David Bensimon, ENS, Paris, France
The maintenance of cooperation in populations where public goods are equally accessible to all but inflict a fitness cost on cooperators is a long-standing puzzle of economics, philosophy and evolutionary biology. In the first two contexts, sanctions are often used to ensure cooperation: would people pay taxes if there was no sanction against tax evasion?
The situation is less obvious in a biological context. A typical example of a public good in biology is provided by iron chelators (siderophores). Iron is an essential co-factor for many biological reactions but is rather insoluble. Bacteria therefore synthesize and secrete siderophores into their environment to fetch iron which then becomes available to all. The maintenance of their costly production is thus a major question in ecology and evolution. In a well mixed liquid culture, these iron chelators diffuse homogeneously so that their secretion by a few bacteria benefits the whole colony, resulting in a situation where non-producers are likely to invade and eventually crush the population.
To overcome this apocalyptic scenario, it has been theoretically shown that local interactions (rather than sanctions) by limiting public good dispersal may stabilize the population of producers. Yet, no experimental evidence supports their existence. In this talk I will present results on the dynamics of siderophore usage by individual bacteria in wild-type clonal micro-colonies of P. aeruginosa growing in 2D, on solid agar gels. I will show that this dynamics is driven by local exchanges between contacting cells, rather than within the whole colony. I will argue that this mode of local exchange affects siderophore trafficking within the colony and impacts the fitness of individual cells, as would be expected in a continuous variant of a spatial tit-for-tat game. Simulations derived from the experimental data indicate that these local interactions are sufficient to ensure the maintenance of producers against non-producers (with no need for sanctions).