A small transparent nematode worm widely used as a model organism in biology.Caenorhabditis elegans is a free-living nematode, a roundworm about one millimetre long that lives in soil and feeds on bacteria. It is transparent at every stage of its life, which means its cells can be watched under a microscope while the animal is still alive and developing. It grows quickly, reproduces in a few days, and can be kept in large numbers on simple agar plates, so it is cheap and convenient to study in a laboratory. These properties made it one of the standard model organisms of modern biology, alongside the fruit fly, the zebrafish and the mouse.
Its importance comes from the fact that its body is built from a small, fixed number of cells. An adult hermaphrodite has exactly 959 somatic cells, and the lineage of every one of them — which cell divides into which — has been mapped from the fertilised egg onwards. The nervous system is similarly compact: 302 neurons with a wiring diagram that has been traced in full. Because the animal is transparent and its cell divisions are invariant between individuals, researchers can follow a single cell from birth to its final role, or watch a specific neuron fire, without dissecting anything.
C. elegans therefore serves as a testing ground for questions that are hard to ask in more complex animals: how a fertilised egg produces a fixed set of tissues, how genes control programmed cell death, how a nervous system produces behaviour, and how ageing proceeds at the level of individual cells. Findings made in the worm have repeatedly turned out to apply to humans, which is why a soil nematode with no eyes and no blood sits in so many genetics and neuroscience laboratories.
Sydney Brenner proposed C. elegans as a model organism in the 1960s, looking for a simple animal in which the development and nervous system could be traced cell by cell. The first genetic and anatomical work appeared in the 1970s, and the complete cell lineage was published by John Sulston and colleagues in the 1980s. The genome was sequenced in 1998, the first of any multicellular animal. Brenner, Sulston and Robert Horvitz shared the 2002 Nobel Prize in Physiology or Medicine for this work.
The worm is raised on agar plates seeded with bacteria, or in liquid culture, at temperatures around 20 °C. Its life cycle runs from embryo to larva through four larval stages to adult in about three days. Under a dissecting microscope, individual animals can be picked with a wire or a pipette and moved to fresh plates. Because the body is transparent, fluorescent proteins can be attached to chosen genes, and the resulting glow is visible through the cuticle in a living animal.
Genetic work is straightforward: the worm can be fed bacteria that produce double-stranded RNA matching a chosen gene, which silences that gene — a technique called RNA interference, first discovered in this organism. Mutants are easy to isolate and cross, and the whole genome has been sequenced. The combination of a mapped cell lineage, a complete wiring diagram and simple genetics lets a researcher connect a specific gene to a specific cell to a specific behaviour.
Used in genetics, developmental biology, neurobiology and ageing research as a fast, cheap system for testing how genes shape cells and behaviour; also a standard organism for studying RNA interference and for screening potential drugs.
Every adult hermaphrodite has exactly 959 body cells, and 131 cells are deliberately killed by programmed cell death during normal development.