![]() ![]() Individuals secrete diffusible signals into the environment and regulate gene expression in response to the concentration of signal. QS is a form of cell–cell communication observed across diverse bacterial species ( Rutherford and Bassler, 2012). We argue that the efficiency gains of QS extend to the context of genetically mixed groups. It has been suggested that quorum sensing (QS), a common regulatory architecture governing secreted protein production, has evolved in part to restrict secretion investments when they are inefficient ( Redfield, 2002 Hense et al., 2007 Darch et al., 2012 Cornforth et al., 2014). The cost of secreting proteins is considerable, and has resulted in selection for cheaper amino-acid residues in secreted proteins ( Nogueira et al., 2009). In bacteria, co-operative interactions are commonly mediated by secreted factors that individual cells produce at a cost to provide shared benefits for a local neighbourhood of cells, for example, iron-scavenging siderophores ( Griffin et al., 2004 Kuemmerli and Brown, 2010) or secreted digestive enzymes ( Diggle et al., 2007 Sandoz et al., 2007). This form of reciprocity works by individuals increasing their level of co-operation as there are more co-operators in their group, removing the requirement for the recognition of individuals or extensive memorisation of past events ( Pfeiffer et al., 2005), and has been observed in both humans ( Stanca, 2009) and rats ( Rutte and Taborsky, 2007). However, one form of reciprocity, which has been suggested to be simple enough to be achievable by most organisms, is ‘generalised reciprocity' ( Pfeiffer et al., 2005). Mechanisms of reciprocity are often suggested to be cognitively complex ( Milinski and Wedekind, 1998 Stevens and Hauser, 2004). ![]() The importance of behavioural control of co-operative effort has been emphasised since Trivers' (1971) pivotal work on reciprocity highlighted that behavioural feedbacks can allow individuals to match their investment in co-operation to the investment of others (that is, creating phenotypic assortment), and thus protect co-operative strategies from exploitation even in well-mixed groups ( Trivers, 1971 Nowak and Sigmund, 1998 Pfeiffer et al., 2005 Fletcher and Zwick, 2006). If individuals have fixed strategies (constitutively co-operative or non-co-operative), then co-operators can only outcompete cheats if any net costs of co-operation are sufficiently offset by increased rates of interaction with fellow co-operators (positive genetic assortment or relatedness, Hamilton, 1964 Frank, 1998).Īlthough much theory has been built on the assumption of constitutive strategies, behavioural plasticity in social traits is increasingly recognised as the norm, not only in vertebrates ( Rutte and Taborsky, 2007), but also in microbes ( Kuemmerli and Brown, 2010 Parkinson et al., 2011 Xavier et al., 2011) and even in viruses interacting with conspecifics ( Leggett et al., 2013). Across biological scales, co-operative individuals face the challenge of competition with non-co-operative ‘cheats' that reap the rewards of co-operation without paying the full costs ( Ghoul et al., 2014). The co-operative provision of help to other individuals is a ubiquitous feature of life, from viruses to vertebrates ( Turner and Chao, 1999 Bshary and Grutter, 2002 Griffin et al., 2004 Rand et al., 2009 Melis et al., 2011). Our results suggest that mechanisms of reciprocity are not confined to taxa with advanced cognition, and can be implemented at the cellular level via positive feedback circuits. Similar behavioural responses have been described in vertebrates under the banner of ‘generalised reciprocity'. ![]() ![]() We demonstrate mathematically and experimentally that the observed response rule of ‘co-operate when surrounded by co-operators' allows bacteria to match their investment in co-operation to the composition of the group, therefore allowing the maintenance of co-operation at lower levels of population structuring (that is, lower relatedness). Using high-density populations of the opportunistic pathogen Pseudomonas aeruginosa we map per-capita signal and co-operative enzyme investment in the wild type as a function of the frequency of non-responder cheats. We show that the canonical QS regulatory architecture allows bacteria to sense the genotypic composition of high-density populations, and limit co-operative investments to social environments enriched for co-operators. Quorum sensing (QS) is a cell–cell communication system found in many bacterial species, commonly controlling secreted co-operative traits, including extracellular digestive enzymes. ![]()
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