SEX AND THE EVOLUTION OF INTRAHOST COMPETITION IN RNA VIRUS PHI-6

Sex allows beneficial mutations that occur in separate lineages to be fixed in the same genome. For this reason, the Fisher-Muller model pre dicts that adaptation to the environment is more rapid in a large sexu al population than in an equally large asexual population. Sexual repr oduction occurs in populations of the RNA virus phi 6 when multiple ba cteriophages coinfect the same host cell. Here, we tested the model's predictions by determining whether sex favors more rapid adaptation of phi 6 to a bacterial host, Pseudomonas phaseolicola. Replicate popula tions of phi 6 were allowed to evolve in either the presence or absenc e of sex for 250 generations. All experimental populations showed a si gnificant increase in fitness relative to the ancestor, but sex did no t increase the rate of adaptation. Rather, we found that the sexual an d asexual treatments also differ because intense intrahost competition between viruses occurs during coinfection. Results showed that the de rived sexual viruses were selectively favored only when coinfection is common, indicating that within-host competition detracts from the abi lity of viruses to exploit the host. Thus, sex was not advantageous be cause the cost created by intrahost competition was too strong. Our fi ndings indicate that high levels of coinfection exceed an optimum wher e sex may be beneficial to populations of phi 6, and suggest that gene tic conflicts can evolve in RNA viruses.