"Nematoda. The roundworms and threadworms, a phylum of smooth-skinned, unsegmented worms with a long cylindrical body shape tapered at the ends; includes free-living and parasitic forms both aquatic and terrestrial."It is small, growing to about 1 mm in length, and lives in the soil - especially rotting vegetation - in many parts of the world, where it survives by feeding on microbes such as bacteria. It is of no economic importance to man.
(Academic press Dictionary of Science and Technology)
Why study C. elegans?
Around the world many hundreds of scientists are working full time investigating the biology of C. elegans. Between October, 1994 and January, 1995 73 scientific articles about this creature appeared in international science journals. Currently an international consortium of laboratories are collaborating on a project to sequence the entire 100,000,000 bases of DNA of the C. elegans genome. Why invest so much effort into the study of such an insignificant organism?
C. elegans is about as primitive an organism that exists which nonetheless shares many of the essential biological characteristics that are central problems of human biology. The worm is conceived as a single cell which undergoes a complex process of development, starting with embryonic cleavage, proceeding through morphogenesis and growth to the adult. It has a nervous system with a 'brain' (the circumpharyngeal nerve ring). It exhibits behavior and is even capable of rudimentary learning. It produces sperm and eggs, mates and reproduces. After reproduction it gradually ages, loses vigor and finally dies. Embryogenesis, morphogenesis, development, nerve function, behavior and aging, and how they are determined by genes: the list includes most of the fundamental mysteries of modern biology. (We must, alas, assume that the greatest biological enigma of all, consciousness, is absent from C. elegans - although this remains to be demonstrated!) C. elegans exhibits these phenomena, yet is only 1 mm long and may be handled as a microorganism - it is usually grown on petri plates seeded with bacteria. All 959 somatic cells of its transparent body are visible with a microscope, and its average life span is a mere 2-3 weeks. Thus C. elegans provides the researcher with the ideal compromise between complexity and tractability.
A biological thumbnail sketch of C. elegans
C. elegans is a free-living nematode. There are two sexes: a self-fertilizing hermaphrodite and a male. The adult essentially comprises a tube, the exterior cuticle, containing two smaller tubes, the pharynx and gut, and the reproductive system. Most of the volume of the animal is taken up by the reproductive system. Of the 959 somatic cells of the hermaphrodite some 300 are neurons. Neural structures include a battery of sense organs in the head which mediate responses to taste, smell, temperature and touch - and although C. elegans has no eyes, it might respond slightly to light. Among other neural structures is an anterior nerve ring with a ventral nerve cord running back down the body. (There is also a smaller dorsal nerve cord.) There are 81 muscle cells. C. elegans moves by means of four longitudinal bands of muscle paired sub-dorsally and sub-ventrally. Alternative flexing and relaxation generates dorsal-ventral waves along the body, propelling the animal along. The development and function of this diploid organism is encoded by an estimated 17,800 distinct genes.
The L2d may, if conditions remain crowded and/or starved, develop into a dauer larva or if conditions improve, switch to the direct developmental route to the adult. The word 'dauer' comes from the German: 'Dauer' (noun), meaning 'endurance' or 'duration'. Whereas adult worms have an average life span of 2-3 weeks, dauer larvae survive for months. If given food after such a period they develop into adults that have a normal life span. For this reason the dauer larva is considered non-aging. Dauers look different from the other larval stages: they are thinner relative to their length and their bodies are darker (due to nutrient storage granules in the intestinal and hypodermal cells). They also have relatively impermeable cuticles, and are non-feeding.
Genes and dauer formation
Mutant genes controlling the ability to form dauer larvae interact in a way that allows them to be ordered into pathways. Two alternative pathways have been proposed (Fig. 2). The mutant phenotypes suggest that the pathway corresponds to neural processing of environmental stimuli. A pheromone that influences both entry into and exit from the dauer larva stage has been characterized, and specific chemosensory functions have been correlated with both entry and exit pathways.
Why study dauer larvae?
During dauer larva development, information about the environment, received by sensory organs, is transduced by the nervous system and somehow translated into a developmental transformation. In studying these processes a number of questions central to biology can be addressed. These include: