Topological Phase Transitions and Foraging in the Slime Mold Physarum polycephalum

点击翻译按钮获取中文摘要

The true slime mold Physarum polycephalum grows as an amoeboid plasmodium containing multiple nuclei. It can reach a size up to square meters and usually forms an extended transport network as a foraging strategy. The structure and dynamics of foraging units are dependent on environmental conditions and corresponding life stage. Physarum polycephalum transportation networks have been shown to possess the ability to solve complex tasks like maze solving and network optimization in terms of path length and efficiency. In addition, these networks appear to exhibit certain learning capabilities while exploring their environment. It is largely unknown how this emergent behavior develops from basic cellular processes. We have designed experiments and theoretical analyses in order to bridge the gap between the behavior of the organism and the local dynamics of its effective soft matter machinery. Briefly, we disrupt a plasmodium by shear to create disconnected microplasmodia (MPs), which strive to reunite when placed in patches on a 2-dimensional agar substrate. Given abundant nutrients, these MPs fuse in a percolation transition directly into an extended network. In contrast, under specific starvation conditions MPs combine first into satellites, which radiate away from the original patch. Subsequently, these satellites develop numerous holes and transition into a network. The final state of the organism in both scenarios is the same. Both pathways involve a sequence of topological transitions where the number of nodes and links changes in a characteristic way. It is the aim of this proposal to describe the statics and dynamics of these processes. The plasmodium network structure in the vicinity of the respective transitions and beyond will be characterized using elements from graph theory and scaling analysis. Global network dynamics will be modeled by a Master equation. We will perform experiments as a function of nutrient history, initial density, nutrient content and substrate stiffness. The plasmodium is comprised of external or internal veins serving as (peristaltic) transport pipelines for nutrients and chemical signals as well as general cellular material. We aim to unravel the reciprocal interaction of cytosolic flow within veins with geometry and topology of the network. This requires an overview of global network structure and microscopic details of cytoskeletal organization including flow patterns within veins. A motorized zoom microscope equipped with an environmental chamber allows prolonged observation at high temporal resolution with alternating magnification. It will enable us to link global network structure and dynamics orchestrating foraging behavior to local intra cellular interactions.