Fine structure of the thraustochytrid Ulkenia amoeboidea. I. Vegetative thallus and formation of the amoeboid stage

Abstract

Development of the thraustochytrid Ulkenia amoeboidea was investigated at the ultrastructural level. The mature thallus possesses a lamellate wall, a nucleus with intranuclear vesicles and lamellae, several Golgi bodies, mitochondria, bundles of microfilaments, multivesicular bodies, dilated perinuclear continuum with filamentous material, endoplasmic reticulum, sagenogenetosomes, and two centrioles. Several unit membrane bounded, variously electron-dense inclusion bodies with electron-dense globular units are present. Wall scales are produced in Golgi cisternae which inflate to form vesicles. These vesicles deposit the wall material to the outside by exocytosis. An aggregate of unit membrane bounded electron-dense cisternae (paranuclear body) is found adpressed to the nucleus. A close association between the paranuclear body and the mitochondria, the former often producing finger-like projections in mitochondrial vicinity, is present. A protocentriole-like structure is seen near the nucleus of young thalli. At later stages, the ectoplasmic net elements disappear. Closely adpressed membrane arrays appear between the cell wall and plasmalemma. These are accumulated in bundles at various places in the cell and are later found in presumed autophagic vacuoles. Before the cell contents escape as an amoeboid mass, the cell wall becomes thinner owing to the peeling off of wall scales and the cell contents round up, with the plasmalemma becoming detached from the cell wall. Various vesicles are closely associated with the plasmalemma.