Abstract
BackgroundExplaining public-goods cooperation is a challenge for evolutionary biology. However, cooperation is expected to more readily evolve if it imposes a smaller cost. Such costs of cooperation are expected to decline with increasing resource supply, an ecological parameter that varies widely in nature. We experimentally tested the effect of resource supply on the evolution of cooperation using two well-studied bacterial public-good traits: biofilm formation by Pseudomonas fluorescens and siderophore production by Pseudomonas aeruginosa.ResultsThe frequency of cooperative bacteria increased with resource supply in the context of both bacterial public-good traits. In both cases this was due to decreasing costs of investment into public-goods cooperation with increasing resource supply.ConclusionOur empirical tests with bacteria suggest that public-goods cooperation is likely to increase with increasing resource supply due to reduced costs of cooperation, confirming that resource supply is an important factor in the evolution of cooperation.
Highlights
Explaining public-goods cooperation is a challenge for evolutionary biology
To test whether selection for cooperation increased with increasing resource supply we calculated the proportion of biofilm-forming WS within each population (i.e., WS density/total density)
Consistent with our predictions, the proportion of biofilm-forming WS increased with increasing resource supply (Figure 1; founding genotype, F3,15 = 1.01, P = 0.4; log2(resource supply), F1,15 = 24.26, P < 0.0001)
Summary
Public-goods cooperation is widespread in nature but explaining this is a challenge for evolutionary biologists [1,2,3]. Because investment of resource into growth gives diminishing returns, the cost of diverting resources away from growth and into cooperative public goods production will be higher when resources are scarce and lower when resources are not limiting Put another way, high resource supply is likely to reduce the relative cost of cooperation. While overexpression of cellulosic polymer is individually costly (as demonstrated by the reduced exponential growth rate of WS relative to SM [11,19]), its production provides a group benefit to WS because colonisation of the air-broth interface niche allows improved access to oxygen, a limiting resource [11]. After 16 days, samples were first taken from the broth phase of cultures and plated onto agar This allowed us to identify the frequency of bacteria that inhabited the broth, and were asocial with relation to biofilm formation.
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