Ultrastructural Study of the Relationships between the Gregarine Schneideria schneidrae Da Cunha et al. 1975 and the cells of the host Trichosia pubescens Morgante 1969 (Sciaridae, Diptera)

Abstract

Summary Schneideria schneiderae is a gregarine symbiont of the sciarid Trichosia pubescens. The trophozoites grow from sporozoites to adults inside the cytoplasm of cells of the intestinal caecum of the larva of the fly. The symbiont induces hypertrophy of the host cells. Control of DNA synthesis in the host cells is altered by the presence of symbionts, so that the polytene chromosomes grow to giant sizes. At later stages, the component fibres of the chromosomes are separated, giving rise to a polyploid nucleus. Electron microscope studies of consecutive stages of the host cell-symbiont system were made, from the entry of the sporozoites into the host cell up to the time of release of the adult trophozoites. The sporozoites enter the cell by endocytosis or by injection through a tube produced by the sporocysts. Endocytosis seems to be induced by the sporozoites without physical contact being made with the cells of the caecum. The structure of the host cell cytoplasm is changed soon after penetration by the symbiont. The trophozoites grow inside the parasitophorous vacuole. The studies show that metabolic exchanges take place between the symbiont and the cell nd that anatomical structures are adapted for this purpose. In particular, there is a very close relation between the symbiont and the host cell nucleus. Early in its development, once inside the host cell, the trophozoite moves towards the cell nucleus. During most of trophozoite development, the narrow space between its pellicle and the nuclear envelope of the host cell is occupied by a vacuolated cytoplasmic layer which is between 0.31 µm and 0.12 µm thick. The gregarine symbiont transforms the epithelial secretory cell into a nurse cell and constitutes with it a very well adapted system. The life cycles of Trichosia pubescens and Schneideria schneiderae are fully synchronized. The gregarine harbours in its cytoplasm a bacterium-like symbiont which is always present and which is transmitted by the sporozoite.