Caenorhabditis Elegans


C. elegans is a vermiform and bilaterally symmetrical nematode with a cuticular integument, four main epidermal muscle cords and a fluid-filled pseudocoelom. In the wild, they feed on bacteria growing on decaying vegetable matter. They are generally hermaphrodite but occasionally males also occur. In 1960’s Sydney Brenner began using it for the study of genetics of development and neurobiology.

BIOLOGY

C. elegans eggs are laid by the hermaphrodite forms. After hatching, they larvae pass through 4 larval stages. 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 sperms in the L4 stage (150 sperms per gonadal arm) and then switch over to producing oocytes. The sperms are stored in the same area of the gonad as the oocytes until the first oocyte pushes the sperm into the spermatheca. The male can inseminate a hermaphrodite, which prefers to use male sperms (both types of sperms 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. Hermaphrodites can mate with males 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).

SPECIAL FEATURES

Strains are cheap to breed and can be frozen. When subsequently thawed they remain viable, allowing long-term storage.

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 that is simple enough to be studied in great detail. The developmental fate of every single somatic cell has been mapped out. In both sexes, a large number of additional cells (131 in the hermaphrodite) are eliminated by programmed cell death (apoptosis).

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.

A useful feature of C. elegans is that it is relatively easy to disrupt the function of specific genes by RNA interference (RNAi). Gene silencing in this way can sometimes allow a researcher to infer what the function of that gene may be. The nematode can be soaked in 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

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.

Caenorhabditiselegans is a microscopic (1 mm) nematode that normally lives in soil. It has become one of the “model” organisms in biology because:

  • It is eukaryotic animal with physiological systems, namely, feeding, nervous, muscle, reproductive etc. found in higher animals.
  • Large numbers of nematodes can be raised in petri dishes by feeding on E. coli which is another model organism.
  • It reproduces rapidly because of the generation time of only 4 days.
  • It is transparent and hence cells in the living animal can be seen under the microscope from the fertilized egg to the 556 cells of the newly-hatched larva and later the 959 somatic cells and a variable number of germ cells of the adult worm.
  • It can be easily transformed with transgenes by injecting DNA into the animal.
  • It can also be treated with antisense RNA.
  • It shows signs of aging before death and thus may provide general clues as to the aging process.
  • Its cells contain 5 pairs of autosomes and, usually 2 X chromosomes which are easy to study.
  • These animals are hermaphrodite, producing both sperm and eggs so reproduction is easy. Most of the time they fertilize themselves, so that any recessive alleles quickly become homozygous and express in the phenotype.
  • On rare occasions, nondisjunction occurs during meiosis with the loss of one X chromosome. Animals with a single X are males and are able to fertilize the eggs of the hermaphrodites.
  • C. elegans was the first multicellular eukaryote to have its entire genome sequenced. It contains some 19 – 20,000 protein-encoding genes incorporated in 100,258,171 base pairs of DNA.
  • In contrast to other eukaryotes, some 13 – 15% of its genes are grouped in operons containing 2 – 8 genes each.

CONTRIBUTION TO SCIENCE

C. elegans was the first multicellular organism to have its genome completely sequenced in 1998. The C. elegans genome sequence is approximately 100 million base pairs long and contains approximately 20,000 genes

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.

In 2002, the Nobel Prize in Physiology and Medicine was awarded to SydneyBrenner, 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 AndrewFire and Craig C. Mello, for their discovery of RNA interference in C. elegans.

In 2008 MartinChalfie shared a Nobel Prize in Chemistry for his work on green fluorescent protein (GFP) in C. elegans.

The complete sequence of all 100,269,912 bases was finished in 2002, making it the first genome of a multi-cellular eukaryote in which every base is known. It is high-quality sequence with an estimated error rate of less than 1 in 100,000. The sequence was produced jointly by the Sanger Institute in Hinxton, England and the GenomeSequencing Center in St. Louis. Funding for the project was provided to the Genome Sequencing Center by The National Human Genome Research Institute (NHGRI), National Institutes of Health (NIH).




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