Presence of a loner strain maintains cooperation and diversity in well-mixed bacterial communities

Cooperation and diversity abound in nature despite cooperators risking exploitation from defectors and superior competitors displacing weaker ones. Understanding the persistence of cooperation and diversity is therefore a major problem for evolutionary ecology, especially in the context of well-mixed populations, where the potential for exploitation and displacement is greatest. Here we demonstrate that a “loner effect”, described by economic game theorists, can maintain cooperation and diversity in real-world biological settings. We use mathematical models of public-good-producing bacteria to show that the presence of a loner strain, which produces an independent but relatively inefficient good, can lead to rock-paper-scissor dynamics, whereby cooperators outcompete loners, defectors outcompete cooperators, and loners outcompete defectors. These model predictions are supported by our observations of evolutionary dynamics in well-mixed experimental communities of the bacterium Pseudomonas aeruginosa. We find that the coexistence of cooperators and defectors, which respectively produce and exploit the iron-scavenging siderophore pyoverdine, is stabilized by the presence of loners with an independent iron-uptake mechanism. Our results establish the loner effect as a simple and general driver of cooperation and diversity in environments that 40 would otherwise favour defection and the erosion of diversity.Publisher PDFPeer reviewe

A Good Opening: The Key to Make the Most of Unilateral Climate Action

In this paper we argue that when a subgroup of countries cooperate on emission reduction, the optimal response of non-signatory countries reflects the interaction between three potentially opposing factors, the incentive to free-ride on the benefits of cooperation, the incentive to expand the demand of fossil fuels, and the incentive to adopt cleaner technologies introduced by the coalition. Using an Integrated Assessment Model with a game theoretic structure we find that cost-benefit considerations would lead OECD countries to undertake a moderate, but increasing abatement effort (in line with the pledges subscribed in Copenhagen). Even if emission reductions are moderate, OECD countries find it optimal to allocate part of their resources to energy R&D and investments in cleaner technologies. International spillovers of knowledge and technology diffusion then lead to the deployment of these technologies in non-signatory countries as well, reducing their emissions. When the OECD group follows more ambitious targets, such as 2050 emissions that are 50% below 2005 levels, the benefits of technology externalities do not compensate the incentives deriving from the lower fossil fuels prices. This suggests that, when choosing their unilateral climate objective, cooperating countries should take into account the possibility to induce a virtuous behaviour in non-signatory countries. By looking at a two-phase negotiation set-up, we find that free-riding incentives spurred by more ambitious targets can be mitigated by means of credible commitments for developing countries in the second phase, as they would reduce lock-in in carbon intensive technologies.Technology Spillovers, Climate Change, Partial Cooperation

Biofilm formation and toxin production provide a fitness advantage in mixed colonies of environmental yeast isolates

Microbes can engage in social interactions ranging from cooperation to warfare. Biofilms are structured, cooperative microbial communities. Like all cooperative communities, they are susceptible to invasion by selfish individuals who benefit without contributing. However, biofilms are pervasive and ancient, representing the first fossilized life. One hypothesis for the stability of biofilms is spatial structure: Segregated patches of related cooperative cells are able to outcompete unrelated cells. These dynamics have been explored computationally and in bacteria; however, their relevance to eukaryotic microbes remains an open question. The complexity of eukaryotic cell signaling and communication suggests the possibility of different social dynamics. Using the tractable model yeast, Saccharomyces cerevisiae, which can form biofilms, we investigate the interactions of environmental isolates with different social phenotypes. We find that biofilm strains spatially exclude nonbiofilm strains and that biofilm spatial structure confers a consistent and robust fitness advantage in direct competition. Furthermore, biofilms may protect against killer toxin, a warfare phenotype. During biofilm formation, cells are susceptible to toxin from nearby competitors; however, increased spatial use may provide an escape from toxin producers. Our results suggest that yeast biofilms represent a competitive strategy and that principles elucidated for the evolution and stability of bacterial biofilms may apply to more complex eukaryotes