nuclear polyhedrosis virus was highly pathogenic to both and . Plaque-purified variants from two of the original isolates showed much greater differences in pathogenicity to the two insect species than the original isolates. Plaque-purified variants from one of the isolates (D) nearly lost pathogenicity for while remaining pathogenic for . Some of the plaque-purified variants produced atypical symptoms, even in . These variants did not liquefy larvae and release polyhedra when the larvae died as is typical for nuclear polyhedrosis infections in Lepidoptera. These variants also produced fewer polyhedra per g larval weight and often had fewer virions per polyhedron. Light and electron microscopical studies of and infected with one of the original isolates (A) and two of the plaque-purified variants (B2 and D7) indicated that only infected with isolate A had the highly productive infection and viral morphogenesis typical of nuclear polyhedrosis viruses. Exposure of to isolate B2 or D7 resulted in a slightly delayed infection characterized by increased amounts of abnormal viral morphogenesis and polyhedra of decreased size. infected with isolate A or B2 had greatly reduced and delayed infections that were accompanied by highly variable abnormal viral morphogenesis; virtually no normal polyhedra were produced in these instances. Isolate D7 produced neither nucleocapsids nor polyhedra in . Large paracrystalline aggregates of nucleocapsids were common in infected with isolates B2 or D7 and in infected by isolate A. Infection of by isolate A or B2 was typified by the accumulation of large amounts of excess envelope membrane in the form of strands and vesicles of various sizes. Normal virogenic stromata were characteristics only of infected with isolate A or B2. Polyhedra produced in by even the most pathogenic isolates (A and B2) contained few if any normal virions and were not infective for either or .


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