Seeing without eyes

Soil-dwelling roundworms avoid light by using nerve cells and pathway reminiscent of vertebrate vision

A tiny, eyeless, roundworm that lives underground can see the light.

Research reported online July 6 in Nature Neuroscience identifies several nerve cells that appear to act as the worm’s light receptors and elucidates how these light-sensitive cells pass environmental information to the worm.

It turns out that the lowly roundworm trips the light fantastic via a cellular messaging system that is similar to the light-sensing pathway in vertebrates. This finding suggests that the worm’s light-sensitive nerve cells are possible precursors to receptors found in vertebrate eyes, says Russell Fernald of StanfordUniversity, who was not involved in the research.

“This is really quite interesting,” he says.

The roundworm Caenorhabditis elegans, a soil-dwelling nematode, is transparent and has only 302 nerve cells, making it the teacher’s pet of researchers trying to understand nervous system genetics and development patterns in animals. But while C. elegans has been the subject of intense scrutiny, no one had looked closely at the worm’s relationship to light.

Shawn Xu, who for years studied vision in another model organism — the fruit fly Drosophila — decided to investigate how the eyeless C. elegans knew how to stay in the dark. Extended amounts of sunlight kill these dirt dwellers, and Xu, of the University of Michigan in Ann Arbor, figured light must somehow repel the worms.

So along with Alex Ward, Jie Liu and other colleagues, Xu did some laser experiments, directing light of different wavelengths at the nematode’s head or tail. The worm always wriggled off in the direction opposite the light source, even if that meant moving backwards, the researchers report.

The research team then destroyed particular nerve cells in the worm, one by one, to determine which cells gave the creature its visual capability. The nerve cells responsible, known as ciliated neurons, play a role in perception in many animals.

“A light sensitive neuron has to catch a photon and then turn it into something useful for the animal,” Fernald says. The pathway the worm uses to pass that light information is similar to the vertebrate pathway, rather than the pathway found in most invertebrates. Scientists have long debated the origin of vision in animals — Charles Darwin himself speculated that the earliest eye could have been a bit of pigment and a cell that detects light.

“Darwin was probably right — you start with a simple eye and later you can add more stuff, like a cornea or lens,” Xu says.

C. elegans doesn’t have any pigment cells, which would shade incoming light, conveying information about the direction the light is coming from. Xu speculates that soil acts as that pigment, shading incoming light in such a way that the animal figures out which direction to move to stay in the dark. “They have the photoreceptors — the pigment is in their environment,” he says.

The work suggests that there is more to these nematodes than meets the eye, and they could serve as a model for studying animal vision pathways.

It also serves as a reminder to scientists to look at how animals actually live in their environment, which can reveal overlooked processes and pathways, says neurobiologist Fernald.

“Animals have evolved really exquisite sensory systems that match their ecological and evolutionary needs,” Fernald says. “That’s where the coolest stuff is found.”

Watch the eyeless roundworm C. elegans respond to light:

SEEING THE LIGHT from Science News on Vimeo.

Video courtesy of Shawn Xu

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