Detailed analysis of the selection process in serial co-infections of cell cultures by wild-type nuclear polyhedrosis virus (AcNPV) strain E2 (AcNPV/E2) and Ac360-β-gal, a genetically engineered strain, shows that the unaltered strain was clearly dominant even when it initially constituted the minority component in the inoculum. A method of calculating a selection coefficient that quantifies the relative advantage of one strain of virus over the other under specific culture conditions is described. Calculated selection coefficients were relatively homogeneous and almost exclusively favoured the progenitor. Selection pressure was not influenced by the relative proportions of the two strains in the population. Selection coefficients, as determined in the present study, may be useful for evaluating the effect of a genetic alteration on viral fitness under specified conditions. Unexpected high frequencies of mixed phenotype plaques were observed during infectivity titrations of media from early serial passages of co-infected cultures. Statistical evaluation implicates some non-heritable combinational phenomenon. Virus plated from mixed phenotype plaques show high segregation of phenotypes implying that genetic recombination does not contribute in a major way to the high mixed phenotype frequencies. Electron microscopic examination of virion pellets from infected 72 h cell culture media similarly argue against co-envelopment as a major contributory factor to the high frequency of mixed phenotype plaques. The cause remains undetermined.


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