Characterization of two variants of Panolis flammea multiple nucleocapsid nuclear polyhedrosis virus

The baculovirus Panolis flammea multiple nucleocapsid nuclear polyhedrosis virus (PfMNPV) was originally isolated from a natural virus epizootic and shown to consist of a mixture of variants. Two subclasses of variants (PfMNPV A and B) were identified by Southern blot hybridization, their polyhedrin genes being located on different restriction fragments. The proportion of the A and B variants changed according to the larval host in which the virus was propagated; PfMNPV(B) predominated in P. flammea but PfMNPV(A) was predominant in Mamestra brassicae. Bioassays of the two pure virus variants in M. brassicae larvae have shown the LD50 values to be 4610 polyhedron inclusion bodies (pibs) for PfMNPV(A) and 5937 pibs for PfMNPV(B). Genomic DNA from the two variants was compared using restriction endonuclease analysis, and dot blot and Southern blot hybridization. Reciprocal quantitative dot blot hybridization analysis in 50% formamide showed PfMNPV(A) and PfMNPV(B) to be only distantly related to Autographa californica MNPV (less than 1%) and more closely related to M. brassicae MNPV (21 to 26%). The two PfMNPV variants exhibited a very high degree of identity to each other (nearly 100%) and therefore are very closely related. This was confirmed by physical mapping of the virus genomes. The nucleotide sequence of the polyhedrin gene of PfMNPV(B) was determined and compared with the published DNA sequences of other polyhedrin genes.