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.

Modulation of Reversal Frequency by Light and Temperature Changes

A.H. Burr

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.,