About C. elegans
About C. elegans
Caenorhabditis elegans (pronunciation: /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 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. 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. elgans, 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. Nematodes are capable of surviving desiccation, and in C. elegans the mechanism for this capability has been demonstrated to be Late Embryogenesis Abundant (LEA) proteins.
In 1963 Sydney Brenner proposed using Caenorhabditis 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. C. elegans is studied as a model organism for a variety of reasons. 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. In addition, 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 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). This aspect has been thoroughly studied in this organism, specifically because of this "apoptotic predictability", which has contributed to the elucidation of some apoptotic genes, mainly through observation of abnormal, apoptosis-surviving nematodes. Wild-type C. elegans hermaphrodite stained with the fluorescent dye Texas Red to highlight the nuclei of all cells 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, 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 of C. elegans is that it is relatively straightforward 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 that the researcher wishes to disable. Alternatively, worms can be fed on genetically transformed bacteria which express the double stranded RNA of interest. C. elegans has also been useful in the study of meiosis. As sperm and egg nuclei move down the length of the gonad, they undergo a temporal progression through meiotic events. This progression means that 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. The organism has also been identified as a model for 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, 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. 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.
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 adult hermaphrodite has 959 somatic nuclei. Its gene density is about 1 gene/5kb. Introns are 26% of the genome. There are some large intergenic regions containing 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 C. elegans genome sequence is approximately 100 million base pairs long. The genome consists of 6 chromosomes (named I, II, II, IV, V and X) and a mitochondrion. 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 gene, 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.