The zoospore ofOlpidium brassicae

Summary The ultrastructure of the zoospore ofOlpidium brassicae is described and compared with observations made of other zoospores of the uniflagellatePhycomycetes. The zoospore ofO. brassicae is characterized by an extensive, cone-shaped rhizoplast and a lack of a nuclear cap, as well as a side-body complex or a rumposome. Vacuoles which contain osmiophilic material are termed gamma-like particles. Three-dimensional reconstructions based on serial sectioning were made of the organelles in the region of the nucleus, showing that the zoospore ofO. brassicae contains one or at most two elaborately branched mitochondria. Microbodies have a high degree of interconnection and are in intimate association with the mitochondrion, lipid drops, and the nuclear envelope.     

CELL DIVISION IN THE FILAMENTOUS PLEURASTRUM AND ITS COMPARISON WITH THE UNICELLULAR PLATYMONAS (CHLOROPHYCEAE)1

At prophase in Pleurastrum, extranuclear spindle microtubules develop from the region of centrioles, which lie lateral to the nucleus midway between the future sites of the metaphase spindle poles. The microtubules then move laterally to overarch the nucleus and finally become incorporated into the spindle. The centrioles do not migrate and therefore lie in the same plane as the chromosomes at metaphase. At telophase, 2, more different systems of microtubules develop from the vicinity of the centrioles—a phycoplast and extensive arrays of microtubules that ensheath the daughter nuclei. Cell division in the filamentous Pleurastrum is compared to that in the green flagellate, Platymonas. The similarities between cell division in the 2 algae are interpreted as evidence: (i) that rhizoplasts (which in Platymonas resemble myofibrils) are somehow homologous to microtubules; and, (ii) that cell division in Pleurastrum differs from cell division in other examined filamentous chlorophycean genera because Pleurastrum has an independent evolutionary origin from a monad with Platymonas-like characteristics.     

The fine structure of Leishmania enriettii in the guinea pig

A study of the fine structure of Leishmania enriettii in the guinea-pig has been presented. There was close similarity to other members of the same genus and the finding of 2–3 axonemes (rhizoplasts) reported previously by other workers in non-dividing protozoa of the same species has not been confirmed. The functions of the main organelles and the morphological differences observed in comparison with those of other species have been reviewed.     

MITOSIS AND CELL DIVISION IN HYDRURUS FOETIDUS (CHRYSOPHYCEAE)1

Mitosis and cell division have been examined ultrastructurally in the vegetative cells of Hydrurus foetidus (Vill) Trev. and found to resemble that of Ochromonas in two important aspects. First, the rhizoplast acts as the spindle organizing body and second, the spindle elongates considerably during anaphase. It differs from Ochromonas in that there is no movement of the basal bodies and flagella towards the poles. Moreover, the nuclear envelope remains relatively intact throughout early stages of mitosis, with gaps developing at the poles during prophase to permit entry of spindle microtubules. Disruption of the nuclear envelope does not occur in the equatorial plane until late anaphase. The spindle persists into telophase and is bent towards the posterior of the cell by the ingrowing edge of the cleavage furrow. Persistence of the spindle and lack of Ochromoms-type cell elongation may be related to the constricting presence of the sheath during cell division—a completely different strategy to that adopted by the green algae under conditions of similar constraint.     

Ultrastructural and immunocytological studies on the rhizoplast in the chrysophycean alga Ochromonas danica

The rhizoplast, a striated band elongating from the flagellar basal body to the nucleus, is conspicuous in cells of Ochromonas danica Prings. In interphase cells, it runs from the basal body of the anterior flagellum to the space between the nucleus and the Golgi body. In O. danica, the rhizoplast duplicates during mitosis and the two rhizoplasts serve as mitotic poles. In the present study, we reinvestigated mitosis of O. danica using transmission electron microscopy and immunofluorescence microscopy, especially focusing on the rhizoplast. The nuclear envelope became dispersed during metaphase, and the rhizoplasts from two sets of the flagellar basal bodies functioned as the mitotic poles. Immunofluorescence microscopy using anti‐α‐tubulin, anti‐centrin and anti‐γ‐tubulin antibodies showed that centrin molecules were localized at the flagellar basal bodies, whereas γ‐tubulin molecules were detected at the rhizoplast during the whole cell cycle.     

Ultrastructure of Allapsa vibrans and the body plan of Glissomonadida (Cercozoa)

Biciliate, gliding zooflagellate Cercozoa are globally the most abundant and genetically diverse predators in soil (glissomonads and cercomonads). We present the first detailed ultrastructural study of a phylogenetically well-characterized glissomonad, Allapsa vibrans. There are two ventral posterior centriolar roots as in Cercomonadida, but fewer other microtubular roots. Allapsa's centriolar roots and rhizoplast basically resemble those of the less well studied glissomonads Bodomorpha and Neoheteromita. The posterior centriole of Allapsa attaches laterally to the base of the anterior centriole and to the nucleus by striated fibrillar connectors and nests in a shallow cup-like ventrolateral depression; two broad fans of single microtubules line the cup's posterior and inner side. The anterior centriole has a dorsal two-microtubule root and probably also a singlet root. Its medium-length ciliary transition zones have a proximal hub-lattice and a prominent dense distal transverse plate/collar complex. Golgi bodies are anterior/paranuclear; isodiametric extrusomes are anterior mid-ventral. Tubulicristate mitochondria attach to the nucleus, as do prominent microbodies. We characterize the body plan of glissomonads, comparing it with other Sarcomonadea: their sister group (Pansomonadida) and the phylogenetically more distant Cercomonadida. We discuss glissomonad radiation into families Sandonidae, Proleptomonadidae, Dujardinidae, Bodomorphidae and Allapsidae, establishing Aurigamonadidae fam. n. for the amoeboflagellate pansomonad Aurigamonas.     

The flagellar apparatus and striated rhizoplast of the zoospore ofOlpidium brassicae

Summary The flagellar apparatus and its associated structures of the zoospore ofOlpidium brassicae are described and compared with observations of other zoospores of the uniflagellatePhycomycetes. The zoospore ofO. brassicae is shown to have an extensive cone-shaped rhizoplast fused to both the functional and the vestigial kinetosomes. Three-dimensional reconstructions were made of the kinetosomal region. The vestigial kinetosome differs from the functional, as it only has triplet bundles of microtubules and it lacks a system of props. The proximal termination of the central pair of flagellar microtubules occurs within the axoneme. No terminal plate is observed. The occurrence of dictyosomes in theChytridiales, Monoblepharidales, andHyphochytriales is discussed and it is concluded that a dictyosome may be present in the encysting zoospore and the maturing zoosporangium ofO. brassicae but only vestiges of a dictyosome are to be found in the free-swimming zoospore.     

A RHIZOPLAST IN CARTERIA RADIOSA (CHLOROPHYCEAE)12

A rhizoplast or rhizoplast-like structure was observed with the electron microscope in Carteria radiosa. The cross-banded structure extends from the proximal end of each of at least 2 of the basal bodies and extends toward, although does not make contact with, the nucleus. The rhizoplast terminates in a ribosome-free area composed of fine granules and microfibrils. This is the first ultrastructural verification of a rhizoplast in a volvocalean alga.