Electron Microscopy of a Poxlike Virus Infecting an Invertebrate Host.

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The fine structure and development of a poxlike virus of insects were studied by electron microscopy. The virus multiplied in the cell cytoplasm and immature forms of the virus were formed at the periphery of viroplasms. Immature virus particles were oval in shape and measured approximately 3500 Å in diameter. Each immature particle was surrounded by two structures which resembled unit membranes. Immature particles underwent a phase of maturation during which the structures of mature virus particles were formed. Mature virus particles measured 3500 Å × 2500 Å and possessed a beaded coat composed of spherical units approximately 400 Å in diameter. Each virion had a core or nucleoid, the coat of which was composed of two layers, an outer layer 110 Å wide and an inner one 75 Å wide. The inner layer was smooth, but the outer layer was composed of subunits 110 Å in length and 60 Å in diameter. The virus core contained a fibrous material, but, occasionally, rodlike structures 300 Å in diameter and parallel to the long axis of the core were seen. Between the outer beaded coat and the viral core there was an electron dense area which appeared to be similar to the lateral bodies of animal poxviruses. Mature virions were found either free in the cell cytoplasm or occluded within ovoid, protein inclusion bodies 2–6 μ in length. The classification of this and other poxlike viruses of insects as a subgroup of the poxvirus family is discussed and the name “insectpox” is proposed for the subgroup.

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