Associative memory is the main type of learning by which complex organisms endowed with evolved nervous systems respond efficiently to certain environmental stimuli. It has been found in different multicellular species, from cephalopods to humans, but never in individual cells. Here we describe a motility pattern consistent with associative conditioned behavior in the microorganism Amoeba proteus. We use a controlled direct-current electric field as the conditioned stimulus, and a specific chemotactic peptide as the unconditioned stimulus. The amoebae are capable of linking two independent past events, generating persistent locomotion movements that can prevail for 44 min on average. We confirm a similar behavior in a related species, Metamoeba leningradensis. Thus, our results indicate that unicellular organisms can modify their behavior during migration by associative conditioning.
Migration trajectories of Amoebae proteus under three basic and independent experimental conditions: without stimulus, galvanotaxis and chemotaxis. a Trace – without stimulus. Without stimulus the cells practically explored all the directions of the experimentation chamber. b Trace – galvanotaxis. Under galvanotaxis conditions practically all the amoebae migrated towards the cathode. c Trace – chemotaxis. Under chemotaxis conditions, 86% of the cells migrated towards the chemotactic gradient. d Plot of displacement angle for a. Distribution of displacement angles (i.e., the angle formed between the origin and the end of the movement, measured in radians) for the trajectories without stimulus (a). No preference towards a certain direction was appreciated. e Plot of displacement angle for b. Distribution of the cosines of displacement angles for the trajectories under galvanotaxis (b). 100% of the displacement cosines were bigger than 0, indicating a strong directionality towards the cathode. f Plot of displacement angle for c. Distribution of the cosines of displacement angles for trajectories under chemotaxis (c). “N” total number of cells, “Er” experimental replications, “nr” number of cells per replication, “t” time of galvanotaxis or chemotaxis, “p” chemotactic peptide (nFMLP), “ + ” anode, “−” cathode. Both the x and y axis show the distance in mm, and the initial location of each cell has been placed at the center of the diagram.