90 — C. elegans discriminate colors without eyes or opsins

If one should put a C. elegans worm in a blender — and generally, one shouldn’t — one would find that passing the resultant sludge through some molecular tests would yield no opsins. If one were to refrain from such blending protocols but were instead to sequence the genome of the worm, one would indeed find no evidence of photoreceptive or opsin genes.

Nevertheless.

Nevertheless, the C. elegans worm will very clearly avoid the blue toxins emitted by such bacteria as P. aeruginosa. And that very same worm will abandon the blue bacterial lawn far sooner in the presence of bright white light. In other words, the worm appears to detect the blue color itself. If this were not enough, C. elegans will also avoid ultraviolet light in the total absence of bacteria.

So there’s something photoreceptive going on here.

The lite-1 gene encodes a transmembrane protein similar to insect olfactory chemoreceptors that appears to control that UV photoresponse. In lite-1 knockouts — worms lacking this gene — the bright white light (with no UV component) fails to evoke the same lawn-abandonment behavior.

This means that lite-1 is responsible for this light/blue response.

If one eliminates the blue pigment (pyocyanin) from the bacterial genome (a mutant known as PA14∆phzM) but leaves in the toxin — that is, the bacteria are still poisonous to the worm but are no longer blue — the worm fails to avoid the bacteria.

Both the lite-1 pathway as well as the blue pyocyanin in the bacteria are required for the worm to exhibit avoidance behavior.

By altering the blue:amber ratio of the light reaching a foraging worm, it is possible to modulate the avoidance response.

This is fascinating, because it means that the higher-level concept of “vision” and phototropism that we think of when imaginging sight is very fundamentally different from the basis of phototropism in a worm. Though these chemical responses seem like a slower, ersatz substitute for the high-speed photoreceptor polarization in retina, it’s very possible that other animals — indeed, even humans — might have similar photochemical pathways at work in non-vision-related organs.