The tiny, single-celled creatures obviously don’t have room for brains to tell them how to move in complex ways, so to get around they usually roll, slide, or swim.
But microscopic pond dwellers called Euplotes eurystomus have mastered a brainless way of walking: crawling like insects, with their 14 tiny appendages.
They appear to move a bit like Dutch-designed kinetic sculptures called Strandbeasts, with clock-like connections that cycle them through a pattern of set states that can be adjusted in response to their surroundings.
“There seemed to be this sequential logic going on with the movements,” says biophysicist Ben Larson of the University of California, San Francisco (UCSF). “They were not random, and we began to suspect that there was some kind of information processing going on.”
These protozoa, single-celled organisms with animal-like characteristics, have 14 stinging bundles of cilia that work together as legs called cirri. They can use these cirri to swim and walk while actively hunting prey.
This all started in the 2016 @MBLPhys course during my PhD with @Choano_Lab. I had noticed predatory creatures eating the choanoflagellates I was trying to isolate from field samples. Knowing that Wallace was a mad microorganism expert, I struck up a conversation…
2/n pic.twitter.com/R4jRwOAWhQ
— Ben Larson (@BEuplotes) March 1, 2021
Larson and his colleagues captured microscopic images of these small predators to study their movements in slow motion. The researchers identified 32 different combinations of leg movements and found that certain combinations were more likely to follow each other.
Cirri are made of tubulin fibers, like the rest of the cell’s scaffolding structures (its cytoskeleton). These fibers also act as a support structure between the different cirri so they also work as a kind of mechanical communication.
“Euplotes uses these connections to facilitate an elaborate walking movement,” explains UCSF biophysicist Wallace Marshall.
Computer modeling revealed that the tension and tension of the fibers dictated which set pattern of cirri positions was possible at any given time. Some cirri store stress in different stages of the march; when this stress is released, it prompts the cell to advance to the next state, causing a cyclical transition between these states.
“The fact that Euplotes appendages move from state to state in a non-random way means that this system is like a rudimentary computer,” says Marshall.
When researchers exposed Euplotes to a drug that disrupts the synchronous reactions of tubulin fibers, it disrupted the cell’s gait, causing the poor creatures to walk in useless circles.
His gait was still regular, but it was no longer coordinated in a way that allowed for effective movement. The clockwork connections between the appendages could no longer be terminated and reset to keep the cell moving forward.
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So rather than brains and nerves, these single-celled creatures are controlled by networks of signaling molecules. We have previously seen how these systems can achieve surprisingly complex behaviors in microbes such as decision making, learning, and maze navigation.
“This is a really fascinating biological phenomenon, but it could also highlight more general computational processes in other cell types,” says Larson.
There is still much more to understand about the mechanistic operation of this locomotor system, but we can now add walking to the list of examples of how random molecular processes can be harnessed to create sequential behaviors.
This research was published in Current Biology.