When grown at a temperature from 16°to 250 and placed on a thermal gradient, the nematode Caenorhabditis elegans migrates to its growth temperature and then moves isothermally. Behavioral adaptation to a new temperature takes several hours. Starved animals, in contrast, disperse from the growth temperature. Several mutants selected for chemotaxis defects have thermotaxis defects as well; these behaviors depend on some common gene products. New mutants selected directly for thermotaxis defects have unusual phenotypes which suggest mechanisms for thermotaxis.Mutants with specific behavioral defects are useful in understanding the mechanisms by which genes direct the formation of a nervous system (1-4). Such mutants have been isolated in the small soil nematode Caenorhabditis elegans and, because the nervous system is simple (<300 cells), the mutant defects may be identified by serial section electron microscopy (1, 5-8). Mutants can also be analyzed by formal genetic techniques, such as dominance testing, complementation, and epistatic ordering, to gain insight into the structure of behavioral pathways and their development.Mutants of C. elegans with general behavioral defects (1, 9) and specific chemotaxis defects (5) have been described. This paper describes some mutants with specific defects in thermotaxis. These mutants are particularly interesting because thermotaxis can be modified by experience.MATERIALS AND METHODS Nematodes. Caenorhabditis elegans [var. Bristol (strain N2)] was used for behavioral studies and mutant selection. Worms were grown monoxenically in petri plates containing nematode growth minimal medium (NGMM) agar (5) preseeded with Escherichia coli strain OP50 (1).Temperature Gradients. A stable and reproducible linear temperature gradient was established by connecting two thermostatically regulated water baths, 50 and 350, by a 61 X 10 X 1.3 cm aluminum slab tightly bolted at each end to a 10-cm aluminum cube immersed in a bath. The temperatures of the baths were stable to 0.10. The room temperature varied from 19.50 to 20.50. Plastic petri plates (9-cm) confaining 35 ml of agar culture medium (NGMM) were placed on the aluminum gradient slab at regular intervals and the agar temperature was monitored with a glass probe thermistor. The agar surface established a uniform gradient of 0.5°/cm with an equilibration half-time of 5 min. The gradient was undistorted by edge-effects to within 1 cm of the petri plate wall. For graphical analysis, each plate was scored for three classes of nematodes: those migrating to the warmer half of the plate (H), those migrating to the colder half (C), and those never fully leaving the point of application (usually less than 10%). The results can be expressed by the percentage going to the warmer half 100 X H/(H + C) or by a "thermal preference" scale defined as 100 X (H -C)/(H + C) (Fig. 3). The second scale has the advantage that 0 indicates no preference, while positive and negative values indicate preferences for higher and lower temperatures, respect...
The general organization and structure of the nerve ring, the main mass of central nervous system neuropil, in the small soil nematode Caenorhabditis elegans is described. The nerve ring receives sensory input from the anterior tip of the animal by means of six nerve bundles, all nerve fibers of which have centrally located cell bodies. The anterior sensory structures are classically divided into two types, papillary and amphidial, and are assumed responsible for mechano-and chemoreception, respectively. Papillary fibers enter directly into the nerve ring, whereas amphidial fibers enter the ventral ganglion, a posterior extension of the nerve ring, in a circuitous manner which is not discussed in detail. Of those papillary fibers which project into the nerve ring neuropil, 22 end in easily characterized sensory structures whereas 14 terminate distally near sensory organs but have no function which can be deduced on the basis of comparative morphology. After entering the ring the fibers maintain their identity and do not anastamose with one another. Cell bodies of each papillary sensory neuron have been mapped around the nerve ring. The cephalic musculature is shown to consist of 32 muscle cells which form four longitudinal submedial groups of eight muscles each. Innervation of this musculature occurs wholly within the CNS by means of processes of the muscle cells which are sent centrally. The anterior 16 cephalic muscle cells are innervated by the ring only, in well delimited regions termed muscle plates. The posterior 16 are dually innervated by means of processes sent both to the nerve ring plates and to their nearest medial longtitudinal nerve cord. The nerve ring neuropil is characterized as having fibers containing one of four morphologically distinct vesicle types. Gap junction contacts are observed within the main neuropil involving one of these fiber types and within the muscle plate regions among muscle processes, which do not contain vesicles. An evolutionarily primitive sensory-motor synapse within the nerve ring is described from an identified sensory neuron onto an identified cephalic muscle cell process. Comparisons are made with the nervous system of Ascaris lumbricoides, the only other nematode to be extensively studied, to illustrate the conservativeness of the nemic nervous system.
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