Caenorhabditis elegans

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{{Taxobox | color = pink | name = Caenorhabditis elegans | image = enlarged_c_elegans.jpg | image_width = 250px | regnum = Animalia | phylum = Nematoda | classis = Secernentea | ordo = Rhabditida | familia = Rhabditidae | genus = Caenorhabditis | species = C. elegans | binomial = Caenorhabditis elegans | binomial_authority = Maupas, 1900 }}

Caenorhabditis elegans ((IPA: Template:IPA) is a free-living nematode (one of the roundworms), about 1 mm in length, which lives in a temperate soil environment. Research into the molecular and developmental biology of C. elegans began in 1965 by Sydney Brenner.

C. elegans is vermiform, bilaterally symmetric, with a cuticle integument, no segmentations, with four main epidermal cords and a fluid-filled pseudocoelomate cavity. Members of the species have many of the same organ systems as other animals. They feed on microorganisms such as Escherichia coli bacteria. C. elegans has a hermaphrodite sex, and a very rare male population, which makes up 0.05% of the total C. elegans on average. The basic anatomy of C. elegans includues 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.

A basic description of the organisms’ life cycle is that C. elegans eggs are laid by the hermaphrodite. After hatching, they pass through four larval stages (L1-L4). When crowded or in the absence of food, C. elegans can enter an alternative third larval stage called the dauer larva. Dauer larva are stress-resistant and do not age. Hermaphrodites produce sperm during the L4 stage, and lay eggs as adults. The male can inseminate the hermaphrodite, which will use male sperm preferentially. The average life span of the laboratory strain of C. elegans at 20 °C is about 2-3 weeks, and the generation time is only a few days.

C. elegans is used as a model organism. Specimens are cheap and easy to maintain in the laboratory. C. elegans has been especially useful for studying cellular differentiation, and was the first multicellular organism to have its genome completely sequenced. The finished genome 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 and contains more than 19,000 genes. Scientific curators continue to appraise the set of known genes, such that new gene predictions continue to be added and incorrect ones 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.

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).

From a research perspective, C. elegans has the advantage of being a multicellular eukaryotic organism which is simple enough to be studied in great detail. The developmental fate of every single somatic cell (959 in the adult hermaphrodite; 1031 in the adult male) has been mapped out. 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). Image:C elegans stained.jpg In addition, C. elegans is one of the simplest organisms with a nervous system. In the hermaphrodite, this comprises 302 neurons whose pattern of connectivity has been completely mapped out, and proven to be a small-world network. Research has explored the neural mechanisms responsible for several of C. elegans' more interesting behaviors, including chemotaxis, thermotaxis, mechanotransduction, and male mating behavior. Interestingly, the neurons fire no action potentials.

In 2002, the Nobel Prize for 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 (PCD) in C. elegans.

C. elegans made news when it was discovered that specimens had survived the Space Shuttle Columbia's disintegration in February, 2003.

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References

(2005) The Evolution of Self-Fertile Hermaphroditism: The Fog Is Clearing. PLoS Biol. 3(1): e30.
Riddle, D.L., Blumenthal, T, Meyer, R. J., and Priess, J.R., 1997. C. elegans II. Cold Spring Harbor Laboratory Press, New York, pp 1-4, 679-683.
Hope, I.A. 1999. C. elegans: A practical approach. Oxford University Press, New York, pp 1-6.
Bird, A.F, and Bird, J., 1991. The Structure of Nematodes. Academic Press, Inc., San Diego, pp 1, 69-70, 152-153, 165, 224-225.
Avery, Leon. September 19, 2003. Caenorhabditis elegans WWW Server
The C. elegans Sequencing Consortium, 1998. Genome sequence of the nematode C. elegans: a platform for investigating biology. Science 282: 2012-2018.
Stein, L.D. et al., 2003. The Genome Sequence of Caenorhabditis briggsae': A Platform for Comparative Genomics. PLoS Biol. 1: 166-192.
Brenner, S., 1974. The Genetics of Caenorhabditis elegans. Genetics 77: 71-94.
Watts DJ, Strogatz SH. Collective dynamics of 'small-world' networks. Nature. 1998 Jun 4;393(6684):440-2.