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Strain selection during serial passage of Trichoplusia in nuclear polyhedrosis virus
Abstract:
Two strains of a nuclear polyhedrosis virus (NPV) of Trichoplusia ni were isolated on the basis of plaque morphology. They are designated as MP (having greater than 30 polyhedra per nucleus) and FP (having fewer than 10 polyhedra per 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 homologus and heterologous antisera. The FP phenotype was observed when FP virus was grown in culture at 17, 22, and 27 C. Hence, the FP phenotype was not considered to be the result of temperature-inhibited crystallization of polyhedrin under standard tissue culture conditions. The NOV of both strains killed insects when injected directly into the hemocoele of T. ni larvae. Only MP inclusion bodies were virulent per os. 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.
Insights
Two strains of Trichoplusia ni nuclear polyhedrosis virus (NPV) were identified: MP and FP. Serial passage in cell culture shifted MP to the FP phenotype, which showed faster release but lacked oral pathogenicity in insects.
Area of Science:
- Virology
- Insect Pathology
- Molecular Biology
Background:
- Nuclear polyhedrosis viruses (NPVs) are significant pathogens of insects.
- Two distinct plaque morphologies, MP and FP, were observed in Trichoplusia ni NPV isolates.
- Understanding viral strain variation is crucial for biological control applications.
Purpose of the Study:
- To characterize two NPV strains (MP and FP) of Trichoplusia ni based on plaque morphology and pathogenicity.
- To investigate the phenotypic transition from MP to FP during serial passage in cell culture.
- To elucidate the underlying mechanisms for differences in pathogenicity between the MP and FP strains.
Main Methods:
- Isolation and characterization of NPV strains based on plaque morphology.
- Serial undiluted passage of nonoccluded virus (NOV) in tissue culture.
- Comparative growth curve analysis of MP and FP NOV.
- Plaque neutralization assays.
- Pathogenicity assays via injection and oral feeding in Trichoplusia ni larvae.
- Electron microscopy of polyhedra.
Main Results:
- Serial passage of MP NOV in tissue culture induced a shift to the FP phenotype, which became predominant.
- FP NOV exhibited faster release kinetics compared to MP NOV.
- While both MP and FP NOV were lethal upon injection, only MP inclusion bodies were virulent per os.
- Electron microscopy revealed heterogeneous material in FP polyhedra, contrasting with the normal nucleocapsid bundles in MP polyhedra, correlating with the lack of oral pathogenicity.
Conclusions:
- The FP phenotype arises from serial passage of MP NPV in cell culture and is associated with altered polyhedra structure and reduced oral pathogenicity.
- The observed phenotypic changes suggest a potential mechanism for viral adaptation or attenuation in cell culture.
- Differences in polyhedra composition significantly impact the route of infection and overall virulence of Trichoplusia ni NPV.