slime mold builds subway

Comparison of the physarum network with the tokyo railroad network. Without the influence of light (a), the physarum network emerged from a uniform exploration of the available space. An exposure mask simulates geographic boundaries in (b) – the mushroom reacts negatively to the influence of light. (c) shows the resulting network, which looks remarkably similar to the tokyo light rail plan (d). (e) shows the smallest tree connecting all stations, in (f) additional connections are shown.

What can we learn from a unicellular organism??

Physarum polycephalum is small but fascinating. It belongs to the group of slime molds – these are single-celled organisms that have characteristics of animals and of fungi, but without being animals or fungi. Physarum has to go through three essentially different phases in his life, against which human puberty seems like child’s play.

After slime mold spores release up to four protoplasts, they develop into mononuclear protozoa that move by crawling (dry) or paddling (wet), depending on their environment. Bacteria and spores serve as food, and reproduction occurs through cell division. Bei veranderten bedingungen konnen die zellen ihre struktur auch verandern. When two cells of the same kind meet, the nuclei and cells fuse together to form a so-called plasmodium. This in turn expands more and more, with its nucleus dividing again and again. The cell eventually becomes so coarse that it is visible to the naked eye and can occupy an area of one square meter. The slimy cell (hence the name) is still mobile. Under certain conditions, it eventually begins to form fruiting bodies. The cell body forms a style with a cap on top, on which the spores are located – and the cycle begins anew.

What is new, and thus the reason for this article, is not the life cycle of physarum, but its capabilities. A few years ago, for example, a british-japanese research team succeeded in using the slime mold to pilot a robot. To do this, they exploited the fact that physarum tries to avoid the influence of light. All that was left to do was to transfer these movements to the machine, and the light-shy robot was ready. At the time, the researchers were unable to find a practical application for this, but they promised to have taken an important step towards an autonomous robot.

A bit closer to the real thing is the artifice that another japanese-british team (but not the same one) has now taught the slime mold. In the scientific magazine science the group reports about it. The fungus is used again in its second stage, when it visibly spreads on food lakes. Nature has cleverly, i.E. Sparingly, solved the way of this spreading. Physarum spreads uniformly in all directions. Behind the front line, however, the valuable cell material is no longer coarsely available. Instead, small tubes connect the food sources found in nature with each other. Additional intermediate stages reduce the time needed to transport food.

An interesting experiment showed that these purely biological mechanisms provide for a very efficient construction of the resulting network of connections. The researchers represented tokyo’s suburban train stations in the form of food piles. The fungus incorporated this into its body. Taking geography into account, the result was astonishingly similar graphs to the tokyo suburban rail network – and no less efficient. But this does not mean that the slime mold can or should replace a route planner. However, the researchers also succeeded in grasping the mathematics underlying self-organizing optimization. Thanks to the experience of evolution, the scientists hope to find more efficient structures for other self-organized networks without central control.

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