THE ADVANTAGE OF SEX IN THE RNA VIRUS PHI-6

When laboratory populations of the RNA bacteriophage phi 6 are subject ed to intensified genetic drift, they experience a decline in fitness. These experiments demonstrate that the average effect of mutations is deleterious, and they are used to suggest that Muller's ratchet can o perate in these viruses. However, the operation of Muller's ratchet do es not alone guarantee an advantage of sex. When phi 6 populations wer e subjected to a series of bottlenecks of one individual and then cros sed, the measured advantage of sex was not significant. To determine w hether a small sample size, as opposed to allelism or another explanat ion, can account for the negative result, we repeated the phi G experi ments by crossing a larger set of populations. We found that bottlenec ked populations of phi 6 could recover fitness through mutations. Howe ver, hybrids produced by crossing the populations recovered an additio nal amount over the contribution of mutations. This additional amount, which represents an advantage of sex to phi 6, was determined to be s ignificantly gr-eater than zero. These results provide indirect suppor t for an advantage of sex through Muller's ratchet. However, we also u se our experimental design and results to propose an alternative to Mu ller's ratchet as a model for the evolution of sex.