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To Discover Irreducibly Complex Mechanisms, Biologists Reverse Engineer a Worm

Casey Luskin

geomagnetic-field-orig_full.jpg

My master’s degree research focused on paleomagnetism and I’ve always been fascinated by the earth’s magnetic field. So naturally I was interested in new research by biologists at the University of Texas, Austin, published in the journal eLife, “Magnetosensitive neurons mediate geomagnetic orientation in Caenorhabditis elegans.” They explored how the nematode worm C. elegans (a favorite model organism for research) orients itself to the earth’s magnetic field.

Many organisms have such an internal compass, which serves them as an aid in purposes like feeding and migration. But this is the first time that the molecular mechanism that’s involved has been identified, one that might work across other species as well.

As the paper’s title indicates, C. elegans has “magnetosensitive neurons” that allow them to burrow vertically in search of food. Now, there’s another way that animals can orient themselves with respect to the earth’s surface: gravity. So how did the investigators determine that this worm uses the magnetic field instead?

First, they created a magnetic coil system capable of generating a magnetic field in any orientation. They then observed the worms’ behavior. And indeed, when generating a magnetic field where “up” was the opposite of gravity, they were able to “trick” the worms into migrating in the opposite direction of gravity, not down but up. And again, when they generated a field that cancelled out the earth’s magnetic field, the worms seemed lost and migrated in random directions.

The researchers proved their point in another way too. With respect to the earth’s surface in different hemispheres, the direction of the magnetic field points in opposite directions. In the Northern Hemisphere the dip of the magnetic field points towards the earth, while in the Southern Hemisphere it points away. But in both hemispheres the worms manage to orient themselves vertically. Thus if they use the magnetic field to orient themselves, then when exposed to the same magnetic field, worms from the Southern Hemisphere should migrate in opposite directions compared to those from the Northern Hemisphere. They took samples of worms from Australia (Southern Hemisphere) and the United Kingdom (Northern Hemisphere), and that’s exactly what they found.

It turns out, however, that the worms don’t migrate exactly along the field lines. The earth’s magnetic field can be understood mathematically as a vector that points in different directions at different positions on the surface of the earth. Since it is a vector, one can calculate the component of the field that aligns with the horizontal surface of the earth, or the component that is perpendicular to the earth’s surface. C. elegans seems able to detect this vertical component of the field that is perpendicular to the earth’s surface, because they orient themselves vertically while burrowing.

So what’s the biological mechanism at play here? The research team answered that question by reverse engineering the worms. They took mutant worms that were known to lack the ability to use magnetic orientation, and found that they had mutations in the genes used to produce amphid finger neurons — also called AFD neurons, due to the finger-like dendrites with which these cells transmit signals. (See this diagram for a nice picture of what they look like.) The scientists then used a technique called genetic ablation to switch off the genes that produce AFD neurons, and compared them to normal worms with functioning AFD neurons. It turned out only the normal worms could orient themselves using the magnetic field. Indeed, they found that ablating other types of neurons did not affect magneto-orientation (also called “magnetotaxis”). All of this suggests that AFD neurons are involved in magneto-orientation.

The researchers then sought to understand how the AFD neurons allow this behavior. They explain:

The sensory ending of the AFD neurons consists of dozens of villi arranged anterior-to-posterior (in an antenna-like formation) imbedded inside glial cells (Perkins et al., 1986). Genetic ablation of the glia surrounding these structures, results in worms with viable AFD neurons but lacking villi (Bacaj et al., 2008). These worms were unable to orient to artificial magnetic fields 279 (Figure 6A). This supports the idea that the villi may be the site of magneto-transduction (and/or that the glia themselves contribute to this sense). Taken together, our results demonstrate that the AFD sensory neurons are required for magnetotaxis.

In other words, the finger-like extensions in the dendrites of the AFD neurons somehow mediate magnetotaxis. But how? To find out, the investigators deleted a gene, tax-4, which helps produce an ion channel in AFD neurons, the TAX-4 cGMP-gated channel. Worms without a functional tax-4 gene could not orient themselves using a magnetic field, but worms with a functional tax-4 gene could. As they put it, “These results support the hypothesis that the cGMP-gated ion channel TAX-4 plays an important role in the AFD sensory neurons for orientation to magnetic fields.”

Then, they performed an experiment using genetically fluorescent AFD neurons, which light up when activated. When the neurons were in the presence of a magnetic field, the researchers could visually detect activity. It’s still not completely understood how exactly the process works, but this work allowed the investigators to identify some of the precise cell-types and ion channels responsible for magneto-sensitivity.

So what is necessary for this behavior? According to the paper:

The AFD sensory neuron pair is necessary for magnetic orientation and for vertical migrations. Similarly, a cGMP-gated ion channel in the AFD neurons, TAX-4, is also necessary and sufficient for these behaviors.

But it’s not enough to have these cells and ion channels, which are able to sense the magnetic field. The worms also need the proper response behavior encoded in their neurons, enabling them to burrow downward when feeding. Because worms in different hemispheres respond differently to the field, the team proposed that this response behavior is genetically encoded. This sounds like an irreducibly complex system — that is, a system where multiple parts are needed to enable some function: the AFD neurons, the cGMP-gated ion channel, and the genetically encoded behavior response.

These authors don’t say anything about irreducible complexity or intelligent design, and I don’t at all mean to claim them as ID advocates. But consider this. Scientists use engineering methods to understand how living systems work. That means they are treating biological systems as purpose-driven and directed towards an engineered goal. In that sense, I think it’s fair to say this research lends support to an observation of ID: Biological systems look engineered because they really are.

Image credit: NASA.

 

Casey Luskin

Associate Director, Center for Science and Culture
Casey Luskin is a geologist and an attorney with graduate degrees in science and law, giving him expertise in both the scientific and legal dimensions of the debate over evolution. He earned his PhD in Geology from the University of Johannesburg, and BS and MS degrees in Earth Sciences from the University of California, San Diego, where he studied evolution extensively at both the graduate and undergraduate levels. His law degree is from the University of San Diego, where he focused his studies on First Amendment law, education law, and environmental law.

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