Circular competition among three strains of E. coli helps maintain a stable community.

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In nature, competition usually means dominant species exclude others. If species are to keep from wiping each other out, they need to have something that helps to equalize them. One option is to specialize in a particular resource, such as a type of food, that their competition doesn’t eat. Another way is to have a relationship where each organism can restrict the other in some way to ensure that no one organism takes over. This is known as a rock-paper-scissors relationship.

In the children’s game of rock-paper-scissors, rock smashes scissors, scissors cut paper, and paper covers rock. There can always be different winners and losers, but no one person can win outright every time. Scientists have found species in nature that also compete this way, including some lizards and plants. One example involves three strains of the bacteria E. coli that scientists have nicknamed C, S, and R (see illustration). Each has its own strengths but also its own weaknesses. C (rock) produces a toxin (called colicin, hence the nickname C) that can kill S (scissors). However, producing that toxin slows the growth of C. R (paper) is resistant to the toxin and grows faster than C, taking over its area. S (scissors) can outgrow R but is sensitive to the toxin.

These strains can exist together as long as they live in patches where there is space to grow uninhibited by the others. If R grows into C’s patch, some of C might die out but C in turn can expand into S’s patch, while S can grow into R’s patch, etc.

There are many ways in nature to cooperate rather than compete. Learning from the rock-paper-scissors strategy, we might want to try maintaining stable relationships with others who we usually think of as competitors. By using a ‘give-and-take’ strategy and not over-using resources, everyone in the relationship  can prosper.

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“The classic non-transitive system involves a community of three competing species satisfying a relationship similar to the children’s game rock–paper–scissors, where rock crushes scissors, scissors cuts paper, and paper covers rock. Such relationships have been demonstrated in several natural systems…Colicinogenic [E. coli] bacteria possess a ‘col’ plasmid, containing genes that encode the colicin (the toxin), a colicin-specific immunity protein (which renders the cell immune to the colicin) and a lysis protein (which is expressed when the cell is under stress, causing partial cell lysis [the disintegration of a cell by rupture of the cell wall or membrane]and the subsequent release of the colicin). In general, only a small fraction of a population of colicinogenic cells will lyse and release the colicin. Colicin-sensitive bacteria are killed by the colicin but may occasionally experience mutations that render them resistant to the colicin. The most common mutations alter cell membrane proteins that bind or translocate the colicin. In some cases, the growth rate of resistant cells (R) will exceed that of colicinogenic cells (C), but will be less than the growth rate of sensitive cells (S). This occurs because resistant cells avoid the competitive cost of carrying the col plasmid but suffer because colicin receptor and translocation proteins are also involved in crucial cell functions such as nutrient uptake. In such cases, S can displace R (because S has a growth-rate advantage), R can displace C (because R has a growth-rate advantage) and C can displace S (because C kills S). That is, the C–S–R community satisfies a rock–paper–scissors relationship.” (Kerr et al. 2002:171)

Journal article
Kerr B, Riley MA, Feldman MW, Bohannan BJM. 2002. Local dispersal promotes biodiversity in a real-life game of rock-paper-scissors. Nature 418:171-174. NatureKerr B, Riley MA, Feldman MW, Bohannan BJM

Elmore H. 2005. Real-life rock-paper-scissors. EvoBites EvoBitesElmore H

Journal article
Nahum JF, Harding BN, Kerr B. 2011. Evolution of restraint in a structured rock-paper-scissors community. PNAS 108(2):10831-10838. PNASNahum JF, Harding BN, Kerr B

Web page
University of Washington. 2011. Bacteria develop restraint for survival in a rock-paper-scissors community. EurekaAlert! EurekaAlert!University of Washington

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