Virus morphogenesis of Helicoverpa armigera nucleopolyhedrovirus in Helicoverpa zea serum-free suspension culture

Helicoverpa armigera single nucleopolyhedrovirus (HaSNPV) replication in Helicoverpa tea serum-free suspension culture was studied in detail and the sequence of virus morphogenesis was determined by transmission electron microscopy, By 16 h post-infection (p.i.), virus replication was observed in the virogenic stroma by the appearance of nucleocapsids. Polyhedron formation was detected by 24 h p.i. and the polyhedron envelope (PE) was completely formed by 72 h p.i. PE morphogenesis of HaSNPV is significantly different compared to the extensively studied Autograph californica (Ac)MNPV, In AcMNPV-infected cells, fibrillar structures are found in both cytoplasm and nuclei, and the fibrillar structures in nuclei are in close association with maturing polyhedra during PE formation. Fibrillar structures that resemble the AcMNPV fibrillar structures were detected only in the cytoplasm of HaSNPV-infected cells and appeared to interact with calyx precursors there, but their role in PE formation is unclear. However, prominent calyx precursor structures of various shapes and sizes were observed in the nuclei of HaSNPV-infected cells as well, and they appeared to interact with polyhedra during PE formation. Both the calyx precursor structure and the cytoplasmic fibrillar structure were detected only after HaSNPV virion occlusion had started, indicating that they might have a role in formation of PE, Similar calyx precursor structures and cytoplasmic fibrillar structures were observed in both serum-supplemented and serum-free suspension cultures, as well as in HaSNPV-infected larval tissues, indicating that the structures observed are not cell culture artefacts.