Choice of a unicellular organism: Physarum polycephalum

Abstract

Physarum polycephalum, a slime mould, is a unicellular organism with unique biological characteristics. One of its characteristics is the plasmodium, a vegetative state of P. polycephalum, which can be fragmented into microplasmodia and fuse back while maintaining its integrity as a unicellular organism. Starvation and a lack of glucose resulted in the formation of globular motile bodies that radially expanded, named 'Satellites', from microplasmodia, instead of a network. How these isolated fragments coordinate and reconstitute larger structures, and how this occurs without a centralized control were the main focus of this thesis. Two different approaches were taken to investigate satellite growth. The first approach was to construct models to describe the satellite growth pattern with physical parameters. The distance travelled by the satellites showed a saturating increase and was consistent between the satellites from the same patch. Therefore, negative chemotaxis from a signal molecule was considered as a mechanism of propagation. The model was constructed based on diffusion of a signal molecule from the patch, and the length scale of the diffusion front as a function of the patch size matched the displacement of the satellites. Using this model, the diffusion coefficient of the signal molecule was calculated, which was within the range of known biological signalling molecules. Also, a scaling relationship was derived based on the maximization principle, assuming that a starving organism would maximize the search area. This assumption is influenced by the optimal foraging theory, which predicts the foraging behaviour of organisms by assuming that the net energy intake is maximized. This scaling was refined with a fusion probability function, calculated based on diffusion and a typical distance between microplasmodia, as well as possible collisions between satellites. The refined scaling equations accurately described the observed number and the sizes of satellites. Based on these models, it is shown that a unicellular organism maximizes the search area even when they are fragmented, and uses a signal molecule to coordinate their behaviour.