Identification of a novel Lymantria dispar nucleopolyhedrovirus mutant that exhibits abnormal polyhedron formation and virion occlusion

In previous studies on the formation of Lymantria dispar nuclear polyhedrosis virus (LdMNPV) few polyhedra (FP) mutants, several polyhedron formation mutants (PFM) were identified that appeared to be unique. These viral mutants are being characterized to investigate the processes of polyhedron formation and virion occlusion, LdMNPV isolate PFM-1 is one of these mutants, and is described in this report. Genetic techniques were used to determine if isolate PFM-1 contained a mutation in the polyhedrin or 25K FP gene. Wild-type viruses were recovered after coinfection of Ld652Y cells with isolate PFM-1 and a FP mutant, and with isolates PFM-1 and PFM-C (isolate PFM-C contains a mutation in the polyhedrin gene). These viruses were analyzed by genomic restriction endonuclease digestion and found to be chimeras of the original PFMs used in the coinfections. Marker rescue studies mapped the mutation in isolate PFM-1 to a genomic region that does not include the polyhedrin or 25K FP genes. Isolate PFM-1 produced approximately 14-fold fewer polyhedra than LdMNPV isolate A21-MPV, an isolate that produces wild-type levels of polyhedra, and approximately a-fold more polyhedra compared to the FP isolate 122-2, Polyhedra generated by isolate PFM-1 were normal in size and shape but contained very few viral nucleocapsids. The same amount of budded virus (BV) was released from cells infected with isolates PPM-I and A21-MPV. In contrast, isolate 122-2 yielded significantly more BV than isolates PFM-1 and A21-MPV.