Observations on the morphogenesis and structure of a hemocytic poxvirus in the midge Chironomus attenuatus

An account is given of the ultrastructure of a poxvirus infecting larvae of a midge, Chironomus attenuatus. The virus appears to replicate only in hemocytes, and is of the occluded type. Immature virions are formed in association with an amorphous virogenic stroma, portions of which are sequestered into the developing particles. Further maturation of virions occurs either free in the cytoplasm or during (or after) occlusion within the crystalline matrix of polyhedra. A new intermediate in the development of cores from nucleoids is described. In addition, the presence of a unit membranelike structure in the poxvirus core is indicated. A possible nucleation site for the deposition of polyhedron protein is described.