How to have memories without having a brain
A single-celled slime mold has no nervous system, but remembers food locations
Having a memory of past events enables us to take smarter decisions about the future. Researchers from the Max Planck Institute for Dynamics and Self-Organization (MPIDS) and Technical University of Munich (TUM) identify the basis for forming memories in the slime mold Physarum polycephalum - despite its lack of a nervous system.
The ability to store and recover information gives an organism a clear advantage when searching for food or avoiding harmful environments, and has been traditionally linked to organisms that have a nervous system. A new study authored by Mirna Kramar (MPIDS) and Prof. Karen Alim (TUM and MPIDS) challenges this view by uncovering the surprising abilities of a highly dynamic, single-celled organism to store and retrieve information about its environment.
Window into the past
The slime mold Physarum polycephalum has been puzzling researchers for many decades. Existing at the crossroads between the kingdoms of animals, plants and fungi, this unique organism provides insight into the early evolutionary history of eukaryotes. Its body is a giant single cell made up of interconnected tubes that form intricate networks. This single amoeba-like cell may stretch several centimeters or even meters, featuring as the largest cell on earth in the Guinness Book of World Records.
The striking abilities of the slime mold to solve complex problems such as finding the shortest path through a maze earned it the attribute “intelligent”, intrigued the research community and kindled questions about decision making on the most basic levels of life. The decision-making ability of Physarum is especially fascinating given that its tubular network constantly undergoes fast reorganization - growing and disintegrating its tubes - while completely lacking an organizing center. The researchers discovered that the organism weaves memories of food encounters directly into the architecture of the network-like body and uses the stored information when making future decisions.
Decisions are guided by memories
“It is very exciting when a project develops from a simple experimental observation”, says Karen Alim, head of the Biological Physics and Morphogenesis group at the MPIDS and TUM Professor on Theory of Biological Networks, “We followed the migration and feeding process of the organism and observed a distinct imprint of a food source on the pattern of thicker and thinner tubes of the network long after feeding. Given P. polycephalum's highly dynamic network reorganization, the persistence of this imprint sparked the idea that the network architecture itself could serve as memory of the past. However, we first needed to explain the mechanism behind the imprint formation.”
To find out what is going on, the researchers combine microscopic observations of the adaption of the tubular network with theoretical modeling. An encounter with food triggers the release of a chemical that travels from the location where food was found throughout the organism and softens the tubes in the network, making the whole organism reorient its migration towards the food.
“The gradual softening is where the existing imprints of previous food sources come into play and where information is stored and retrieved”, says Mirna Kramar, first author of the study. “Past feeding events are embedded in the hierarchy of tube diameters, specifically in the arrangement of thick and thin tubes in the network. For the softening chemical that is now transported, the thick tubes in the network act as highways in traffic networks, enabling quick transport across the whole organism. Previous encounters imprinted in the network architecture weigh into the decision about the future direction of migration.”
Universal principles inspire design
The authors highlight that the ability of Physarum to form memories is intriguing given the simplicity of this living network. “It is remarkable that the organism relies on such a simple mechanism and yet controls it in such a fine-tuned way. These results present an important piece of the puzzle in understanding the behavior of this ancient organism and at the same time point to universal principles underlying behavior. We envision potential applications of our findings in designing smart materials and building soft robots that navigate through complex environments”, concludes Karen Alim.