A worm that lives underground seemingly has little need to detect light. But now researchers have uncovered this overlooked talent in the soil-dwelling nematode Caenorhabditis elegans. Their findings may help illuminate the evolutionary history of vertebrate eyes.
C. elegans is a favorite subject of neurobiologists because it has a simple nervous system. Over the years, researchers have figured out how the worm detects chemicals, heat, and other clues to its environment. However, researchers generally assumed that the worm, which lacks eyes, was insensitive to light, says Shawn Xu, a neurobiologist at the University of Michigan, Ann Arbor. But Xu and his colleagues noticed that the millimeter-long worms sometimes seemed to wriggle away from the bright light of their microscope.
Investigating further, Xu's graduate student Alexander Ward and postdoc Jie Liu found that flashes of light aimed at a swimming worm caused it to reverse course (see video). The worms were most sensitive to long ultraviolet (UV-A) light, and additional experiments indicated that this wavelength, which damages skin in sun-worshiping people, is downright lethal to the worms, the researchers report online 6 July in Nature Neuroscience. The scientists reason that the ability to detect UV-A may help C. elegans stay safely underground.
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To figure out how the sensory system works, the researchers turned lasers on the worm's neurons. C. elegans has just 302 neurons, whose functions and connections have been well studied. Destroying four pairs of sensory neurons eliminated the worm's ability to respond to light. Other experiments revealed that flashes of light spur at least one of these sensory neurons to produce weak electrical currents. These currents didn't flow without two molecular components also found in vertebrate eyes: cyclic nucleotide-gated channels, which let ions pass across a cell membrane, and a messenger molecule called cGMP.
The results suggest that some molecular components of rod and cone photoreceptor cells in the vertebrate eye may have been present in the first animals with bilateral symmetry, which appeared at least 540 million years ago, before the first vertebrates, says Todd Oakley, an evolutionary biologist at the University of California, Santa Barbara. A few previous studies have hinted at this early origin, Oakley says, but to the best of his knowledge the new paper furnishes the first demonstration that these components underlie behavioral responses to light in such a distant relative of vertebrates.