Slimy, smart and naked

What allows the network-forming slime mold Physarum polycephalum to find the shortest path through a maze - despite lacking a nervous system? The key is a simple feedback: the slime mold sends information in the form of signaling molecules throughout its network of veins. Signaling molecules are transported by flowing fluids and cause fluid flow to increase. This positive feedback loop, on the one hand, speeds up information transfer, but at the same time fosters the growth of veins, precisely those that are tracing the shortest path between stimuli. With their project, the scientists from Harvard, Madison and the Max Planck Institute for Dynamics and Self-Organization, illuminate the mystery behind the intelligent slime mold. They hope that the feedback mechanism found in nature may in future help develop artificial systems permitting self-organizing adaptation.

HFSP Program Grant holders Michael Brenner and Anne Pringle and colleagues
authored on Thu, 08 June 2017

Without a brain, but smart!

Complex behavior is typically associated with animals. But apparently simple organisms like slime molds and fungi can process information and coordinate organism-wide decisions. The slime mold Physarum polycephalum uses its network-forming body to solve complex problems, for example finding the shortest route between food sources, even though the organism lacks any neural circuitry and is growing as a single cell.

Figure: A small drop of liquid nutrient stimulant on the network of a slime mold, Physarum polycephalum. Despite the lack of any neural circuitry, P. polycephalum can solve complex problems. Alim et al. use the network’s responses to stimuli to develop a mathematical model and identify a simple mechanism underpinning the organism’s emergent, complex behaviors. © Natalie Andrew.

Within the slime mold liquid cytoplasm is shuttled back and forth through its network of veins. Cross-sectional contractions of veins drive the rhythmic, peristaltic flows. To identify how information is sent around this network, the scientists first tracked the slime mold’s response to a localized nutrient stimulus and observed an increase in contractions that spreads out throughout the network. The increase in contractions propagates with a velocity comparable to the flow-driven dispersion of particles. The team built a mathematical model based on these observations and identified the mechanism of information transfer in the slime mold: the nutrient stimulus triggers the release of signaling molecules. The molecules are spread by the fluid flow but simultaneously control flow generation by causing local increases in contraction, which move with the flow. The molecule is initiating a feedback loop to promote its own movement. This very simple mechanism explains previously puzzling phenomena, including the adaptation of the peristaltic flows to organism size and the slime mold’s ability to find the shortest route between food sources.

“The mechanism of communication is based on a signaling molecule, fluid flows, and an interaction between the signal and fluid flows,” says Karen Alim first author of this study. “The mechanism is likely to be a general one and may serve as a broad explanation for the complex behaviors of many organisms without nervous systems. For us, as scientists, it is fascinating to transfer this mechanism of communication to technological applications to implement self-organized adaptation.”

Reference

Mechanism of signal propagation in Physarum polycephalum, Karen Alim, Natalie Andrew, Anne Pringle, and Michael P. Brenner, Proc. Natl. Acad. Sci. U.S.A.,114(20), 5236-5141 (2017), doi: 10.1073/pnas.1618114114.

PubMed link