The effect of cheats on siderophore diversity in Pseudomonas aeruginosa

Cooperation can be maintained if cooperative behaviours are preferentially directed towards other cooperative individuals. Tag-based cooperation (greenbeards) - where cooperation benefits individuals with the same tag as the actor - is one way to achieve this. Tag-based cooperation can be exploited by individuals who maintain the specific tag but do not cooperate, and selection to escape this exploitation can result in the evolution of tag diversity. We tested key predictions crucial for the evolution of cheat-mediated tag diversity using the production of iron-scavenging pyoverdine by the opportunistic pathogen, Pseduomonas aeruginosa as a model system. Using two strains that produce different pyoverdine types and their respective cheats, we show that cheats outcompete their homologous pyoverdine producer, but are outcompeted by the heterologous producer in well-mixed environments. As a consequence, co-inoculating two types of pyoverdine producer and one type of pyoverdine cheat resulted in the pyoverdine type whose cheat was not present having a large fitness advantage. Theory suggests that in such interactions, cheats can maintain tag diversity in spatially structured environments, but that tag-based cooperation will be lost in well-mixed populations, regardless of tag diversity. We saw that when all pyoverdine producers and cheats were co-inoculated in well-mixed environments, both types of pyoverdine producers were outcompeted, whereas spatial structure (agar plates and compost microcosms), rather than maintaining diversity, resulted in the domination of one pyoverdine producer. These results suggest cheats may play a more limited role in the evolution of pyoverdine diversity than predicted.