![]() ![]() To date, the most comprehensive evolutionary study on siderophore-mediated interactions used a combination of whole-genome sequencing and phenotype screening to show that many marine Vibrio strains have lost the siderophore-synthesis cluster, but kept the receptor for uptake 26. Moreover, some strains might produce exclusive pyoverdine types, which remain inaccessible for competing non-isogenic strains because they lack a matching receptor-a scenario that could confer resistance to cheating 17, 23.Īlthough it has long been conjectured that the above-mentioned interactions could be important drivers of ecological and evolutionary dynamics in microbial communities 17, 22, 24, 25, 26, 27, there is a lack of studies that have systematically examined siderophore-mediated social interactions, and the resulting fitness consequences among natural isolates in replicated communities. Alternatively, pyoverdine non-producers could gain a foothold by exploiting foreign pyoverdines (i.e. ![]() For instance, pyoverdine-producing strains could exploit each other’s pyoverdines 19, 20. This pyoverdine-receptor diversity could facilitate different types of social interactions among co-occurring species. Moreover, while pyoverdine and its cognate receptor are typically specific, strains can also have less specific and/or several different receptors allowing the uptake of heterologous pyoverdines 16, 17, 18, 19. The different Pseudomonas strains often produce slightly different pyoverdines, varying in the length and the composition of the peptide chain 15. The molecule consists of a conserved chromophore (making this molecule naturally fluorescent), an acyl side chain linked to the chromophore, and a variable peptide chain (6–12 amino acids) 13, 14. ![]() Pyoverdine is a secondary metabolite produced via non-ribosomal peptide synthesis. ![]() Laboratory experiments have shown that pyoverdine is a public good that can be shared among cells, and be exploited by cheating mutants 2, 12. Albeit diverse, many fluorescent pseudomonads share an important trait: they can produce and secrete pyoverdine, a siderophore that scavenges insoluble or host-bound iron from the environment 11. soil, aquatic ecosystems and animal hosts) 11. Pseudomonas is a diverse genus of γ-proteobacteria, occupying a wide range of habitats (e.g. Here, we tackle this question by examining the potential for public-goods cooperation and cheating among pseudomonads from natural communities both at the genetic and behavioural level. While highly influential as a general proof of social evolution theory, a key open question is whether cheating and the public-goods dilemma also occur in natural microbial communities 8, 9, 10. This body of work has become a paradigm for the public-goods dilemma, showing how a trait that is beneficial for the group can be selected against by the spread of selfish individuals 7. Many laboratory studies focused on the problem of cheating, a scenario where mutants that no longer contribute to public goods undermine cooperation by capitalising on the public goods secreted by others 2, 3, 4, 5, 6. Such public goods include matrix components to build up biofilms, enzymes to digest food, biosurfactants for cooperative swarming and iron-scavenging siderophores 1. Perhaps the most common form of microbial cooperation is the secretion of so-called public goods-compounds that are costly to produce but generate benefits for other cells in the vicinity of the producer 1. While microbes have become model organisms to study the evolution of cooperation in laboratory settings, we still know little about the role of microbial cooperative interactions in complex natural communities. Our findings indicate that there is both selection for cheating and cheating resistance, which could drive antagonistic co-evolution and diversification in natural bacterial communities. While non-producers have genes coding for multiple pyoverdine receptors and are able to exploit compatible heterologous pyoverdines from other community members, producers differ in the pyoverdine types they secrete, offering protection against exploitation from non-producers with incompatible receptors. We find that pyoverdine non- and low-producers co-occur in many natural communities. Here, we show that social interactions mediated by a single shareable compound necessary for growth (the iron-scavenging pyoverdine) have important consequences for competitive dynamics in soil and pond communities of Pseudomonas bacteria. Although bacteria have become model organisms to study social dilemmas in laboratory systems, we know little about their relevance in natural communities. All social organisms experience dilemmas between cooperators performing group-beneficial actions and cheats selfishly exploiting these actions. ![]()
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