Worm Positioning Systems

It’s great to find a scientific paper that reports something really new — something interesting whose very existence as a substance or behavior or phenomenon was unexpected. It’s doubly great if the paper itself is clear, thorough, and convincing. And it’s almost too much to ask, in addition, for for the topic to be biophysical — something that illustrates the connections between living organisms and physical forces or mechanisms. All of these stars aligned a few weeks ago when I came across the following paper in eLife:

Magnetosensitive neurons mediate geomagnetic orientation in Caenorhabditis elegans (http://dx.doi.org/10.7554/eLife.07493)

C. elegans is a soil-dwelling roundworm. It’s an immensely popular model organism and has been intensely studied for decades. It was the first multicellular organism to have its genome sequenced, the connectivity between each of its few hundred neurons is known, and the pattern of divisions that give rise to each cell in its body have been thoroughly mapped. One would think that every aspect of its sensory capabilities would have been noticed and remarked upon by now. But no: no one had looked at whether it can navigate using magnetic fields. The authors of the paper noted that this may be worth investigating: in real life, these worms burrow through soil, and might need a way to distinguish up from down that local magnetic fields might provide. Moreover, the mechanisms by which other animals sense magnetic fields (various birds, sea turtles, and others) remain quite mysterious, so finding this ability in an experimentally tractable creature would be useful.

The authors constructed simple, elegant experiments monitoring the direction C. elegans travel under various applied magnetic fields. (I recommend reading the paper itself, but here’s a summary of the findings.) They discovered that magnetic fields strongly guide the worms, and more strikingly, that the worms do not travel along the field vector, but rather at an angle to the field that corresponds to the angle between the local magnetic field direction and the vertical in Bristol, England, where the organisms are from. C. elegans from Australia traveled preferentially at a nearly opposite angle to the field as their British counterparts, corresponding to the nearly opposite field angle down under. Specimens from all over the globe migrate at angles in accord with their local field, strongly implying that they can use the magnetic field to distinguish up and down.

In itself, these experiments and measurements would be wonderful, indicating a previously unknown “sense” in these animals. The authors went even further, however, and screening various mutants they were able to identify particular sensory neurons that are necessary for the magnetic field sensing, and even visualize (with calcium imaging) these neurons “lighting up” when magnetic fields were applied!

What exactly these neurons are (physically) doing is a mystery. Apparently, they have lots of rod-like villi at their end, and one might imagine that subtle motions or deflections induced by magnetic fields trigger the activation of membrane channels, rather similarly to the mechanism behind hearing. What the motions and deflections are, and what materials transduce them, would be fascinating to uncover.

In all, it’s a wonderful paper, and one of my favorites that I’ve read in the past year. The only sour note struck is not in the article itself, but in the university (UT Austin) press release about the work, which notes that it “might open up the possibility of manipulating magnetic fields to protect agricultural crops from harmful pests” — apparently even the most elegant and insightful science needs a ridiculous comment about “practical” applications.

Note added: See the next post for more on this topic!