Ecology drives the evolution of diverse social strategies in Pseudomonas aeruginosa

Abstract

Bacteria often cooperate by secreting molecules that can be shared as public goods between cells. Because the production of public goods is subject to cheating by mutants that exploit the good without contributing to it, there has been great interest in elucidating the evolutionary forces that maintain cooperation. However, little is known about how bacterial cooperation evolves under conditions where cheating is unlikely to be of importance. Here we use experimental evolution to follow changes in the production of a model public good, the iron‐scavenging siderophore pyoverdine, of the bacterium Pseudomonas aeruginosa. After 1200 generations of evolution in nine different environments, we observed that cheaters only reached high frequency in liquid medium with low iron availability. Conversely, when adding iron to reduce the cost of producing pyoverdine, we observed selection for pyoverdine hyperproducers. Similarly, hyperproducers also spread in populations evolved in highly viscous media, where relatedness between interacting individuals is increased. Whole‐genome sequencing of evolved clones revealed that hyperproduction is associated with mutations involving genes encoding quorum‐sensing communication systems, while cheater clones had mutations in the iron‐starvation sigma factor or in pyoverdine biosynthesis genes. Our findings demonstrate that bacterial social traits can evolve rapidly in divergent directions, with particularly strong selection for increased levels of cooperation occurring in environments where individual dispersal is reduced, as predicted by social evolution theory. Moreover, we establish a regulatory link between pyoverdine production and quorum‐sensing, showing that increased cooperation with respect to one trait (pyoverdine) can be associated with the loss (quorum‐sensing) of another social trait.

Evolution of the quorum sensing systems in Pseudomonas aeruginosa can involve both loss of regulon function and network modulation

“…In our case, the QS mutants emerged from wild type populations that had been experimentally evolved in casamino acid medium, predominantly consisting of digested amino acids, an environment in which classic QS-regulated traits such as proteases and rhamnolipids are not needed. Thus, disuse is a plausible explanation for the selective spread of these mutants, even more so because the loss of QS fostered increased production of the siderophore pyoverdine, which was beneficial in the context of the initial study (Figueiredo, Wagner and Kümmerli, 2021). Taken together, whether loss of the Las system is caused by disuse or cheating is often context-dependent and spurred by the relative costs and benefits of the QS system in the respective environment.…”

“…We analyzed a collection of 61 experimentally evolved P. aeruginosa clones from Figueiredo, Wagner and Kümmerli (2021) (Table S1). All clones have a common ancestor, the PAO1 wild type strain (ATCC 15692).…”

“…The protocol of the experimental evolution study is described in detail elsewhere (Figueiredo, Wagner and Kümmerli, 2021). Briefly, experimental cultures were initiated with ancestral PAO1, and evolving populations were propagated for 200 consecutive days, during which approximately 1,200 generations occurred.…”

“…While these environmental conditions were important for the initial study design, they do not serve purpose for the current study, as QS mutants arose in all nine environments. Figueiredo et al (2021) sequenced the whole genome of 119 evolved clones, among which 61 (51.2%) had mutations in genes of the QS regulons. Experimentally evolved clones were sequenced on the Illumina NovaSeq6000 platform (paired-end, 150 base-pair reads).…”

“…To obtain a deeper understanding of how mutations in the QS regulon affect downstream phenotypes and QS network topology, we used a set of 61 experimentally evolved QS mutants to investigate whether these mutants have lost the ability to produce QS-regulated traits (supporting either the first or the second hypotheses) or if they show an altered QS-regulated trait expression portfolio (supporting the third hypothesis). The mutant collection stems from an experimental evolution study performed in our laboratory that focused on the evolution of iron uptake systems under various in vitro conditions (Figueiredo, Wagner and Kümmerli, 2021). The experiment was initiated with P. aeruginosa PAO1 wild type populations and ran for 200 consecutive days.…”