An ultrastructural study of the marine diatom Licmophora hyalina and its parasite Ectrogella perforons. I. Infection of host cells

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Abstract
An EM study was made on the infection and penetration of the marine diatom L. hyalina Agardh by E. perforans Petersen, an obligate fungal parasite of diatoms. The zoospores encyst on the host cell wall. The nucleus of the cyst may be situated proximal or distal to the host wall. A germ tube is produced from the side where the nucleus is situated. The germ tube may be branched or unbranched. The penetrating germ tube swells distally, develops an appressorium at the site of penetration of the host wall and pierces the host wall in the form of an infection peg. The infection peg is smaller in diameter than the germ tube and the appressorium. Successful infection takes place always at the areolae of the diatom wall. The infection peg may directly inject its contents by piercing the subfrustular layer of the diatom wall or may grow for some distance beneath the subfrustular layer. At the site of entry the host plasma membrane invaginates and surrounds the fungal protoplast. Initially, the host-parasite interface consists of a 2-layered envelope, of which the outer layer is the host plasma membrane and the inner one the fungal plasma membrane.