Like the marauding sandworms in Dune that swallowed everything that came between their giant jaws, the 1-millimeter-long lab worm Caenorhabditis elegans lumbers up to masses of bacteria and gobbles them whole. But some of the worms are a bit more selective than others. Researchers have now discovered a gene that helps some worms to stay away from bacteria that might be bad for their health.
The C. elegans immune system is very rudimentary, says geneticist Dennis Kim of the Massachusetts Institute of Technology in Cambridge. So, like humans recoiling from foul-smelling objects in the fridge, it's important that they learn to avoid dangerous things before those things kill them. But with just a handful of neurons, how do they do it?
To find out, Kim and colleagues studied 162 strains of C. elegans that another lab had collected from the wild. They placed the worms in plastic dishes that contained a patch, or lawn, of the bacterium Pseudomonas aeruginosa, which can cause severe infections in humans. It's dangerous to C. elegans, too. At first, the worms approached the Pseudomonas. But when the researchers came back to the plates 24 hours later, they found that most of the strains had moved away from the bacteria. The worms had grown a little bigger, however, indicating that they had taken a bite and learned not to take another. One strain, however, was still eating, and many of its members were dying.
To figure out why some worms were faster learners than others, the researchers compared the genomes of the various strains. In the worm strain that stayed on the lawn, they spotted mutations in a gene called HECW1, which is involved in relaying signals through the nervous system. Two of C. elegans' sensory neurons have HECW1 in them, the researchers report online today in Nature. When the researchers zapped these two neurons with lasers in slow learners, those worms started hanging out on the Pseudomonas. It seems that HECW1 has a role learning, and in passing along a message: Don't touch the bacteria.
Kim says that this mechanism might be triggered only by Pseudomonas, or bacteria similar to it. If so, the strain that was slower on the uptake probably doesn't have to deal with those bacteria in its environment, so it never needed to learn to avoid them. It's probably good that they didn't, he adds; with only 302 neurons to learn with, worms can't become too picky, lest they starve because of too little "adventurous eating."
The study nicely highlights how C. elegans, which does little more than squirm around, can be a model for studying the genetic basis of behavior, says neuroscientist Mario de Bono of the University of Cambridge in the United Kingdom. Even though humans have 300 million times as many neurons as these tiny organisms, he notes, some of the pathways may be the same. "That's the power of the worm."