Bottleneck size drives the evolution of cooperative traits in an aggregative multicellular myxobacterium

Repeated population bottlenecks influence the evolution and maintenance of cooperation. 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. Using laboratory evolution experiments, 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 selected against. The pattern was reversed in the relaxed regimen. Populations propagated under the relaxed regimen also exhibited greater fitness across the entire life cycle and maintained higher trait variations, including coexistence of cooperative and exploitative strategies. Genomic analyses identified mutations in σ54 interacting protein and DNA binding response regulator protein associated with adaptations in stringent and relaxed regimens, respectively. Furthermore, similar trade-offs, for example, between sporulation and germination, are also seen among natural populations of M. xanthus. Overall, we demonstrate that different bottleneck sizes drive the evolution of cooperative life history traits in distinct ways, often via trade-offs that constrain their joint optimization.