It is generally believed that C.elegans is not sensitive to the human-visible light. We have found two novel light responses in worms: light avoidance and light sensitivity. For light avoidance, we developed two assays: an acute avoidance assay and a dispersal assay. In the former, the latency of the backing response upon light stimulation is scored; in the latter, worms" dispersal from the central illuminated area of the plate to the shadowed periphery is quantified. In both assays, responses to the blue part of the human-visible spectrum (400-500 nm) were far more pronounced than to the green (500-550 nm); responses to red light (above 600 nm) were undetectable. This wavelength specificity strongly suggests that this response is not heat avoidance. If it were heat avoidance, a much stronger correlation with the light intensity, rather than wavelength, would be expected. In addition, we used a small temperature probe to confirm that heating of the plate did not occur at light intensities used. To elicit acute avoidance, a blue light intensity of 2000 Wm-2 was needed (about twice that of sunlight), while dispersal was observed starting at 200 Wm-2.
The light sensitivity phenotype was studied by continuously illuminating animals at different stages of development. Blue light (450-490 nm) of 100 Wm-2 arrested egg development and caused a growth delay in larval development. Higher intensities caused other distinct phenotypes: arrest of pharyngeal pumping, developmental arrest, and, finally, death.
In attempt to identify neurons involved in light avoidance, we laser ablated dye-filling neurons: ASH, ADL, ASI, ASK and ASJ; all together or in subsets, and in all cases, performance in the acute avoidance assay was normal. This was consistent with a normal performance of amphid-defective mutants,
osm-6,
osm-2 and
che-2, in the light dispersal assay. (Amphid-defective mutants were previously found to be defective in temperature avoidance). We found, however, that worms lacking TAX-2, a worm ortholog of the vertebrate rod photoreceptor cyclic nucleotide gated channel, were defective in light dispersal. In addition to amphid neurons,
tax-2 is also expressed in the neurons exposed to the body cavity, where it is involved in social and oxygen avoidance behaviors and is activated by soluble guanylyl cyclases (sGCs). We are currently testing whether
tax-2 also functions in these neurons to mediate light avoidance. sGCs are hemoproteins, and therefore may be direct light targets.