Genetic evidence for variant selection in the course of dilute passaging of mumps vaccine virus.

Mumps vaccine viruses, Leningrad-3 (L-3) strain, harvested at the 8th (8P) and 38th (38P) passage levels, were compared by nucleotide sequencing of the fusion (F) and the phosphoprotein (P) genes, and for replication efficiency in cell culture. Sequencing revealed only one clear base substitution throughout the entire F gene, and no substitutions in the variable 183-nucleotide-long region of the P gene. However, the 8P virus, unlike the 38P variant, contained multiple "ambiguous" nucleotide regions, i.e., additional bases positioned at the level of the principal ones. The 38P variant replicated faster and appeared more homogeneous by its plaque character compared to the 8P virus. The results indicate that the 8P progenitor virus consisted of more than one viral variant and that one of these was selected on repeated passage due to its higher replication efficiency.