Many Cells Make Light Work: Self-Generated Gradients Organize <i>Dictyostelium</i> Aggregates and Neutrophil Swarms

Cells work together to accomplish complex tasks. For example, both neutrophils and Dictyostelid collectives use self-generated multicellular signaling gradients to coordinate aggregation over large areas through local interactions. However, these aggregation programs occur for different reasons that necessitate different implementations. Dictyostelids are soil-dwelling amoeba that aggregate when starving to facilitate dispersal to new locations. These aggregates do not require specific locations or group sizes. In contrast, neutrophils are innate immune cells that collectively migrate to sites of injury and infection. These swarms need to occur in specific locations and must be constrained in size to avoid collateral damage to the host. Here, we review how these evolutionarily divergent systems sculpt long-range gradients at the molecular and cellular levels, discussing their similarities and differences in light of their distinctive goals. Convergence on self-generated gradients for aggregation despite different goals suggests that it is an optimal strategy to bring individuals together in complex environments.