A single-celled organism that walks using 14 “legs” seems to control these legs with a mechanical computer made of fibres called microtubules. The finding might help explain how many other single-celled organisms engage in extraordinarily sophisticated behaviours despite having no brain or nervous system.
“If you can make a computer out of microtubules, you can make the case for looking for them in many other cell types,” says team member at the University of California, San Francisco.
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The study began when , also at UCSF, noticed that the cells he was trying to study kept getting eaten by predators that could scurry along surfaces with an extraordinary, insect-like gait. He knew Marshall was an expert on unusual microorganisms, so he got in touch.
The pair identified the predators as single-celled organisms belonging to the genus Euplotes. These predatory protists are found in both the sea and fresh water. They can swim but also walk on surfaces underwater using around about 14 cirri or legs on their underside (the number varies depending on the species).
Biologists have been baffled since the early 20th century about how single-celled creatures with no nerves can coordinate the movements of so many legs. So Larson, Wallace and their colleagues decided to study Euplotes in more detail.
Larson began by taking videos of 13 Euplotes cells walking on a glass microscope coverslip. Then he painstakingly annotated each cirrus in each video frame so the team could analyse the gait in detail. This revealed that Euplotes have a very unusual gait.
Animals with brains, such as millipedes, typically repeat the same pattern of leg movements over and over again. Euplotes‘ legs instead move in lots of different patterns.
“It doesn’t have this well-defined, exact sequence of steps that it takes, there’s a lot of variability,” says Larson.
This could be because it isn’t capable of regular, repetitive patterns of leg movement, he says, but might also be an adaption to walking on rough surfaces it may not be able to sense. Designers of walking robots typically add in some variability to prevent their machines getting stuck, Wallace points out.
However, the movement of Euplotes‘ legs isn’t entirely random either. This indicates that some internal system controls and coordinates leg movements to some extent. “There is some kind of information that is conveyed,” says Larson.
The prime candidate is the bundles of microtubules that extend internally from where each leg joins the cell and which interlink with each other. So the team added a drug known to disrupt microtubule formation to the water. Sure enough, this altered the cells’ gait, making the legs move in different patterns and leaving the Euplotes cells turning in circles.
The results suggest that the microtubule network acts as a simple kind of mechanical computer known as a .
“Our data shows you need the microtubules for the computation to happen. The simplest explanation is that those are the computing elements,” says Marshall. “Have we proven it? No.”
“It’s a very solid scientific work,” says at the University of Hamburg in Germany, who designs nanomechanical computers. “The aspects of mechanics are undervalued in biology and even in biophysics.”
The team needs to do more to show definitely that the microtubules act as a finite state machine, says Blick. “If that’s the case, it’s a possible game changer.”
Many single-celled organisms are capable of highly sophisticated behaviours. One even appears to spot prey with an eye that can move in its socket and fire a harpoon to catch them. But how creatures with no nerves or brain manage this remains an almost complete mystery.
“These behaviours are all still mysterious,” says Wallace.
Microtubules are found in all complex cells, so they might be controlling behaviour in other cells as well.
The researchers compare Euplotes to created by artist Theo Jansen. “They are the perfect paradigm,” says Wallace. “You can make these quite complex movements using mechanical linkages.”
Jansen, for his part, is also amazed by Euplotes. “Indeed it’s impressive how these small creatures walk,” he says.
Reference: bioRxiv,
