solution is used for the freezing of C. elegans. Samples are cooled at 1°C per minute. Freshly starved young larvae survive freezing best. About 35% to 45% of the worms stored in liquid nitrogen survive. The worms can also be stored at −80°C for over ten years, but survival is not as great as for worms stored at −196°C, the temperature of liquid nitrogen.
Space travel research
C. elegans made news when it was discovered that specimens had survived the Space Shuttle Columbia disaster in February 2003. Later, in January 2009, it was announced that live samples of C. elegans from the University of Nottingham would spend two weeks on the International Space Station that October as part of a project to explore the effects of zero gravity on muscle development and its physiology. The emphasis of the research was to be on the genetic basis of muscle atrophy. This has relevance to space travel, but also to individuals who are bed-ridden, geriatric or diabetic. Descendants of the worms aboard Columbia in 2003 were launched into space on Endeavour for the STS-134 mission.
C. elegans was the first multicellular organism to have its genome completely sequenced. The sequence was published in 1998, although a number of small gaps were present; the last gap was finished by October 2002. The C. elegans genome sequence is approximately 100 million base pairs long. The genome consists of six chromosomes (named I, II, III, IV, V and X) and a mitochondrial genome. Its gene density is about 1 gene/5kb. Introns, or non-expressed sequences, are 26% of the genome. Some large intergenic regions contain repetitive DNA sequences. Many genes are arranged in operons: polycistronic series that are transcribed together. C. elegans and other nematodes are among the few eukaryotes currently known to have operons.
The genome contains approximately 20,470 protein-coding genes. 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, and the genome is now believed to contain more than 16,000 RNA genes, up from as few as 1,300 in 2005. Scientific curators continue to appraise the set of known genes, such that new gene predictions continue to be added and incorrect ones modified or removed.
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. Work is now ongoing to determine the genome sequences of more nematodes from the same genus, such as C. remanei, C. japonica and C. brenneri. These newer genome sequences are being determined using the whole genome shotgun technique, which means they are likely to be less complete and less accurate than that of C. elegans, which was sequenced using the "hierarchical" or clone-by-clone approach.
The official version of the C. elegans genome sequence continues to change as new evidence reveals errors in the original sequencing; DNA sequencing is not an error-free process. Most changes are minor, adding or removing only a few base pairs (bp) of DNA. For example, the WS202 release of WormBase (April 2009) added two base pairs to the genome sequence. Occasionally, more extensive changes are made, as in the WS197 release of December 2008, which added a region of over 4,600 bp to the sequence.
A small number of conserved protein sequences in the far more distantly related sponges have been shown to be more similar to humans than to C. elegans. This suggests an accelerated rate of evolution has occurred in the C. elegans lineage. The same study found that several phylogenetically ancient genes are not present in C. elegans.
RNA interference (RNAi) has been used extensively in C. elegans because it can be done by simply feeding the worms transgenic bacteria expressing double stranded RNA complementary to the gene of interest. Their relative simplicity makes gene loss-of-function experiments in C. elegans the easiest of all animal models, and thus, scientists have been able to knock down 86% of the ~20,000 genes in the worm, establishing a functional role for 9% of the genome.
Incidentally, environmental RNAi uptake does not work nearly as well in other species of worm in the Caenorhabditis genus. Although injecting RNA into the body cavity of the animal induces silencing in most species, only C. elegans and a few other distantly related nematodes can uptake RNA from the bacteria they eat for RNAi. This ability has been mapped down to a single gene, sid-2, which when inserted as a transgene in other species, allows them to uptake RNA for RNAi the way C. elegans does.
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. In 2008, Martin Chalfie shared a Nobel Prize in Chemistry for his work on green fluorescent protein in C. elegans.
Because all research into C. elegans essentially started with Sydney Brenner in the 1970s, many scientists working in this field share a close connection to Brenner, having either worked as a post-doctoral or post-graduate researcher in Brenner's lab or in the lab of someone who previously worked with Brenner. Because most people who worked in his lab went on to establish their own worm research labs, there is now a fairly well documented "lineage" of C. elegans scientists. This lineage was recorded in some detail at the 2003 International Worm Meeting and the results were stored in the WormBase database.
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