Lay summary
The occurrence of cooperation is one of the greatest challenges for evolutionary biology. The problem is why should individuals carry out cooperative behaviours that are costly to perform but benefit others? Theory has shown that natural selection can favour cooperation if actors receive direct fitness benefits from the cooperative act, or indirect fitness benefits, whereby cooperators pass their genes to the next generation indirectly by helping relatives to reproduce. This theoretical framework has proofed extremely successful in explaining the evolution of cooperation in insects and mammals. Recently, a great variety of cooperative traits have been described in microbes. Intriguingly, it could be shown that the existing theory also successfully explains cooperation in microbes and therefore represents a general theory of social evolution. Consequently, microbial cooperative systems have opened a completely new research area because: (1) microbes offer exciting experimental possibilities to test aspects of theory that has not been possible to test with higher organisms; (2) experimental evolution approaches allow observing the evolution of cooperation in real-time; and (3) many cooperative traits are involved with virulence in infections of humans such that the understanding of their evolution has medical relevance.

This work focuses on the production of iron-scavenging siderophore molecules in the opportunistic human pathogen Pseudomonas aeruginosa. Iron is a major limiting factor for bacterial growth because most iron is in an insoluble form. In response to iron deficiency, P. aeruginosa releases siderophores to scavenge iron, making it available for bacterial metabolism. Siderophore production is a cooperative behaviour because neighbouring cells can take up iron bound to siderophore produced by others. Consequently, mutants that do not produce siderophores can avoid the metabolic cost of its production, whilst still gaining the benefit and can therefore be considered as cheats. I will use siderophore production to investigate the ecological and social conditions required for cooperation to be favoured. Specifically, I will study: (i) the cooperative properties of siderophore molecules and the resulting fitness consequences for cooperators and cheats; (ii) conduct experimental evolution studies that investigate the dynamics of cooperators and cheats under different environmental conditions; (iii) study adaptive responses of cooperators to the presence of cheats at the behavioural and genetic level. In addition, I will complement the experimental work with theoretical models to predict the evolutionary stability of siderophore production.