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RESEARCH PRODUCT

Frequency-Dependent Selection in a Mammalian RNA Virus

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RNA viruses have been used as experimental systems to test evolutionary hypotheses such as Muller's ratchet (Chao 1990; Duarte et al. 1992, 1993; Clarke et al. 1993), the Red Queen hypothesis (Clarke et al. 1994), the nature of the adaptive topography (Elena, 1995; Elena et al. 1996), and the dynamics of adaptive evolution (Elena 1995; Novella et al. 1995). Two hypotheses which have received attention in virus studies are the competitive exclusion principle and frequency-dependent selection. The competitive exclusion principle (Hardin 1960) states that two populations or species competing for the same limiting resource cannot stably coexist because one competitor will displace the other. An exception to the competitive exclusion principle can occur under frequency-dependent selection, in which it is possible to find a frequency at which two populations may coexist stably and indefinitely despite the fact that they are competing for the same resource (Ayala and Campbell 1974). Frequency-dependent selection has been demonstrated to occur in organisms as different as Drosophila (Wright and Dobzhansky 1946; Ayala 1971; Kojima 1971; Van Delden et al. 1978) and Escherichia coli (Helling et al. 1987; Chao et al. 1977; Lenski and Hattingh 1986; Turner et al. 1996) among others. Evidence for the applicability of the competitive exclusion principle to viruses has been reported by Clarke et al. (1994) in their work with vesicular stomatitis virus (VSV). These authors observed that two virus populations of approximately equal fitness can coexist during prolonged replication in the same stable environment; but stochastic changes in the population balance did eventually occur, and their impact on the competition was sudden and decisive. They justified their observations by the stochastic fixation of highly advantageous or deleterious mutations in variants of one of the two competing populations. One alternative explanation for the Clarke et al. (1994) observations is the absence of any stable coexistence equilibrium points. The clear and simple objective of the present paper was to determine if frequency-dependent selection acting on viral populations can promote the stable coexistence of two genotypes competing for the same resource.