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The evolution of cooperative behaviour: Experimental tests of adaptive explanations and development of evolutionary theory

Final Activity Report Summary - EVOLUTIONOFSOCIALITY (The evolution of cooperative behaviour: Experimental tests of adaptive explanations and development of evolutionary theory)

The occurrence of cooperation is one of the greatest challenges for evolutionary biology. The problem is why should an individual carry out a cooperative behaviour that is costly to perform, but benefits other individuals? To date, most of the research on cooperation has been focussed on social insects and vertebrates. However, a great variety of cooperative behaviours have recently been discovered in microbes and it is mainly unknown whether the theoretical framework developed for higher organisms also applies to microbes.

In this project we investigated siderophore production - a cooperative behaviour - in the bacterium Pseudomonas aeruginosa (an opportunistic human pathogen). Siderophores are agents produced by bacteria in response to iron deficiency. They are released into the environment to get hold of insoluble iron making it available for bacterial metabolism. Siderophore production is a cooperative behaviour because it is costly for the individual to produce, but provides a group benefit, as other individuals can also take up the siderophore-iron complex. This cooperative behaviour is exploitable by individuals who do not produce siderophores, whilst still gaining the benefit by taking up siderophores produced by others. Such non-siderophore producing individuals can be regarded as cheats.

We carried four different projects to investigate the biology of cooperating and cheating individuals and to assess the ecological conditions that are favourable for one or the other type.

We assessed the costs and benefits of siderophore production in environments differing in their ecological and social compositions. We found that bacteria can sense changes in their ecological and social environment and adjust cooperative siderophore production accordingly. Specifically, cooperators decrease siderophore production with more free iron available (i.e. when siderophore are not needed) and at higher cell densities (i.e. when the sharing of siderophores becomes more efficient). Moreover, we found that cooperators increase siderophore production in the presence of cheats because cheats take the siderophores away from the co-operators, which is detrimental for cooperators.

We assessed the success of co-operators and cheats in viscous medium, which represents one of the natural habitats of this species. We found that co-operators out compete cheats in viscous medium because viscosity limits the dispersal of bacteria and the diffusion of siderophores. Both factors prevent cheats from accessing the siderophores, which limits their growth. This contrasts with the results obtained in liquid medium, where cheats displace cooperators as the two bacterial types mix well and cheats have free access to siderophores.

We conducted an experimental evolution study to investigate the effects of dispersal on the evolution of cooperation in patch-structured populations. Consistent with theory, we found that high dispersal between patches disfavoured cooperation because cooperators were efficiently exploited by cheats. However, limited dispersal between patches was also detrimental for co-operators because the extra offspring produced due to cooperation could not be exported to other patches. In contrast, we show that cooperators are favoured if bacteria disperse in clonal groups because this limits the possibilities for cheats to exploit co-operators, and at the same time enables co-operators to export their extra offspring.

We carried out another experimental evolution study to investigate the yet untested prediction of whether repression of competition among group members favours cooperation. Mechanism of competition repression such as punishment, policing and rewarding have long been suggested to be favourable for cooperation. Consistent with theory, we found that when competition between cooperative bacteria and cheats was repressed, cooperators took over the population. This was because cheats could no longer exploit cooperators, whilst the benefit of cooperation (i.e. producing extra offspring) remained.