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Prisoner's dilemma in an RNA virus


The evolution of competitive interactions among viruses1 was studied in the RNA phage φ6 at high and low multiplicities of infection (that is, at high and low ratios of infecting phage to host cells). At high multiplicities, many phage infect and reproduce in the same host cell, whereas at low multiplicities the viruses reproduce mainly as clones. An unexpected result of this study1 was that phage grown at high rates of co-infection increased in fitness initially, but then evolved lowered fitness. Here we show that the fitness of the high-multiplicity phage relative to their ancestors generates a pay-off matrix conforming to the prisoner's dilemma strategy of game theory2,3. In this strategy, defection (selfishness) evolves, despite the greater fitness pay-off that would result if all players were to cooperate. Viral cooperation and defection can be defined as, respectively, the manufacturing and sequestering of diffusible (shared) intracellular products. Because the low-multiplicity phage did not evolve lowered fitness, we attribute the evolution of selfishness to the lack of clonal structure and the mixing of unrelated genotypes at high multiplicity4,5,6.

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Figure 1: Expected and observed fitness values for a game in which opponents use conflicting strategies of cooperation and defection.
Figure 2: The fitness of derived high MOI phage relative to the ancestor is a decreasing function of initial frequency in competition.


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We thank C. Burch, K. Hanley, S. Lance, U. Mueller, J. Smale and G. Wilkinson for useful comments and enlightening discussion. This work was supported by fellowships from the NSF and University of Maryland to P.E.T.

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Correspondence to Paul E. Turner.

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Turner, P., Chao, L. Prisoner's dilemma in an RNA virus. Nature 398, 441–443 (1999).

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