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Caenorhabditis elegans

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iCaenorhabditis elegans
Image:Adult worm.jpg
Scientific classification
Kingdom: Animalia
Phylum: Nematoda
Class: Secernentea
Order: Rhabditida
Family: Rhabditidae
Genus: Caenorhabditis
Species: elegans
Binomial name
Caenorhabditis elegans
Maupas, 1900

Caenorhabditis elegans (IPA: [ˌsiːnəʊræbˈdaɪtɪs ˈelegænz]) is a free-living nematode (roundworm), about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner <ref>Brenner, S. (1974). The Genetics of Caenorhabditis elegans. Genetics 77: 71–94.</ref> and it has since been used extensively as a model organism.

Contents

[edit] Biology

C. elegans is unsegmented, vermiform, 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 vegetal matter. 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.

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 state. Dauer larvae are stress-resistant and do not age. Hermaphrodites produce sperm during the L4 stage, and lay eggs as adults <ref>Nayak, S., J. Goree & T. Schedl (2004). fog-2 and the Evolution of Self-Fertile Hermaphroditism in Caenorhabditis. PLoS Biology 3 (1): e6. DOI: 10.1371/journal.pbio.0030006.</ref>. The male can inseminate the hermaphrodite, which will use male sperm preferentially. 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. Hermaphrodites can mate with others or self-fertilize.

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).

[edit] C. elegans as a model organism

C. elegans is used as a model organism for a variety of reasons, including economy, and the ease of maintaining a population in the laboratory. Worms can be frozen and subsequently thawed and remain viable, thus ensuring easy long-term storage of different worm strains. Because the complete cell lineage of the species has been determined, C. elegans has proven especially useful for studying cellular differentiation.

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 shown to be a small-world network <ref>Watts D. J. & S. H. Strogatz (1998). Collective dynamics of 'small-world' networks. Nature 393 (6684): 440–442.</ref>. 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. Interestingly, the neurons fire no action potentials.

The organism has also been identified as a model for nicotine dependence as it has been found to experience the same symptoms humans experience when they quit smoking. <ref>Feng et al. (2006). A C. elegans Model of Nicotine-Dependent Behavior: Regulation by TRP-Family Channels. Cell 127: 621-633.</ref>

As for most model organisms, there is a dedicated online database for the species that 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.

[edit] The Genome

C. elegans was the first multicellular organism to have its genome completely sequenced. The finished genome sequence was published in 1998,<ref>The C. elegans Sequencing Consortium (1998). Genome sequence of the nematode C. elegans: a platform for investigating biology. Science 282: 2012–2018.</ref> 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 approximately 20,000 genes. The vast majority of these genes encode for proteins but there are likely to be as many as 1,000 RNA genes. 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 <ref>Stein, L. D. et al. (2003). The Genome Sequence of Caenorhabditis briggsae: A Platform for Comparative Genomics. PLoS Biology 1: 166–192.</ref>. Work is now ongoing to determine the genome sequences of more nematodes from the same genus such as C. remanei [1] and C. japonica [2]. These newer genome sequences are being determined by using the whole genome shotgun technique which means that the resulting genome sequences are likely to not be as complete or accurate as C. elegans (which was sequenced using the 'hierarchical' or clone-by-clone appoach).

[edit] C. elegans scientists

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. 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.<ref>A. Fire, S.Q. Xu, M.K. Montgomery, S.A. Kostas, S. E. Driver, C.C. Mello: Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. In: Nature. 391/1998, S. 806-811, ISSN 0028-0836</ref>

Because all research into C. elegans essentially started with Sydney Brenner in the 1970's, many scientists working in this field share a close connection to Brenner (they 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.

[edit] C. elegans in the media

C. elegans made news when it was discovered that specimens had survived the Space Shuttle Columbia disaster in February 2003.<ref>"Worms survived Columbia disaster", BBC News, 2003-05-01.</ref>

[edit] References

<references/>

[edit] Relevant publications

  • Riddle, D.L., T. Blumenthal, R. J. Meyer & J. R. Priess (1997). C. elegans II. Cold Spring Harbor Laboratory Press, New York, pp 1-4, 679–683.
  • Bird, A. F & J. Bird (1991). The Structure of Nematodes. Academic Press, Inc., San Diego, pp 1, 69–70, 152–153, 165, 224–225.
  • Hope, I. A. (1999).  C. elegans: A practical approach. Oxford University Press, New York, pp 1–6.

[edit] Online resources

[edit] External link

Major Model Organisms in Genetic Studies
Lambda phage | E. coli | Chlamydomonas | Tetrahymena | Budding yeast | Fission yeast | Neurospora | Maize | Arabidopsis | C. elegans | Drosophila | Zebrafish | Rat | Mouse
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