Environmental feedback maintains cooperation in viruses

Cooperators that pay a cost to provide benefits to others are vulnerable to exploitation by defectors that reap the benefits while avoiding the costs. Thus selection on the individual level can lead to loss of cooperators while lowering overall population fitness. Accordingly, the maintenance of cooperation is a key problem in evolutionary biology. The puzzle of cooperation extends to viruses: cooperative viruses produce gene products that can be shared, while defector viruses produce less and instead use products made by cooperators. In coinfection, defectors are always advantaged, predicting the loss of cooperation. However, the fitness of cooperator and defector phenotypes is context dependent. Though defectors are advantaged in coinfection, they suffer reduced replication in single infections. Environmental feedback is the process by which changes in population composition alter viral densities and rates of coinfection, which in turn feed back to affect the fitness of each type. We show that environmental feedback maintains cooperation in viruses. We also find that defector emergence may interfere with phage therapy by disrupting the phage dynamics that cause host extinction, and demonstrate that the introduction of defectors lowers viral densities and drives viral extinction, suggesting that defectors that replicate alone could function as antiviral therapies.