Contingent neutrality in competing viral populations

The replicative fitness of a genetically marked (MARM-C) population of vesi cular stomatitis virus was examined in competition assays in BHK-21 cells. In standard fitness assays involving up to eight competition passages of th e mixed populations, MARM-C competes equally with the wild type (wt), but v ery prolonged competitions always led to the wt gaining dominance over MARM -C in a very slowed, nonlinear manner (J. Quer et al., J. Mol. Biol. 264:46 5-471, 1996). In the present study we show that a number of quite unrelated environmental perturbations, which decreased virus replication during comp etitions, all led to an accelerated dominance of the wt over MARM-C. These perturbations were (i) the presence of added (or endogenously generated) de fective interfering particles, (ii) the presence of the chemical mutagen 5- fluorouracil (5-FU), or (iii) an increase in temperature to 40.5 degreesC. Thus, the "neutral fitness" of the MARM-C population is contingent. We have determined the entire genomic consensus sequence of MARM-C and have identi fied only six mutations. Clearly, some or all of these mutations allowed th e MARM-C quasispecies population to compete equally with wt in a defined co nstant host environment, but the period of neutrality was shortened when th e environment was perturbed during competitions. Interestingly, when four p assages of each population were carried out independently in the presence o f 5-FU (but in the absence of competition), no significant differences were detected in the fitness changes of wt and MARM-C, nor was there a differen ce in their subsequent abilities to compete with each other in a standard f itness assay. We propose a model for this contingent neutrality. The condit ions employed to generate the MARM-C quasispecies population selected a sma ll number of mutations in the consensus sequence. It appears that the MARM- C quasispecies population has moved into a segment of sequence space in whi ch the average fitness value is neutral but, under environmental stress, be neficial mutations cannot be generated rapidly enough to compete with those being generated concurrently by competing wt virus quasispecies population s.