Aspects of the sporogonic development of Leucocytozoon tawaki of the Fiordland crested penguin in its primary vector, Austrosimulium ungulatum: an ultrastructural study.

Early oocysts of Leucocytozoon tawaki Fallis, Bisset and Allison were located between the basal lamina and the midgut epithelium of the vector, Austrosimulium ungulatum. The spherical oocysts were surrounded by an amorphous, electron-dense wall and contained a large, central core of closely spaced dense particles, the crystalloid inclusion. Around the latter were many concentrically arranged cisternae of granular endoplasmic reticulum. A few large, poorly defined nuclei, some of which contained spindle apparatus, were seen. Sporozoite formation occurred around the peripheral cytoplasm of maturing oocysts. The sporozoite pellicle was subtended by 30 microtubules which appeared to originate from the most posterior of 3 dense, polar rings. Each forming sporozoite contained a central nucleus, mitochondrion, and a crystalloid inclusion both anterior and posterior to the nucleus. The nature and significance of the crystalloid in Leucocytozoan species and other apicomplexans is discussed with special reference to similar viruslike inclusions in the sporogonic stages of certain species of Plasmodium.