Persistence of an occlusion-negative recombinant nucleopolyhedrovirus in Trichoplusia ni indicates high multiplicity of cellular infection

We use data from the serial passage of co-occluded recombinant Autographa c alifornica nuclear polyhedrosis virus (AcMNPV) to estimate the viral multip licity of infection of cells within infected insects. Co-occlusion, the inc orporation of wild-type and mutant virus genomes in the same occlusion body , has been proposed as a strategy to deliver genetically modified viruses a s insecticides in a way that contains their spread in the environment. It m ay also serve as a means whereby naturally occurring mutant forms of NPVS c an be maintained in a stable polymorphism. Here, a recombinant strain of Ac MNPV was constructed with a deletion of its polyhedrin gene, rendering it i ncapable of producing occlusion bodies (i.e., occlusion negative). This was co-occluded with wild-type AcMNPV and used to infect fifth-instar Trichopl usia ni larvae. The fate of both genotypes was monitored over several round s of insect infection. Levels of the occlusion-negative virus genome declin ed slowly over successive rounds of infection. We applied these data to a m odel of NPV population genetics to derive an estimate of 4.3 +/- 0.3 Aral g enomes per occlusion body-producing cell.