In the Light of Evolution Volume V: Cooperation and Conflict (2011) / Chapter Skim
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6 Evolution of Restraint in a Structured RockPaperScissors Community--JOSHUA R. NAHUM, BRITTANY N. HARDING, and BENJAMIN KERR
6 Evolution of Restraint in a Structured RockPaperScissors Community--JOSHUA R. NAHUM, BRITTANY N. HARDING, and BENJAMIN KERR
Pages 117-136
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From page 117... ...
Using both computer simulations and evolution experiments with a nontransitive community of Escherichia coli, we find that restrained growth can evolve under conditions of limited dispersal in which negative feedback is present. This research thus highlights a set of ecological conditions sufficient for the evolution of one form of altruism.
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From page 118... ...
In this chapter, we address this question directly by outlining ecological conditions sufficient to favor the evolution of restraint. One ingredient found in most explanations for the evolution of altruism, and thus relevant to the evolution of restraint, is positive assortment.
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From page 119... ...
and a third player possessing mildly costly resistance to the harm. Reminiscent of the children's game rock–paper–scissors, the harmer outcompetes the sensitive player, who outcompetes the resis tant player; in turn, the resistant player outcompetes the harmer.
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From page 120... ...
Thus, these three players constitute a nontransitive community: the sensitive strain outgrows the resistant strain, the resistant strain outgrows the producer, and the pro ducer kills the sensitive strain. Previous work with the three members of the colicin E2 system has shown nontransitivity both in vitro (Kerr et al., 2002)
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From page 121... ...
We initialized the metapopulations with the nontransitive community (Community treatment) or the resistant strain alone (Alone treatment)
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From page 122... ...
We found that isolates from the Restricted Community treatment had the lowest competitive ability [single-factor ANOVA; F2,12 = 9.36, P = 0.0036, multiple comparisons by Tukey's Honestly Significant Difference (HSD)
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From page 123... ...
All three players coexisted in the Community treatments for the duration of the experiment, and the density of the resistant strain was comparable across all three treatments.
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From page 124... ...
Resistant cells in a full community evolved a significantly higher competitive ability under unrestricted migration than under restricted migration (Unrestricted Community vs. Restricted Commu nity in Fig.
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From page 125... ...
Consistent with our empirical results, we found the a verage growth rate of resistant strains from the Restricted Community treatment to be significantly lower than the average growth rate from t he Restricted Alone treatment (Fig.
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From page 126... ...
The fitness of resistant populations from the Restricted Community treatment was significantly lower than that of the other treatments at both time points. This pattern is consistent with the evolution of restrained growth in the Restricted Community treatment.
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From page 127... ...
. In this case, resistant mutants with a higher growth rate r each a lower frequency in the Restricted Community treatment than in the Unrestricted Community treatment by the end of the experiment.
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From page 128... ...
In the Restricted Community treatment, the nontransitivity of the full community provides a form of negative feedback, and the restricted migration ensures a form of positive assortment. We suggest that it is these two factors, negative feedback and positive assortment, that set the stage for the evolution of restraint.
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From page 129... ...
or the positive a ssortment resulting from limited migration (e.g., in the Unrestricted Community treatment) , the evolution of restraint is not expected.
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From page 130... ...
However, spatial structure can be important for the evolution of restraint in other types of communities as well. As an example, limited dispersal can promote restraint within victim–exploiter communities (Boots and Mealor, 2007; Kerr et al., 2006)
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From page 131... ...
. There have also been experimental demonstrations that limited dispersal favors restraint in host–parasite communities in the form of reduced parasite virulence and/or infectivity (Kerr et al., 2006; Boots and Mealor, 2007; Eshelman et al., 2010)
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From page 132... ...
With limited migration, similar types associate into patches that chase one another. The negative feedback resulting from the nontransitivity in our system means that patches filled with unrestrained variants are more prone to extinction.
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From page 133... ...
. In the Community treatments, each metapopulation consisted of two microtiter plates (192 wells with 200 mL growth medium each)
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From page 134... ...
rik ( t ) are the abundances of each of K types of mutant resistant strains.
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From page 135... ...
. After C growth cycles, we measured the expected fitness of a randomly chosen resistant cell relative to the resistant ancestor.
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From page 136... ...
Number of mutant resistant 7 K strains (unitless) { } Ks Kp KR KR KR , mY Maximum growth rate 0.61 0 1 2 (abundance per hour)
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