This brainless mud mold can decide where to go without having been there before

A slippery yellow slime that lives in the damp undergrowth continues to test our understanding of what it means to make decisions.

Polycephalic physarum, AKA the multi-headed slime mold, uses its body to physically sense its surroundings before making a decision about where it wants to go, new research shows. This is the latest in an impressive list of ways the single-celled organism has blown us away in recent times.

“People are more and more interested Physarum because it doesn’t have a brain, but it can still perform many of the behaviors we associate with thinking, ”said neuroscientist Nirosha Murugan of Algoma University in Canada.

“Finding out how proto-intelligent life does this kind of calculation gives us a better insight into the foundations of cognition and behavior in animals, including our own. “

P. polycephalum is a curious little organism. It’s not a mushroom at all; neither is it an animal or a plant. It belongs to the Protist kingdom – basically anything that does not belong to the other three kingdoms. It lives in dark, humid environments like forest soils, helping to break down organic matter and recycle it back into the food web.

Physarum begins its life as so many individual cells, each with its own nucleus. These cells fuse to form Plasmodium, a large single cell containing millions or even billions of nuclei swimming in cytoplasmic fluid. This is the vegetative stage of life, during which the protist moves, feeds and grows.

It is also the stage where P. polycephalum exhibits curious behaviors. Scientists watched him solve mazes and remember new substances for months. He can also remember places he has found food before and share memories with other slime mold spots. It’s pretty amazing for something that doesn’t have a brain or a nervous system.

Most of the previous research has involved some sort of inducement, such as a chemical the protist dislikes, light, or a food reward. Muruga and his team wanted to know how P. polycephalum makes decisions in the absence of these clues, based solely on his physical environment, so they designed an experiment to find out.

(Nirosha Murugan, Levin lab, Tufts University and Wyss Institute of Harvard University)

They placed samples of P. polycephalum in Petri dishes, on a natural agar gel. On one side of the dish, a single small glass disc was placed. On the other, three glass discs were placed side by side. The dishes were placed in a dark room – slime mold’s preferred lighting setting – and left on their own.

During the first 12 hours or so, P. polycephalum grows evenly in all directions. Then, after 24 hours, 70 percent of the samples had all grown to all three disks instead of one.

Further experimentation revealed even more quirks. When the three discs were stacked on top of each other, rather than side by side, the slime mold lost its preference, growing to one side or the other at roughly the same rate. This seemed to suggest that it was not the lump alone causing the slimy mold preference for the three discs side by side.

The additional factor was revealed by computer modeling. When placed side by side on the elastic agar gel, the three discs deformed the gel differently than when placed in a stack, much like three weights placed next to each other on a trampoline will cause a different stress model than a stacked weight.

This stress model in the agar gel, the team determined, is what P. polycephalum Head towards.

“Imagine you are driving on the highway at night and looking for a city to stop. You see two different arrangements of light on the horizon: a single bright spot and a group of dim spots. While the single dot is brighter, the cluster of dots illuminates a larger area which is more likely to indicate a city, ”said engineer Richard Novak of the Wyss Institute.

“The light models in this example are analogous to the mechanical stress models produced by different mass arrangements in our model.”

(Nirosha Murugan, Levin lab, Tufts University and Wyss Institute at Harvard University)

Given that P. polycephalum does not have a nervous system, the next question was, of course, how is slime mold able to detect this stress pattern. It turns out that it is about the movement and the internal communication of the body.

The cytoplasm inside P. polycephalum is not static, but moves in pulses. The walls of its veins contract to act like a peristaltic pump, pushing fluid from one area to another. Other animals, such as mammals, have molecules called TRP proteins in their cell membranes that can sense stretching.

The researchers therefore decided to donate P. polycephalum a TRP blocker to see what would happen.

Indeed, the protist has lost its ability to distinguish between a glass disc and three glass discs side by side. In a new test, 71% of the samples moved to both sides of the Petri dish. This suggests that something like TRP proteins are at play in P. polycephalum.

“Our discovery of the use of biomechanics by this slimy mold to probe and respond to its surrounding environment highlights how early this ability evolved in living organisms and how closely intelligence, behavior, and morphogenesis are intertwined. related, “said biologist Mike Levin of the Wyss Institute.

“This work in Physarum proposes a new model … to explore the ways in which evolution uses physics to implement the primitive cognition that determines form and function. “

The research was published in Advanced materials.