Properties of a unique mutant of Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus that exhibits a partial many polyhedra and few polyhedra phenotype on extended serial passaging in suspension cell cultures.

Serial passaging of wild-type Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV) in H. zea (Hz-AM1) insect cell cultures results in rapid selection for the few polyhedra (FP) phenotype. A unique HaSNPV mutant (ppC19) was isolated through plaque purification that exhibited a partial many polyhedra (MP) and FP phenotype. On serial passaging in suspension cell cultures, ppC19 produced fivefold more polyhedra than a typical FP mutant (FP8AS) but threefold less polyhedra than the wild-type virus. Most importantly, the polyhedra of ppC19 exhibited MP-like virion occlusion. Furthermore, ppC19 produced the same amount of budded virus (BV) as the FP mutant, which was fivefold higher than that of the wild-type virus. This selective advantage was likely to explain its relative stability in polyhedra production for six passages when compared with the wild-type virus. However, subsequent passaging of ppC19 resulted in a steep decline in both BV and polyhedra yields, which was also experienced by FP8AS and the wild-type virus at high passage numbers. Genomic deoxyribonucleic acid profiling of the latter suggested that defective interfering particles (DIPs) were implicated in this phenomenon and represented another undesirable mutation during serial passaging of HaSNPV. Hence, a strategy to isolate HaSNPV clones that exhibited MP-like polyhedra production but FP-like BV production, coupled with low multiplicities of infection during scale-up to avoid accumulation of DIPs, could prove commercially invaluable.