Caenorhabditis elegans

Caenorhabditis elegans (pronounced /ˌsiːnɵræbˈdaɪtɨs ˈɛlɨɡænz/) is a free-living, transparent 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 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 kind of 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 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. elgans is ameboid, lacking flagella and acrosomes.
Longitudinal section through the hermaphrodite C. elegans

Genome

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 and contains approximately 20,100 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 gene,and the genome is now believed to contain more than 16,000 RNA genes, up from as little 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 and when 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 WS169 release of WormBase (December 2006) lists a net gain of 6 base pairs to the genome sequence.Occasionally more extensive changes are made, as in the WS159 release of May 2006, which added over 300 bp to the sequence