Mechanism of signal propagation in Physarum polycephalum

Abstract
Complex behaviors are typically associated with animals, but the capacity to integrate information and function as a coordinated individual is also a ubiquitous but poorly understood feature of organisms like slime molds and fungi. Plasmodial slime molds grow as networks and use flexible, undifferentiated body plans to forage within and across substrates. How an individual communicates across its network to generate sophisticated behaviors remains a puzzle, but Physarum polycephalum has emerged as a novel model and is used to explore these emergent behaviors. Cytoplasm is shuttled throughout a P. polycephalum network in a peristaltic wave driven by the cross-sectional contractions of tubes. Here, we investigate the mechanism of signal propagation within a body by first following P. polycephalum’s response to a localized nutrient stimulus. We observe a front of increased contraction amplitude that propagates with a velocity comparable to the dispersion of particles within the body advected by the cytoplasmic fluid flows. Data suggest the stimulus triggers advection of an unidentified signal that is also advected by the fluid flows and subsequently triggers changes in contraction amplitude which feed back and drive fluid flows. To test this hypothesis, we generate a theoretical model and not only find agreement with the observed front propagation, but also discover an explanation for the so far puzzling adaptation of the peristaltic wave to organism size. Finally we use our findings to explain how P. polycephalum is able to perform complex tasks, for example finding the shortest path between food sources. A simple feedback appears to give rise to P. polycephalum’s complex behaviors, and the same mechanism is likely to function within the tens of thousands of additional species with behaviors like P. polycephalum.

Host: Pieter Rein ten Wolde