Prisoner's dilemma in an RNA virus

The evolution of competitive interactions among viruses(1) was studied in t he RNA phage phi 6 at high and low multiplicities of infection (that is, at high and low ratios of infecting phage to host cells). At high multiplicit ies, 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 study(1) was that phage grown at high rates of co-infection increa sed in fitness initially, but then evolved lowered fitness. Here we show th at the fitness of the high-multiplicity phage relative to their ancestors g enerates a pay-off matrix conforming to the prisoner's dilemma strategy of game theory(2,3). In this strategy, defection (selfishness) evolves, despit e the greater fitness pay-off that would result if all players were to coop erate. Viral cooperation and defection can be defined as, respectively, the manufacturing and sequestering of diffusible (shared) intracellular produc ts. Because the low-multiplicity phage did not evolve lowered fitness, we a ttribute the evolution of selfishness to the lack of clonal structure and t he mixing of unrelated genotypes at high multiplicity(4-6).