Frank O. Perkins

The ultrastructure of holdfasts, “rhizoids”, and “slime tracks” in thraustochytriaceous fungi and Labyrinthula spp.

SummaryThe fine structure of the holdfasts or “rhizoids” is described for the thraustochytriaceous organisms, Thraustochytrium motivum, Schizochytrium aggregatum, and an unidentified organism, denoted T-20, which resembles S. aggregatum and Labyrinthula spp. Labyrinthula algeriensis and L. minuta “slime track” ultrastructure is also described. The holdfasts, rhizoids, and tracks have the same basic fine structure and are collectively termed ectoplasmic nets. They are delimited by a unit membrane which is in continuity with the plasmalemma, contain no cytoplasmic organelles only membrane-limited cisternae, and contain a fibrogranular ground substance. The nets appear to arise from one or as many as 20 organelle complexes consist of an approximately disk-shaped electron-dense granular aggregate in which are embedded portions of cisternae of the endoplasmic reticulum or perinuclear clear continuum. The cisternae appear to contribute small (ca. 17 nm diameter) vesicles to the granular aggregate which coalesce to form internal membranes of the net elements. The sagenogenetosome underlies the plasmalemma where it evaginates to form the delimiting membrane of the main trunk element of the net. No continuous membrane separates the net contents from the cytoplasm, only the granular aggregate.In L. algeriensis, L. minuta, and T-20 the net is necessary for motility of nonflagellated, nonamoeboid cells. Presence of the nets is not associated with motility in S. aggregatum and T. motivum. The possible taxonomic significance of the observations is discussed.

Chromosome Number of a Small Protist: Accurate Determination

Electron micrographs of serial sections through a meiotic prophase nucleus of the mycetozoan Labyrinthula sp. show that there are nine separate and distinct synaptinemal complexes. Since each complex represents a set of paired homologous chromosomes, it follows that the haploid chromosome number of this protist is nine.

Fine structure of the oyster pathogen Minchinia nelsoni (Haplosporida, Haplosporidiidae)☆

Abstract The fine structure of plasmodia and spores of the sporozoan Michinia nelsoni , pathogenic to the oyster Crassostrea virginica , is described. Plasmodial nuclei have fibers of 150 to 180 A attached to centriolar plaques such as have been described for other microorganisms. Electron-dense elements with internal structure were found singly and in groups in the cytoplasm. The groups appeared to be areas of elaboration or formation. Mitochondria showed fibers reminiscent of intramitochondrial DNA fibers described by other investigators. The spores contained a prominent Golgi apparatus which probably corresponds to the anterior cytoplasmic spherule of the literature on Minchinia sp. Spore wall ontogeny is described in detail.

A new species of marine labyrinthulid Labyrinthuloides yorkensis gen. nov. spec. nov. —Cytology and fine structure

SummaryThe structure and life cycle of a new genus and species of labyrinthulid, Labyrinthuloides yorkensis, is described from observations of pure cultures. Uninucleate cells are capable of gliding by means of ectoplasmic nets which push or pull the cells across the substrate. The net elements do not enrobe the cells and motility is reversible. Sporulation occurs by successive bipartition or progressive cleavage of the protoplast. Biflagellated zoospores with an anterior tinseled flagellum and posterior whiplash flagellum are formed as are non-replicating amoebae and plasmodia. The fine structure includes centrioles with a single large granule in the lumen and numerous cytoplasmic inclusion bodies with associated membrane complexes. The membrane-inclusion arrays are each found within a delimiting vesicle membrane.

Ultrastructure of vegetative stages in Labyrinthomyxa marina (Dermocystidium marinum), a commercially significant oyster pathogen.

Abstract The microstructure of the protist Labyrinthomyxa marina as found in a host organism, Crassostrea virginica, is described. Most of the organelles were similar to those of many eucaryotic protists; however, a few unusual structural features were observed. Viruslike particles (460–530 A diameter) were found in the nucleus and cytoplasm of the Virginia, but not the Florida, pathogens. Cell-wall formation appeared to occur, in part, by formation of material in vesicles derived from the endoplasmic reticulum. The vesicles appeared to fuse with the plasmalemma and release material into the cell wall. Funguslike lomasomes were found between the cell wall and plasmalemma. Vacuoplast formation occurred by synthesis of droplets of electrondense material through the membranes of large vacuoles followed by coalescence to form one or two large vacuoplasts. Mitochondria were involved in the formation of vesicles which contained electron-dense material and which were delimited by two membranes. Since L. marina has previously been allied with the Labyrinthulia (Protozoa) and the Saprolegniales (Phycomycetes), comparisons are made with the ultra-structure of representatives from both groups. Affinities with the fungi seem to be the closest on the strength of lomasome formation and the repeated formation of cell walls in all vegetative stages.

Range extension of Bonamia ostreae to Maine, U.S.A.

Flat oysters, Ostrea edulis L., were collected from the Damariscotta River, Maine, in September of 1991 and 1992 and from Tomales Bay, California, in October of 1991 and 1992. Histological examination revealed infections with Bonamia ostreae in 34 and 45% of the flat oysters from Maine and in 31 and 12% of those from California. The history of oyster movements between geographic locations suggested that the source of bonaniasis in flat oysters in Maine was infected flat oysters from California.

Ultrastructure of the Nebenkörper or “secondary nucleus” of the parasitic amoeba Paramoeba perniciosa (Amoebida, Paramoebidae)

Abstract The fine structure of the Nebenkorper or secondary nucleus of Paramoeba perniciosa is described. It consisted of one or two eukaryotic nuclei and a prokaryotic-like nucleoid with cytoplasm surrounding each of the two regions. The cytoplasm contained, as normal constituents, ribosome-like particles and phagosomes with the latter containing cytoplasm of the amoeba. No Golgi bodies, endoplasmic reticulum, or mitochondria were observed. Oval, bipolar Nebenkorpers were observed in which one eukaryotic nucleus was present at each pole, separated by a prokaryotic-like nucleoid. Such Nebenkorpers were derived from division of the eukaryotic nucleus in unipolar forms. On the basis of its structure and division, the Nebenkorper appears to be a discrete organism, not an organelle of the amoeba; however, an accurate determination cannot be made until adequate biochemical and physiological information is available concerning its interactions with the amoeba. If the Nebenkorper is a discrete organism, then its taxonomic affinities are uncertain since there are no known taxa in which it could be placed.

Ultrastructural observations of spore excystment, plasmodial development and sporoblast formation inHaplosporidium lusitanicum (Haplosporida, Haplosporidiidae)

The fine structure of spore excystment, plasmodial development, and sporoblast formation phases of the life cycle of the sporozoanHaplosporidium lusitanicum haplosporidian species recently described by Azevedo (1984), pathogenic to the plate limpetHelcion pellucidus (Gastropoda) is described. After liberation from the spore wall, the uninucleate amoebula penetrates the host and is first found in the connective tissue in contact with the periphery of the ovary and hepatopancreas. The amoebula develops into a plasmodium by enlargement and nuclear multiplication as evidenced by mitotic figures each consisting of a bundle of microtubules attached to spindle pole bodies situated in the nucleoplasm near the nuclear envelope. The cell becomes successively binucleated, tetranucleated, etc.. forming a multinucleated sporont containing several hundred nuclei. Each nucleus contains a nucleolus and obvious chromatin.In the next phase a very irregular membranous system and some Golgi complexes begin to differentiate in the cytoplasm. Then, each nucleus and a portion of surrounding cytoplasm is encircled by a limiting membrane which arises from fusion of Golgi vesicles. In sporonts several hundred sporoblasts are thus formed.

The ultrastructure of fishing tentacle muscle in the jellyfish Chrysaora quinquecirrha: A comparison of contracted and relaxed states

The ultrastructure of fishing tentacle muscle of Chrysaora quinquecirrha in contracted and relaxed states is described. Myofilaments are found only in the ectoderm on the mesogleal side of “T”-shaped epitheliomuscular cells where they are grouped into one or two muscle fibrils per cell. When relaxed to 30 times the contracted tentacle length, the cell membrane between muscle fibrils and mesoglea is smooth, no desmosomes are found along the plasmalemmas of juxtaposed muscle fibrils, myofilaments are parallel to each other and to the longitudinal axis of the fibril, and only two classes of myofilaments, averaging 189 and 80 A in diameter, are found. Upon contraction the fibril decreases in length by about 20% and increases in diameter by a factor of about 5. Myofilaments are thrown into considerable disarray, muscle processes containing myofilaments are extended into the mesoglea, and desmosomes are formed. Two classes of myofilaments are still found but the diameters of the thick ones apparently increase by about 40% to an average of 275 A. A paramyosin-like, third class of myofilament is found in tentacles relaxed to 7.2 to 20 times the contracted length. Extreme tentacle length changes are apparently due primarily to folding of the tentacle, with only a small part resulting directly from shortening of the muscle fibril.

Fine structure of zoospores fromLabyrinthomyxa sp. parasitizing the clamMacoma balthica

Zoospores were obtained from a protist, parasitic in the clamMacoma balthica. It has been previously suggested that the parasite is either a closely related species or is the same species asLabyrinthomyxa marina (Dermocystidium marinum), which is found in the oysterCrassostrea virginica. The demonstration of zoosporulation and the description of zoospore micromorphology are here offered as evidence of similarity between the two parasites. The zoospores were biflagellated, pyriform cells with mastigonemes along one side of the anterior flagellum. Negative staining revealed substructure in the mastigonemes.