Genetics, Vol 147, 953-959, Copyright © 1997

INVESTIGATIONS

The Advantage of Sex in the RNA Virus {phi}6

L. Chao, T. T. Tran and T. T. Tran
Department of Zoology, University of Maryland, College Park, Maryland 20742

When laboratory populations of the RNA bacteriophage {phi}6 are subjected 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 operate in these viruses. However, the operation of Muller's ratchet does not alone guarantee an advantage of sex. When {phi}6 populations were subjected to a series of bottlenecks of one individual and then crossed, the measured advantage of sex was not significant. To determine whether a small sample size, as opposed to allelism or another explanation, can account for the negative result, we repeated the {phi}6 experiments by crossing a larger set of populations. We found that bottlenecked populations of {phi}6 could recover fitness through mutations. However, hybrids produced by crossing the populations recovered an additional amount over the contribution of mutations. This additional amount, which represents an advantage of sex to {phi}6, was determined to be significantly greater than zero. These results provide indirect support for an advantage of sex through Muller's ratchet. However, we also use our experimental design and results to propose an alternative to Muller's ratchet as a model for the evolution of sex.

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