ULTRASTRUCTURAL AND ULTRACYTOCHEMICAL OBSERVATIONS ON BONAMIA SP IN OYSTERS (TIOSTREA-CHILENSIS), WITH A CONSIDERATION OF ORGANELLE FUNCTION

Samples of oysters (Tiostrea chilensis) collected from February to August, when Bonamia is most abundant, were used in a morphological and ultracytochemical study on the inter-relationships of organelles in this pathogen. Early samples contained intermediate density forms (IDF), but plasmodial forms were present and increased from May onwards. In IDF Golgi was membrane-bound, close to, and in contact with, cup-like indentations of the nuclear surface containing material similar in appearance to nuclear material. In some IDF the Golgi/nuclear material complex was detached in the cytoplasm. In plasmodia, the Golgi/nuclear material complexes were indistinguishable from Golgi/multi-vesicular body (MVB) complexes at which haplosporogenesis was evident. Loops of Golgi with dense material on the concave surfaces enclosed nuclear material to form haplosporosomes. Peripheral arrays of smooth endoplasmic reticulum, also thought to be Golgi, were associated with the formation of silky-textured ovoid granules, possibly acid phosphatase containing lysosomes. The formation of haplosporosomes from Golgi/nuclear material complexes, and evidence from other studies, indicate that haplosporosomes are composed of DNA, protein and/or glycoprotein. In composition and structure they are similar to viruses, and it is hypothesised they represent an early viral element in eukaryotic cells.