Signatures of kin selection in a natural population of the bacteria Bacillus subtilis

Laboratory experiments have suggested that bacteria perform a range of cooperative behaviours, which are favoured because they are directed towards relatives (kin selection). However, there is a lack of evidence for cooperation and kin selection in natural bacterial populations. Molecular population genetics offers a promising method to study natural populations, because theory predicts that kin selection will lead to relaxed selection, which will result in increased polymorphism and divergence at cooperative genes. Examining a natural population of Bacillus subtilis, we found consistent evidence that putatively cooperative traits have higher polymorphism and greater divergence than putatively private traits expressed at the same rate. In addition, we were able to eliminate alternative explanations for these patterns, and found more deleterious mutations in genes controlling putatively cooperative traits. Overall, our results suggest cooperation favoured by kin selection, with an average relatedness of r=0.77 between interacting individuals.