SELECTION OF BENEVOLENCE IN A HOST–PARASITE SYSTEM

A paradigm for the evolution of cooperation between parasites and their hosts argues that the mode of parasite transmission is critical to the long‐term maintenance of cooperation. Cooperation is not expected to be maintained whenever the chief mode of transmission is horizontal: a parasite's progeny infect hosts unrelated to their parent's host. Cooperation is expected to be maintained if the chief mode of transmission is vertical: a parasite's progeny infect only the parent's host or descendants of that host. This paradigm was tested using bacteria and filamentous bacteriophage (f1). When cells harboring different variants of these phage were cultured so that no infectious spread was allowed, ensuring that all parasite transmission was vertical, selection favored the variants that were most benevolent to the host—those that least harmed host growth rate. By changing the culture conditions so that horizontal spread of the phage was allowed, the selective advantage of the benevolent forms was lost. These experiments thus support the theoretical arguments that mode of transmission is a major determinant in the evolution of cooperation between a parasite and its host.