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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...

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The gene cul-1 (formerly lin-19) is a negative regulator of the cell cycle in C. elegans. Null mutations cause hyperplasia of all tissues. cul-1 is required for developmentally programmed transitions from the G1 phase of the cell cycle to the GO phase or the apoptotic pathway. Moreover, the mutant phenotype suggests that G1-to-S phase progression is accelerated, overriding mechanisms for mitotic arrest and producing abnormally small cells. Significantly, diverse aspects of cell fate and differentiation are unaffected in cul-1 mutants. cul-1 represents a conserved family of genes, designated cullins, with at least five members in nematodes, six in humans, and three in budding yeast.

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