Synergy and group size in microbial cooperation

Cornforth, D.M., Sumpter, D., Brown, S., & Brännström, Å. (2012). Synergy and group size in microbial cooperation. IIASA Interim Report. IIASA, Laxenburg, Austria: IR-12-032

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Abstract

Microbes produce many molecules that are important for their growth and development, and the consumption of these secretions by nonproducers has recently become an important paradigm in microbial social evolution. Though the production of these public goods molecules has been studied intensely, little is known of how the benefits accrued and costs incurred depend on the quantity of public good molecules produced. We focus here on the relationship between the shape of the benefit curve and cellular density with a model assuming three types of benefit functions: diminishing, accelerating, and sigmoidal (accelerating then diminishing). We classify the latter two as being synergistic and argue that sigmoidal curves are common in microbial systems. Synergistic benefit curves interact with group sizes to give very different expected evolutionary dynamics. In particular, we show that whether or not and to what extent microbes evolve to produce public goods depends strongly on group size. We show that synergy can create an 'evolutionary trap' which can stymie the establishment and maintenance of cooperation. By allowing density dependent regulation of production (quorum sensing) we show how this trap may be avoided. We discuss the implications of our results for experimental desgn.

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