Foraging Strategies for Dictyostelium discoideum

The rate at which organisms are able to forage for food affects the rate at which they can grow, and thus foraging strategies are crucial for almost any organism. In the social amoeba Dictyostelium discoideum (Dicty), it is known that cell-cell communication plays an important role in aggregation of the cells, and thus we ask if it plays a similarly important role in foraging. In particular, we consider the role of repulsion in foraging: it has been observed that Dicty cells repel each when in the unicellular state, but the role of this repulsion is not yet understood. Here, we use computer simulations show that Dicty-Dicty repulsion improves foraging efficiency by allowing the cells to spread out more evenly and thus cover terrain more efficiently. Our results show that for a cooperative homogeneous population, repulsion leads to a significant increase in foraging efficiency; furthermore, an optimal repulsion strength for the population exists. We then show that if we introduce a cost to repulsion, non-repelling cheaters can invade such a homogeneous population. Under certain conditions, the cheaters will not take over entirely but rather enter into a stable coexistence with the repelling Dicty. In such a coexistence the repelling Dicty make up for the cost of repulsion by keeping other cells at a distance and thus obtaining more food than the non-repelling Dicty. This result suggests a possible explanation for the experimental result that Polysphondylium, another species of amoeba, is repelled by but does not repel Dicty or other Polysphondylium amoebae. In particular, our results suggest that the coexistence of Dictyostelium and Polysphondylium as repelling and nonrepelling amoebae may have evolved together as a spatial mixture of amoebae, because the non-repelling strategy is advantageous only in the presence of repelling strains.