The chemosensory System of Caenorhabditis Elegans and Other Nematodes

Olfactory systems allow organisms to detect and discriminate between thousands of low molecular mass, mostly organic, compounds which we call odours. Organisms as diverse as humans and nematodes utilize the same basic mechanisms for this sensory perception. Represented in the olfactory repertoire of both vertebrates and invertebrates are aliphatic and aromatic compounds with diverse functional groups including aldehydes, esters, ketones, alcohols, ethers, carboxylic acids, amines, halides and sulphides. Soil-dwelling nematodes encounter many types of volatile and water-soluble molecules in their environment; successful foraging depends on the animal’s ability to detect a gradient in one odorant while ignoring extraneous odours. Water-soluble chemicals tend to diffuse slowly in the soil and may provide short-range chemosensory cues whereas volatile compounds diffuse more rapidly and thus can be used for long-range chemotaxis to distant food sources. Animals modify their behaviour based on the interpretation of these environmental cues. The biochemical and physiological processes of chemosensory perception involve the recognition of small chemical molecules by specialized transduction pathways in the organism. These pathways are responsible for the transformation of information from extrinsic molecules into signals that the nervous system can interpret. The highly conserved G-protein signalling pathway is used to provide this chemosensory ability. The interaction of an odorant with an olfactory receptor results in the activation of heterotrimeric GTP-binding proteins (G proteins). G-protein signalling has been the subject of intense research over the last two decades. G proteins are present in all eukaryotic cells and signalling through G-protein-coupled receptors and heterotrimeric G proteins is one of the main means of transducing extracellular signals in the cell. Caenorhabditis elegans is an excellent model organism to study the molecular mechanisms behind signalling pathways in that it possesses unique traits amenable to both forward and reverse genetics. Exploiting these traits has shed much light on the mechanisms behind G-protein signalling. As molecular manipulations routinely used for C. elegans are becoming available for other nematodes, an increasing amount of chemosensory information is becoming available for a diverse range of nematodes from an even more diverse range of habitats.