PHYSICAL AND GENETIC STUDIES OF NEWCASTLE DISEASE VIRUS - EVIDENCE FOR MULTIPLOID PARTICLES

Newcastle disease virus (NDV) grown in 10-day-old embryonated eggs was analyzed on sucrose gradients. Approximately 50% of the hemagglutinating particles were noninfectious and sedimented at a much slower rate than the infectious particles. The infectious particles, which exhibited a 2.5-fold range in sedimentation constants, were themselves highly heterogeneous. Further studies showed that the noninfectious particles contained no genomes, the bulk of the infectious particles contained one genome, and the larger infective particles contained two or more genomes. Although the overall ultraviolet sensitivity of NDV is nearly single-hit, the larger infective particles gave multihit inactivation kinetics, confirming that they contain more than one genome. Genetic tests showed that between 11 and 15% of the infective particles in a mixed yield from chick embryo cells were heterozygotes. On the assumption that (1) only integral numbers of genomes occur in NDV particles, and (2) the number of genomes per particle is randomly distributed, the average number of genomes per particle was calculated to lie between 0.5 and 0.8. The size distribution of the particles, and ultraviolet inactivation data on both the whole population and selected size classes are consistent with values in this range. © 1969.