Nematode chemotaxis and possible mechanisms of host/prey recognition

It has long been recognized that free-living nematodes utilize specific recogni­ tion mechanisms for finding their hosts or prey in the soil. One of the earliest observations described the attraction of the plant parasitic nematode Meloidogyne incognita to tomato roots grown in sterile petri plates (62). If nematodes were unable to detect signals emanating from a food source, food finding would be a random, inefficient process. Such is not the case. The general consensus, based on experimental evidence and the morphologic con­ figuration of purported sensory structures located in the cephalic region, is that in nematodes the primary food-finding mechanisms are governed by chemotac­ tic factors emanating from the host or prey (22). Other stimuli, such as thermal, vibratory, or tactile stimuli (29), are believed to play a minor role, if any, in food-finding behavior. Once the nematode reaches the potential food source, it faces further barriers before it can commence feeding. For plant nematodes this includes recognition of an area of the root susceptible to attack. For bacteriophagous nematodes, recognizing certain species of bacteria as good food sources is vital, for these nematodes will not grow and reproduce on all bacteria. Molecular recognition of "good" and "bad" bacteria undoubtedly comes into play, but here again definitive data are lacking.