Although myxomycetes (plasmodial slime molds) are unicellular organisms, they are distinguished by a unique ecological feature in that they grow to huge sizes while remaining a single cell. Even more astonishingly, they can accurately distinguish self from non-self. Even within the same species, plasmodia from different localities may spend several hours assessing one another and, if they are incompatible for fusion, move apart without fusing. They can also distinguish compatible from incompatible individuals without direct contact, apparently using signaling substances in the secreted slime sheath. Understanding allorecognition (self–non-self recognition) in myxomycetes may help clarify the evolutionary origins of multicellularity and what constitutes “self” in an organism.

The yellow, jelly-like bodies that spread over fallen trees and humus are plasmodial slime molds (myxomycetes). These unicellular organisms with neither a brain nor a nervous system solve mazes through the shortest path, forming networks that closely resemble the Kanto region’s rail network. Professor Toshiyuki Nakagaki of Hokkaido University won two Ig Nobel Prizes after demonstrating this behavior in experiments, and is also the author of Kashikoi Tansaibō: Nenkin [Slime Molds: Smart Single-Celled Organisms], a picture book for children that illustrates the wonder of these organisms. Plasmodial slime molds do not “think.” However, the accumulation of local rules produces the optimal solution. This phenomenon leads to the fundamental question of what intelligence actually is.