I have to confess, I’ve been holding out on you. For the last five months, I’ve been sitting on one of the coolest stories in the Volvox world. I learned about this at the Fourth International Volvox Meeting last August in St. Louis, and I’ve been dying to write about it ever since. I didn’t, because the work wasn’t published. Now it is.
Headline writers love to use the word “zombie” as a metaphor, so we have “zombie retailers,” “zombie deer,” “zombie properties,” and even “zombie politicians.” None of those are literally dead things moving around as if they were alive.
That’s precisely what this story is about.
There was a lot of cool science presented at the Volvox meeting, but I don’t think any talk dropped more jaws than Noriko Ueki’s.
In order to explore the mechanisms regulating flagellar activity, Dr. Ueki and Ken-ichi Wakabayashi developed a method to “demembranate” whole Volvox rousselettii colonies. By subjecting the colonies to a nonionic detergent, they were able to dissolve the cell membranes while leaving the flagella intact. Amazingly, when ATP was added to the medium, the dead colonies began to swim around as if they were alive. ATP is adenosine triphosphate, the energy currency of all cells, and hydrolysis of ATP drives the motor proteins that cause eukaryotic flagella to beat.
Here is what live Volvox look like when they’re swimming around (I couldn’t find a video of Volvox rousselettii; these are Volvox aureus):
And here is Ueki and Wakabayashi’s experimental procedure. The colonies stop swimming after the addition of detergent: they ded. Watch what happens after the addition of ATP:
With the addition of ATP, the demembranated (dead) colonies begin to swim very much as if they were alive. In fact, they swim almost as fast, on average, as live spheroids: 0.5 mm/s versus 0.6 mm/s for live spheroids. It wasn’t clear to me whether or not the zombie Volvox were capable of phototaxis (swimming toward the light) as live Volvox do; I assume not.
The purpose of all this wasn’t just so Drs. Ueki and Wakabayashi could yell “It’s alive!” and practice their evil laughs.
Rather, they were studying mechanisms of flagellar control, specifically the role of calcium ions (Ca2+) in phototactic and photoshock responses, in other words the behaviors of swimming toward a suitable light source and away from a sudden, bright light. As you can see in the videos above, Volvox spheroids rotate as they swim. In normal phototaxis, cells facing away from the light beat their flagella normally, that is, toward the posterior (back), while cells facing the light reverse their beating. This has the effect of continually steering the spheroid toward the light source. The reversal of beating occurs most strongly in the anterior (front), so you can imagine this like a canoe: if the person in front paddles backwards on the right and the person in the back paddles forward on the left, the canoe will turn (quickly) toward the right.
In response to a sudden change in light intensity, Volvox spheroids undergo a photoshock response, in which flagella all over the spheroid suddenly reverse the direction of their beating. By adding Ca2+ to the demembranated spheroids (with ATP), Drs. Ueki and Wakabayashi showed that this reversal is not uniform over the surface of the spheroid, but strongest in the front and nearly absent in the back:
In the presence of Ca2+, V. rousseletii flagella near the anterior pole show almost full reversal (rotation of ∼180°) of beating direction. The degree of rotation of beating direction decreases with distance from the anterior pole, and becomes ∼90° at the equator and 0° near the posterior pole.
The response to Ca2+ is not the only thing that changes from the front of a Volvox colony to the back; the eyespots that detect light also get smaller the further they are away from the anterior. The eyespots change the intracellular concentration of Ca2+ in response to light, so it’s reasonable to think that a combination of differently-sized eyespots and different magnitudes of response to Ca2+ explains the different flagellar responses of anterior and posterior cells:
The light sensitivity of an eyespot may increase with size, such that a larger eyespot more readily promotes light-induced in- crease in cytoplasmic Ca2+ concentration. If so, V. rousseletii spheroids may have a strong A–P gradient in photosensitivity, resulting from the combined effects of two kinds of gradients: one in the mechanism that regulates light-induced Ca2+ influx and the other in the flagellar mechanism to change the waveform in a Ca2+-dependent manner.
So aside from the “wow” factor, this is actually really important science, digging deeply into the molecular mechanisms that control flagellar photoresponse.
But my first thought on seeing dead Volvox colonies swimming around as if they were alive was that it was the most powerful argument I’d seen against vitalism. Living systems are chemical systems; there is no élan vital. I can’t think of a better demonstration of that than dead organisms being reanimated by the simple addition of ATP, a chemical that is easily synthesized in the lab.
Ueki, N. and Wakabayashi, K.-I. 2018. Detergent-extracted Volvox model exhibits an anterior-posterior gradient in flagellar Ca2+ sensitivity. Proc. Natl. Acad. Sci. U. S. A., 201715489. doi: 10.1073/pnas.1715489115