Strain selection during serial passage of Trichoplusia in nuclear polyhedrosis virus

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Abstract
Two strains of a nuclear polyhedrosis virus (NPV) of T. ni were isolated on the basis of plaque morphology. They are designated as MP (having greater than 30 polyhedra/nucleus) and FP (having fewer than 10 polyhedra/nucleus). Serial, undiluted passage of plaque-purified MP nonoccluded virus (NOV) in tissue culture led to the production of the FP phenotype detectable at passage 9. With continued serial, undiluted passage, FP became the predominant strain. Comparative growth curves showed that FP NOV are released faster than MP NOV. MP morphology was not observed after 14 serial, undiluted passages of plaque-purified FP. By the plaque neutralization assay, NOV from both strains of virus was neutralized by the homologous and heterologous antisera. The FP phenotype was observed when FP virus was grown in culture at 17, 22 and 27.degree. C. Hence, the FP phenotype was probably not due to temperature-inhibited crystallization of polyhedrin under standard tissue culture conditions. The NOV of both strains killed insects when injected directly into the hemocoel of T. ni larvae. Only MP inclusion bodies were virulent when administered orally. The FP inclusion bodies fed to cabbage looper larvae did not kill, and no infectious agent could be detected in the hemolymph. Electron micrographs of MP polyhedra showed bundles of nucleocapsids of normal length within the polyhedra, whereas FP polyhedra contained heterogeneous, electron-dense material, which could account for their lack of pathogenicity.