An experimental study of strong reciprocity in bacteria

R Fredrik Inglis,Angus Buckling,Stuart West

doi:10.1098/rsbl.2013.1069

Abstract

Strong reciprocity, whereby cooperators punish non-cooperators, may help to explain the evolutionary success of cooperative behaviours. However, theory suggests that selection for strong reciprocity can depend upon tight genetic linkage between cooperation and punishment, to avoid the strategy being outcompeted by non-punishing cooperators. We tested this hypothesis using experimental populations of the bacterium Pseudomonas aeruginosa, which cooperate by producing iron-scavenging siderophores and, in this context, punish non-cooperators with toxins. Consistent with theory, we show that cooperative punishers can indeed invade cheats, but only when the traits are tightly linked. These results emphasize that punishment is only likely to be favoured when the punishment itself leads to a direct or indirect fitness benefit to the actor.

Highlights

The evolution of cooperation in humans has been argued to have been facilitated by strong reciprocity: helping other cooperators while punishing individuals who do not cooperate [1,2,3,4,5,6]
A co-expression of cooperative and harming traits would be analogous to strong reciprocity, if bacteria were able to preferentially harm individuals that & 2014 The Authors
We created replicate metapopulations of bacteriocinsensitive, non-siderophore-producing cheats and tested whether these could be invaded by: (i) a wild-type strain of P. aeruginosa (PAO1) that can be considered a ‘strong reciprocator’ as it produces both cooperative siderophores and harming bacteriocins; (ii) an isogenic bacteriocinknock-out mutant (PAO1150-2) that can be considered a non-punishing cooperator as it only produces cooperative siderophores and (iii) a 1 : 1 mixture of both strains (PAO1 and PAO1150-2), in order to mimic a break in genetic linkage between the cooperating and punishing trait

Summary

Introduction

The evolution of cooperation in humans has been argued to have been facilitated by strong reciprocity: helping other cooperators while punishing individuals who do not cooperate [1,2,3,4,5,6]. If the traits are unlinked, neither strong reciprocity nor cooperation by itself can invade the population, unless other factors are incorporated, such as individuals adjusting conditionally whether they cooperate or defect [9]. We use experimental populations of the bacterium Pseudomonas aeruginosa to test the predicted role of tight linkage between cooperation and punishment in the invasion of strong reciprocity.

Results

Conclusion