Role of the parasporal body in causing toxicity of Bacillus thuringiensis toward Aedes aegypti larvae

A unique type of microorganism has been found causing an unusual disease in larvae of the clover cutworm, Scotogramma trifolii (Lepidoptera; Noctuidae). The organism contains DNA and reproduces exclusively by self-assembly forming enveloped reniform/bacilliform particles which measure 170 × 450 nm in negatively stained preparations. During initial stages of development, the organism apparently reproduces primarily within vesicles in the cytoplasm of a variety of cell types including hemocytes, and epidermal, fat body, and tracheal matrix cells. Most reproduction, however, occurs in vesicles that circulate in the hemolymph. These vesicles, most of which are derived from host cells, measure 2–10 μm in diameter, are highly refractile, reach populations as high as 10⁸/ml of hemolymph, and are diagnostic for the disease. The pathology caused by this organism, its shape and ultrastructure, and reproduction within vesicles indicate it is either a peculiar type of rickettsia, possibly related to those of the genus Rickettsiella, or a new type of invertebrate virus. Among its unusual features are its ability to induce the formation of reproductive vesicles from host cell components, and its apparent control of de novo ribosome and membrane synthesis within these vesicles as it develops. The possible relationship of this organism to baculoviruses and rickettsia is discussed.