Trends in Microbiology
Strategies of microbial cheater control
Section snippets
Cheating load
Cheaters can have substantial negative consequences in the groups in which they are found. A familiar example of this can be observed in tumours and their consequent effects [7]. Tumour cells arise from normal cells as a result of mutation and as normal cells do, they benefit from access to nutrients and protection from the external environment that is provided by the body. However, in contrast to normal cells, tumour cells cheat by proliferating without restriction and without contributing to
Societal control in microbes
An understanding of cheating in social microbes can be obtained by testing specific evolutionary and mechanistic hypotheses. To facilitate the testing of species-specific hypotheses, several mechanisms that might be used to restrain the spread of cheaters are described. Some readers might argue that one or more of the proposed scenarios require an implausible degree of sophistication on the part of microbes. Nonetheless, explicit consideration of a broad range of theoretical possibilities could
Two strategies
Cooperative genotypes have two primary strategies by which they might reduce or eliminate cheaters and their effects within social groups: targeted benefit and targeted punishment. In the benefit strategy, individuals that contribute to the group (cooperators) gain access to group-associated benefits that are not available to cheaters. Therefore, cooperators acquire some benefits that might vary with their social contribution. In the punishment strategy, individuals not contributing to the
Intrinsic defector inferiority
The most direct way for social genes to spread and persist would be to make their loss inherently costly. This could be accomplished by the evolution of a tight coupling of access to group benefits to mechanisms of developmental signal expression, as seen with the csA gene and its product in D. discoidium. Only those genotypes expressing correct developmental signals would promote development or suffer any individual cost to development. However, these would also be the only genotypes to have
Policing
Cooperative genotypes might take more aggressive action against defectors by targeted ‘policing’, where a pro-active punishment harms defectors but not cooperators. This is similar to the way that major defections from tax contribution are sometimes punished by fines or jail-time in human societies. In our definition, such punishment decreases defector fitness more than mere exclusion from access to benefits. For example, a toxin to which only defectors are susceptible might be generated by
Pure colonization
If the effects of cheating are particularly severe, then selection against cheaters can potentially occur as a function of population structure rather than direct selection either for cooperators or against cheaters within populations 48, 49. This mode of selection is somewhat akin to ‘opting out’ or leaving one social group to start or join another, and is a frequently invoked strategy to prevent cheating in human societies 50, 51. However, to be an evolutionarily successful strategy for
Perspective
Just how sophisticated can microbial cheating and anti-cheating strategies be? In principle, there are two modes of defection by which cheating might occur: obligate and facultative. Obligate defectors are unable to alter their defector status as a function of group composition. In other words, obligate defectors do not cooperate any more with fellow defectors than they do with non-defectors, and pure groups of obligate defectors are at a disadvantage to pure cooperator groups (at least when
Acknowledgements
We would like to gratefully acknowledge the many helpful comments by members of our respective laboratories, two anonymous reviewers and J. Strassmann.
Glossary
Glossary
- Altruism:
- Behaviour that has an individual fitness cost but provides a fitness benefit to others. Cooperative behaviour in the presence of cheaters constitutes altruism toward the cheaters.
- Cheating:
- Obtaining benefits from a collectively produced public good (see below) that are disproportionately large relative to a cheater's own contribution to that good.
- Cheating Load:
- The degree to which obligately defecting cheaters decrease the group-level benefits of cooperation in chimeric social groups.
- C
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