Non-kin interactions between Bacillus subtilis soil isolates limit the spread of swarming deficient cheats

Cooperative behaviors in human, animal, and even microbial societies are vulnerable to exploitation. Kin discrimination has been hypothesized to help stabilize cooperation. However, the mechanisms that sustain cooperative behavior remain poorly understood. Here, we investigate the role of kin discrimination in limiting the spread of cheats in adjoining populations during surfactant dependent cooperative swarming over surfaces using the bacterium Bacillus subtilis as a model organism. We show that mixing surfactant secreting cooperators and cheats that do not produce surfactants at 1:1 initial ratio quickly leads to cooperation collapse. However, when such common swarms encounter non-kin B. subtilis swarms, the proportion of the surfactant non-producers decreases, suggesting that kinship dependent interactions may limit cheats’ advantage in an adjoining population. To further validate this finding, we subjected wild-type cooperators to multiple transient encounters with kin and non-kin swarms over 20 cycles of experimental evolution. The evolved populations exposed to non-kin swarms less frequently contained defective swarming phenotypes compared to those encountering kin swarms. Altogether, our results support the prediction that the spread of cheats in an adjoining bacterial population is impeded by kin discrimination interactions which might have a role in stabilizing cooperative behavior in evolving populations.