Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) is a free-living nematode (a roundworm), 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, bilateral in symmetry, with a cuticle integument, no segmentations, with four main epidermal cords and a fluid-filled pseudocoelomate cavity. Members of the species also have an organ system and a closed circulatory system. They feed on microorganisms such as Escherichia coli bacteria. C. elegans has a male and hermaphrodite sex. 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 dauer. Dauers 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.

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 all of its 959 somatic cells has been mapped out. (There are originally 1090 cells but 131, of which most are neurons, are eliminated by apoptosis.)

In addition, C. elegans is one of the simplest organisms with a nervous system. It comprises of 302 neurons and has been completely mapped out. Research has explored the neural mechanisms responsible for two of C. elegans' more interesting behaviors: chemotaxis and thermotaxis. In addition, recent research at the University of Toronto has shown that the worm appears to be capable of a simple form of associative learning, with recall being dependent on the environmental context. This organism is the subject of a proposal which involves cataloguing its glycome.

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 development and programmed cell death (PCD) in C. elegans. There are 3 genes involved in the PCD of the 131 cells previously introduced: ced-3, ced-4 and ced-9. Both ced-3 and ced-4 are actively bringing death to these cells since a knockout of these genes prevents the PCD of the cells. On the other hand, ced-9 is a pro-survival gene because it prevents the stimulation of the activation of ced-3 by ced-4.

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

Loss of function mutations in daf-2, the C. elegans insulin-like receptor gene, and clk-1 a gene required for ubiquinone biosynthesis, have been shown to significantly extend the lifespan of the worm. These genes are believed to be involved in regulating resistance to oxidative stress.

References

1. (2005) The Evolution of Self-Fertile Hermaphroditism: The Fog Is Clearing. (http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371%2Fjournal.pbio.0030030) PLoS Biol. 3(1): e30.
2. Riddle, D.L., Blumenthal, T, Meyer, R. J., and Priess, J.R., 1997. C. Elegans II. (http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=ce2.TOC) Cold Spring Harbor Laboratory Press, New York, pp 1-4, 679-683.
3. Hope, I.A. 1999. C. elegans: A practical approach. Oxford University Press, New York, pp 1-6.
4. 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.
5. Avery, Leon. September 19, 2003. Caenorhabditis elegans WWW Server (http://elegans.swmed.edu/)
6. Ferree, T. C., Marcotte, B. A. and Lockery, S. R., 1997. "Neural network models of chemotaxis in the nematode C. elegans." (http://www.csi.uoregon.edu/projects/celegans/pubs/NIPS1997.pdf) Advances in Neural Information Processing Systems 9:55-61. MIT Press.
7. The C. elegans Sequencing Consortium, 1998. Genome sequence of the nematode C. elegans: a platform for investigating biology. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9851916) Science 282: 2012-2018.
8. Stein, L.D. et al., 2003. The Genome Sequence of Caenorhabditis briggsae': A Platform for Comparative Genomics. (http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0000045) PLoS Biol. 1: 166-192.
9. Brenner, S., 1974. The Genetics of Caenorhabditis elegans. (http://dev.wormbase.org/papers/31_Brenner74.pdf) Genetics 77: 71-94.

External links

• WormBase (http://www.wormbase.org)
• BBC news item about C. elegans specimens surviving the Columbia space shuttle's disintegration (http://news.bbc.co.uk/2/hi/science/nature/2992123.stm).
• Eureka Alert article about context-dependent recall of associative learning in C. elegans (http://www.eurekalert.org/pub_releases/2004-07/uot-eil072704.php)
• November 3, 2003. “Classifmnu.” (http://plpnemweb.ucdavis.edu/nemaplex/Taxadata/Secernea.htm)
• Dept. of Medical Oncology. Online Medical Dictionary (http://cancerweb.ncl.ac.uk/cgi-bin/omd?query=rhabditoidea&action=Search+OMD)
• Undergraduate honor's thesis: a computational model of thermotaxis (http://www.seeelegance.com/Coleman%20-%20Computational%20Models%20of%20Thermotaxis.pdf)
• Wormbase, an online database for C. elegans genes and proteins (http://www.wormbase.org)
• Wormatlas, an online database for behavioral and structural anatomy of C. elegans (http://www.wormatlas.org)
• C. elegans book (http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=ce2.TOC)
• Wellcome Trust Sanger Institute C. elegans page (http://www.sanger.ac.uk/Projects/C_elegans)
• Worm Classroom Education Portal (http://www.wormclassroom.org)de:Caenorhabditis elegans

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