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

by Mirna Elizabeta Kramar
Doctoral thesis
Date of Examination:2020-06-12
Date of issue:2020-06-22
Advisor:Dr. Karen Alim
Referee:Dr. Karen Alim
Referee:Dr. Florian Rehfeldt
Referee:Dr. Hans-Günther Döbereiner
Persistent Address: http://dx.doi.org/10.53846/goediss-8052

 

 

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Abstract

English

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 network-like 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.
Keywords: behaviour; decision making; flow networks; memory; self-organization
 
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