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

Laboratory experiments have suggested that bacteria perform a range of cooperative behaviors, which are favored because they are directed toward 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 the 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 that cooperation is favored by kin selection, with an average relatedness of r = .79 between interacting individuals.Lay SummaryBacteria produce and secrete a wide range of molecules. The benefits of these molecules can be shared by nearby bacterial cells. For example, secreted molecules that deactivate certain antibiotics provide protection to the whole group, including cells that do not produce the molecule themselves. Laboratory experiments have shown that this is a form of cooperation, which evolves because it benefits closely related cells that share the gene for cooperation (kin selection). However, it has been challenging to find evidence for cooperation and kin selection in natural bacterial populations. To address this, we have used a new way of studying cooperation in natural populations. Evolutionary theory tells us that cooperation leaves a distinct footprint in DNA sequence data. We examined the bacterial genomes of the soil-dwelling bacteria Bacillus subtilis taken from a natural population in Dundee, Scotland, and used this theory to look for evidence of cooperation in the DNA sequences. Our results suggest that these shared molecules are indeed cooperative traits that have been favored by kin selection in natural populations.

Automated summary

Laboratory experiments have shown that bacteria exhibit cooperative behaviors directed towards relatives, but natural bacterial populations lack evidence for cooperation and kin selection. By using molecular population genetics, a study of a natural population of Bacillus subtilis found evidence supporting kin selection and cooperative traits through increased polymorphism and divergence at cooperative genes. This study also eliminated alternative explanations and found more deleterious mutations in genes controlling cooperative traits.