Ecology drives the evolution of diverse siderophore-production strategies in the opportunistic human pathogen 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 on how bacterial cooperation evolves under conditions where cheating is unlikely 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 hyper-producers. Similarly, hyper-producers also spread in populations evolved in highly viscous media, where relatedness between interacting individuals is higher. Whole-genome sequencing of evolved clones revealed that hyper-production is associated with mutations/deletions in 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 at one trait (pyoverdine) can be associated with the loss (quorum-sensing) of another social trait.

Fluctuating Environment Can Negate Cheater Success Due to Speed-Agility Trade-Off

“…Additionally, grouping together of cooperative cells, facilitated by spatial structuring, can induce a positive, density-dependent, local effect, with cooperative cells passively forming ‘exclusivity zones’ that halt further proliferation of cheating phenotypes (Bachmann et al, 2013; Cavaliere et al, 2017; Folse III & Allison, 2012; Momeni et al, 2013; Nadell et al, 2009; Stump et al, 2018; Van Dyken et al, 2013). This spatial effect can also be made apparent through non-spatially explicit factors such as viscosity of growth media, with higher viscosity favouring cooperative phenotypes by lowering cell dispersal and public goods diffusion (Figueiredo et al, 2021; Kümmerli et al, 2009). Cheater access to public goods can also be actively undermined via enforcement.…”

“…Cheater access to public goods can also be actively undermined via enforcement. Enforcement mechanisms reward fellow cooperators and/or punish cheaters (Figueiredo et al, 2021; Ho et al, 2013; Schluter et al, 2015; Smukalla et al, 2008; Travisano & Velicer, 2004). An example of enforcement is that of the rhizobia system in root nodules.…”

“…Similarly, Dobay et al (2014) emphasised that interactions between key parameters of public goods cooperation give rise to more complex fitness landscapes, and thus that a further understanding of cooperation in the face of cheating requires multifactorial approaches. Undescribed interactions between multiple cheater-limiting mechanisms (and other (a)biotic processes) may weigh into the many cost-benefit trade-offs involved in microbial community interaction dynamics, and hence alter a cheater’s selective landscape, keeping the role of cheating in natural microbial systems smaller than previously expected (Figueiredo et al, 2021; Waite & Shou, 2012). These additional undescribed and often context-dependent costs thus subject the selective landscape of the cheater strategy to the Cheater’s Dilemma: a set of trade-offs that mitigate the selective benefit imparted by reduced investments in public goods.…”