Ultrastructural changes during sporangium formation and zoospore differentiation in Blastocladiella Emersonii

Samples from synchronized cultures of Blastocladiella emersonii were examined by electron microscopy from the late log phase to the completion of zoospore differentiation. Log-phase plants contain the usual cytoplasmic organelles but also have an unusual system of large tubules ca. 45 mμ diam that ramify in organized bundles throughout the protoplast. After induction, zoosporangium differentiation requires a 2-hr period in which the nuclei divide, a cross wall forms to separate the basal rhizoid region, and an apical papilla is produced. Nuclear division in B. emersonii is intranuclear with a typical microtubular spindle apparatus and paired, unequal, extranuclear centrioles at each pole. The papilla is formed by a process of localized cell wall breakdown and deposition of the papilla material by secretory granules. Differentiation of zoospores begins when one of the two centrioles associated with each nucleus elongates to form a basal body. The flagella fibers arise from the basal body and elongate into an expanding vesicle formed by the fusion of small secondary vesicles. The cleavage planes are formed by fusion of vesicles similar to those associated with flagellum initiation. When cleavage is complete, each sporangium contains ca. 250–260 uninucleate spore units with their flagella lying in the cleavage planes. Probable fusion of mitochondria to produce the single mitochondrion of the zoospore occurs after cleavage; the mitochondrion does not take its position around the basal body and rootlets until just before zoospore release. The ribosomal nuclear cap is organized and enclosed by a membrane formed through fusion of many small vesicles during a short period near the end of differentiation.