Ultrastructural Changes During Sporangium Formation and Zoospore Differentiation in Blastocladiella Emersonii

Abstract

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.

An Albino Mutant of Blastocladiella emersonii: Comparative Studies of Zoospore Behaviour and Fine Structure

“…7) of this association is very similar to one (Fig. 55) published by Lessie & Lovett (1968) which prompted their comment that '. .…”

“…Fine structure of wild-type and mutant zoospores Actively swimming wild-type zoospores of Blastocladiella emersonii are subglobose to ellipsoid. Numerous aspects of the fine structure of such spores have already been descril5ed (Cantino, Lovett, Leak & Lythgoe, 1963;Fuller, 1966;Lessie & Lovett, 1968). When spores were first released from dense populations of OC plants into a surface film of fluid on solid media, they were irregularly elongated rather than Spore structure in a Blastocladiella 375 smoothly ovoid and crawling amoeboids rather than active swimmers.…”

“…; the loose pellet thus produced was fixed as follows. were diluted with 1.8 x I O -~ M-Na veronal+ Na acetate buffer (PH 7-8) to yield 2 %, I % and 2 % solutions, respectively (Lessie & Lovett, 1968). Spores fixed I hr in glutaraldehyde were postfixed I hr in either OsO, or KMnO,; the latter was also used alone (2 hr).…”

“…The biochemistry of RS morphogenesis and the changes in fine structure associated with zoosporogenesis in Blastocladiella emersonii have been studied in some detail (Cantino & Lovett, 1964; Lessie & Lovett, 1968); as yet genetic studies have not been…”

“…The significance of the differences among the granules situated in the doublemembraned lipid sac (Cantino et al 1963, Lessie & Lovett, 1968Reichle & Fuller, 1967) along the outer edge of the mitochondrion is still uncertain. In amoeboid as in swimming spores, these granules always appeared in two forms: one, an often elongated membrane-bound organelle (possibly a continuous matrix) of even electron density; the other, situated among the former, a somewhat polyhedral body with an electron-transparent centre and an electron-opaque limiting region.…”