Timing of an early sporulation sequence of microsporidia in the genus vairimorpha (Microsporidia: burenellidae).

An early sporulation event in the host midgut tissues has been reported for several species of microsporidia infecting Diptera, Hymenoptera, and Lepidoptera. The role of these primary spores, formed between 35 and 96 h postinfection per os, has been suggested to be the cell to cell spread of infection within the host, but the sequence of events during the early sporulation stages has been reported for only a few species of microsporidia. We investigated these early life cycle events for two species of microsporidia, Vairimorpha necatrix and Vairimorpha sp. from Lymantria dispar, tested in the laboratory hosts Spodoptera exigua and L. dispar, respectively. We injected hemolymph drawn from orally infected host larvae into uninfected larvae at time periods from 2 to 96 h postinfection to determine the timing of infection of the hemolymph and target tissues. Our studies demonstrated that, for both Vairimorpha species, the early sporulation in the midgut tissues is a discrete first stage of infection. Invasion of the hemolymph and infection of the target tissues follows maturation of "primary spores" in the midgut and coincides with the germination of these spores beginning approximately 30 h postinfection for hosts held at a constant 24 degreesC. The developmental stages of the microsporidia observed in the target tissues at specific time periods postinfection corresponded to the stages observed in the hemocytes, suggesting that sporoplasms from the germinating primary spores are directly injected into the target tissues and the hemolymph. Copyright 1998 Academic Press.