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Flows, morphology, and memory: study of a living network


Kramar,  Mirna
Max Planck Research Group Biological Physics and Morphogenesis, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society;

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Kramar, M. (2020). Flows, morphology, and memory: study of a living network. PhD Thesis, Georg-August-Universität, Göttingen.

Cite as: https://hdl.handle.net/21.11116/0000-0008-FC03-9
The complex behaviour of the slime mould Physarum polycephalum, a simple eukaryote, has
been puzzling researchers since its discovery. The giant unicellular, but multinucleate organism
is highly successful at tackling complex environments, despite being a very simple life
form. The organism forages for nutrients and flees from threat by reorganizing its networklike
body made of actomyosin-lined tubes. The tubes undergo periodic contractions, causing
a shuttle flow of the cytoplasm inside the tubes which in turn transports nutrients, signals and
redistributes body mass. Often termed intelligent, the organism displays behaviours usually
found in higher species with a nervous system.
In this thesis, we aim to uncover the governing principles behind several phenomena from
P. polycephalum’s abundant repertoire of behaviours. First, we delve into the memory encoding
abilities by studying how the network imprints the location of a nutrient source. Using
theoretical and experimental methods, we show that the nutrient stimulus triggers a release of
a tube-softening chemical agent. The propagation of the agent released at the stimulus site
is flow-based, causing tube dilation downstream. We show that the organism relies on the
hierarchy of the tube diameters in its network to encode and read out memories. Next, we
break down the complex oscillation dynamics of P. polycephalum in pursuit of characteristic
contraction patterns. We decompose the time series of tube contractions and identify combinations
of oscillation patterns that correspond to stereotyped behaviours, such as locomotion
and reaction to nutrient stimuli. Then, we turn to studying the role of calcium, the universal
signalling agent. By establishing experimental protocols for measuring and quantifying calcium
dynamics, we lay the groundwork for investigating calcium-related phenomena in the
plasmodial network. Finally, we focus on the wound healing response in P. polycephalum.
We analyze the contraction dynamics upon mechanical severing of the network and find a
multi-step pattern of tube oscillations accompanying the process of wound healing.
With this work, we uncover previously unidentified functioning principles of the slime
mould P. polycephalum, thereby contributing to the understanding of the apparent intelligent
behaviour of the organism.