Worm Breeder's Gazette 8(3): 55
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
C. elegans can see! That is, its reversal frequency is increased by perpendicularly-incident light. Adult hermaphrodites were tested with 10 s periods of light stimulus, alternating with 20 s periods of dark. Reversals were counted during the 10 s of stimulus and the 10 s preceding the stimulus. Observation was by video camera and continuous near-infrared radiation. For 14 monochromatic stimuli ranging between 420 and 680 nm at a constant 3.4 x10+E13 photons s(-1) cm(-2), only those in the 520-600 nm range caused a significant increase in the frequency of reversals. Various possible radiant- heating effects, investigated in detail in a forthcoming paper (Burr, in press, Photochem. Photobiol.), are ruled out because the temperature changes would be too minute (less than 2x10+E-6 C, which is on the order of natural temperature fluctuations inside the nematode), and the wavelength dependence would be different. Moreover, a larger, 2x10+E-4 C change produced with a 1230 nm monochromatic stimulus had no significant effect on reversal probability. Therefore, the increase in reversal frequency must have been due to light. Much larger temperature changes did have an effect. Increasing or decreasing the temperature by 2 C from the acclimation temperature (20 C) increased average reversal frequency by 0.28 and 0.19 minute(-1), respectively. The responses adapted within 30 minutes. (Note: That the temperature changes affect reversal frequency differently from thermotaxis.) The sign of the latter response is different for increases and decreases of temperature (Hedgecock and Russell, Proc. Nat. Acad. Sci. USA 72, 4061-4065, 1975). The threshold light intensity was about 2x10+E13 photons s(-1) cm(-2) at 540 nm (40 lux or about 1/20 the illuminance of office lighting). At higher light intensities, reversal frequency reached a maximum level about 0.50 minute(-1) higher than the background frequency of spontaneous reversals (0.72 minute(-1)). The threshold intensity and maximum increment in frequency were not affected by increasing or decreasing the temperature by 2 C just prior to the experiment, in spite of the effect on background reversal frequency. Evidently the effects of light and temperature-change on reversal frequency are independent and additive. In another study, responses of C. elegans and Oncholaimus vesicarius were compared under identical conditions at 15 C and an intensity that was saturating for both. For O. vesicarius, background reversal frequency was slightly higher (0.93 vs 0.82 minute( -1)) and the increase due to perpendicularly-incident light was much greater (2.17 vs 0.68 minute(-1)). Oblique illumination of O. vesicarius elicited a strong negative phototaxis as found previously ( Burr, J. Comp. Physiol. 134, 85-93, 1979), whereas C. elegans had no directional tendency. This was as expected, since C. elegans lacks the pigment spots needed for detecting the direction of the light source (See Burr, Photomovement behavior in simple invertebrates, pp. 179-239 in: Photoreception and Vision in Invertebrates, M. A. Ali, ed., 1984).