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

## TL;DR

This study shows how population bottlenecks influence the evolution of cooperation in bacteria by favoring different traits under varying conditions.

## Contribution

The study reveals that different bottleneck sizes selectively favor specific cooperative traits in Myxococcus xanthus through life cycle trade-offs.

## Key findings

- Stringent bottlenecks favor fruiting body formation and growth, while relaxed bottlenecks promote predation and germination.
- Relaxed bottlenecks maintain trait variation and coexistence of cooperative and exploitative strategies.
- Genomic analyses identified mutations in regulatory proteins linked to adaptations under different bottleneck conditions.

## Abstract

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.

Population bottlenecks have long been known to stabilize cooperation. However, we show that repeated bottlenecks affect different cooperative traits in distinct ways. Using an experimental evolution approach to study the effect of population bottleneck sizes on four social traits in Myxococcus xanthus—sporulation, germination, predation, and growth—we show that stringent bottlenecks favor sporulation and growth, while relaxed bottlenecks instead promote predation and germination. Relaxed bottlenecks maintained heterogeneity with coexisting cooperators and exploiters, while stringent bottlenecks produced homogeneous cooperator populations. We also identify mutations in key regulatory proteins driving these adaptations. Furthermore, we show that such trade-offs between social traits are common in natural populations of M. xanthus. Taken together, we demonstrate how population bottlenecks shape the evolution of life history strategies in M. xanthus.

Population bottlenecks shape the evolution of cooperative traits in Myxococcus xanthus through life cycle trade-offs. This study shows that stringent bottlenecks favor growth and sporulation, while relaxed bottlenecks favor predation and germination, revealing how distinct evolutionary pressures influence cooperation across different environmental contexts.