ELECTRON-MICROSCOPIC OBSERVATIONS ON PSEUDAPHELIDIUM-DREBESII SCHWEIKERT AND SCHNEPF, A PARASITE OF THE CENTRIC DIATOM THALASSIOSIRA-PUNCTIGERA

Ultrastructural observations revealed details of the infection process and the fine structure of Pseudaphelidium drebesii Schweikert and Sch nepf, a parasite of the marine centric diatom Thalassiosira punctigera (Castracane) Hasle. After attachment and encystment of the zoospore a n intracellular infection tube is generated. This structure is everted between the overlap of the girdle bands or the gap between the valve and the girdle band of the host diatom. The bulk of the Pseudaphelidiu m protoplast enters the silica shell of the diatom via the infection t ube but does not pierce the host plasma membrane. Parts of the host cy toplasm are phagocytized with microfilament involvement. Pseudaphelidi um drebesii is characterized by unusual microbodies containing tubular inclusions and by closed mitosis with a perinuclear spindle. The taxo nomic position of P. die besii is discussed.