Exploiting new terrain: an advantage to sociality in the slime mold Dictyostelium discoideum

Abstract

Understanding the ecological benefits of social actions is central to explaining the evolution of social behavior. The social amoeba Dictyostelium discoideum has been well studied and is a model for social evolution and development, but surprisingly little is known about its ecology. When starving, thousands of the normally solitary amoebae aggregate to form a differentiated multicellular organism known as a slug. The slug migrates toward the soil surface where it metamorphoses into a fruiting body of hardy spores held up by a dead stalk comprising about one-fifth of the cells. Multicellularity in D. discoideum is thought to have evolved to lift the spores above the hazards of the soil where spores can be picked up for long-distance dispersal. Here, we show that multicellularity has another advantage: local dispersal to new food sources. We find that cells shed by D. discoideum slugs during migration consume and remove bacteria in the path of the slug, although slugs themselves do not breakup. We also show that slugs are adept at local dispersal by comparing migration of slugs with migration of individual cells of the mutant, CAP2, which cannot aggregate and so rely only on cellular movement. In particular, the solitary cells of the aggregation mutant are unable to cross a soil barrier, easily crossed by slugs. We propose that the exploitation of local food patches is an important selective benefit favoring multicellular cooperation in D. discoideum. Key words: altruism, Dictyostelium discoideum, multicellularity, social evolution, sociality. [Behav Ecol]