Differential evolution of cooperative traits in aggregative multicellular bacterium Myxococcus xanthus driven by varied population bottleneck sizes

Repeated population bottlenecks influence the evolution and maintenance of cooperation1,2. However, it remains unclear whether bottlenecks select all cooperative traits expressed by an organism or only a subset of them. Myxococcus xanthus, a social bacterium, displays multiple cooperative traits, including growth, predation, sporulation in multicellular fruiting bodies, and germination 3-6. Using laboratory evolution experiment, we investigated the effect of repeated stringent versus relaxed population bottlenecks on the evolution of these four cooperative traits when they were all under selection. We found that only fruiting body formation and growth were positively selected under the stringent regimen, while the other two traits were negatively selected. The pattern was reversed in the relaxed regimen. Additionally, the relaxed regimen led to a significant increase in fitness when competed against ancestors across the entire lifecycle, whereas the stringent treatment did not change competitive fitness. Genomic analysis revealed that mutations in {sigma}54 interacting protein and DNA-binding response regulator protein are linked with the changes observed in stringent and relaxed regimens respectively. Further, similar trade-offs are also seen among natural populations of M. xanthus. Overall, we demonstrate that different bottleneck sizes drive the evolution of lifecycles in distinct manners, driven by trade-offs between cooperative life history traits.