The isolation and characterization of the MP and FP plaque variants of Galleria mellonella nuclear polyhedrosis virus.

Two plaque morphologies were detected in assays of Galleria mellonella nuclear polyhedrosis virus preparations which had been serially passaged in the TN-368 cell line. The FP (few polyhedra) plaques were larger (0.7-mm average diameter) and the infected cells contained less than 10 polyhedra each. The MP (many polyhedra) plaques were smaller (0.5-mm average diameter), and the infected cells contained greater than 10 each. Only the FP variant could be purified by plaque picking and remained pure through 10 serial injection passages in last-instar Galleria mellonella larvae, while the MP variant always produced FP plaques after serial passage in cell cultures or by injection of larvae. Bioassays of MP and FP polyhedra demonstrated a 350-fold reduction in virulence for FP polyhedra. Electron micrographs of these polyhedra showed that both variants were MNPV type viruses, although the FP polyhedra were predominantly devoid of nucleocapsids. Electron microscopic observations of MP- and FP-infected cell cultures revealed an apparent deficiency in de novo membrane synthesis in FP infections. SDS-polyacrylamide gel electrophoresis of cell-released nonoccluded virus proteins demonstrated several differences in protein composition between the two variants. The evidence indicates that the FP variant of G. mellonella nuclear polyhedrosis virus is a stable genotypic variant which arises from spontaneous mutation of the MP variant.