title: Ultrastructure of zoosporogenesis in the alga Botrydiopsis alpina (Tribophyceae), as revealed by cryofixation and freeze substitution.
abstract: The fine structure of mitosis and cytokinesis leading to formation of zoospores in the unicellular tribophycean alga Botrydiopsis alpina Vischer has been studied by ultra-rapid cryofixation, freeze substitution, and electron microscopy. Preceding zoosporogenesis, the cell usually enlarges considerably and becomes multinucleate. Throughout zoosporogenesis, nuclear divisions are synchronous with centrioles and dictyosomes intimately associated with nuclear poles. During prophase, nuclei migrate towards the periphery of the cell. During metaphase, the spindle becomes aligned parallel to the cell surface. Anaphase separations of the chromosomes are synchronous and telophase is characterized by a persistent, slightly narrowed, elongated interzonal spindle. Formation of flagellar axonemes mainly commences in the central cytoplasm during prophase and, gradually during mitosis, these profiles are shifted towards the periphery of the developing zoosporangium. Cytokinesis occurs by synchronous, centripetal cleavage from the plasma membrane. Throughout mitosis, special flat vesicles are present underneath the plasma membrane and in conspicuous assemblages around extra-plasmatic pockets which contain extruded flagella. The present observations corroborate the view that centrioles may (in)directly determine the origin of cleavage furrows. The possible function of the flat vesicles is evaluated.
authors.affiliation: Rijksherbarium, State University of Leiden, Leiden, NL.
authors.forename: G M
authors.initials: GM
authors.lastname: Lokhorst
authors.forename: P J
authors.initials: PJ
authors.lastname: Segaar