The rise of cheats during experimental evolution is restricted by non-kin interactions between Bacillus subtilis soil isolates

Abstract

Cooperative behaviors in human, animal, and even microbial societies are vulnerable to exploitation. Kin discrimination (KD) has been hypothesized to help stabilize cooperation. However, the mechanisms that sustain cooperative behavior remain poorly understood. We here investigate the role of KD in limiting the rise of cheats during surfactant dependent cooperative swarming over surfaces by bacterium Bacillus subtilis as a model organism. We show that mixing surfactant secreting cooperators and cheats that do not produce surfactants leads to cooperation collapse. However, when such mixed swarms transiently encounter non-kin B. subtilis swarms, the frequency of the surfactant nonproducers decreases, suggesting that kinship dependent interactions may limit cheats advantage. To further validate this hypothesis, we subjected wild-type co-operators to transient encounters with kin and non-kin swarms over 20 cycles of experimental evolution. Evolved populations exposed to non-kin swarms exhibited lower occurrences of genotypes with defective swarming phenotypes compared to those encountering kin swarms. These results provide compelling support for the prediction that the evolution of cheats in bacterial populations is impeded by kin discrimination providing experimental proof of its role in stabilizing cooperative behavior.