Oral Assembly in Left-Handed Tetrahymena thermophila

We have investigated oral development in a non-genically derived left-handed (LH) form of Tetrahymena thermophila , in which the large-scale asymmetry of arrangement of cortical structures is reversed whereas the local asymmetry of ciliary architecture remains normal. Approximately 1/2 of the oral apparatuses (OAs) of LH cells develop in the form of superficial mirror-images of OAs of RH cells. In most of these OAs, membranelles are assembled from the cells'anterior to posterior. Nonetheless, the posterior ends of these membranelles undergo the basal body displacements that lead to a "sculptured" appearance, so that the membranelles of LH OAs become organized as rotational permutations of membranelles of normal RH OAs. Many of these membranelles re-orient to a normal orientation near the end of oral development. Membranelles and undulating membranes (UMs) may develop independently of each other, and formation of postciliary microtubules of UMs is separate from that of ribbed wall microtubules. In some cases, the entire OA develops and remains as a 180° rotational permutation of the normal, resembling the inverted OAs of mirror-image doublets and LH cells of Glaucoma scintillans described by Suhama [36, 37]. We present a model (Fig. 37) for these complex developmental outcomes. These developmental patterns resemble those described previously and less completely for "secondary" OAs of cells with mirror-image global patterns, including janus cells. The present study demonstrates that such alterations in oral development are not a direct outcome of genotypic changes.     

Molecules and Morphologies: The Perpetuation of Pattern in the Ciliated Protozoa*†

SYNOPSIS. Three apparently conflicting generalizations concerning the relationships between molecules and cell structure may be derived from studies on cellular patterning in the ciliates. (A) Cells with identical genes and molecular composition may have different hereditary patterns. (B) Genes, through their prescribed molecular derivatives, constrain the modes of pattern permutation and define the states of greatest stability. (C) Cells with identical hereditary patterns may have entirely different genes and molecular compositions. These principles may be reconciled through the recognition that they are characteristically applicable over different time intervals. Hereditary differences within a clone and without related molecular differences (principle A) may persist for hundreds of cell generations, but they are resolved eventually within a constant environment (principle B) as the configuration of minimal free energy is approached. On an even longer time scale, molecular substitutions have occurred for many or most components of the cell, but these have been constrained by selective pressures on an ancient design (principle C) that disallow substitutions affecting certain form-function relations which have been elevated to an adaptive peak.     

RNA干涉在纤毛虫中的研究进展

RNA干涉是dsRNA介导的基因沉默现象,本文简要介绍了其作用的机制和生物学意义,重点阐述了RNA干涉在原生动物纤毛虫中的发现与应用,比较了RNA干涉与纤毛虫大核基因组重排机理的异同,并对RNA干涉在纤毛虫中传输的技术途径-RNAi喂饲法的原理也做了详细的介绍。     

The Contribution of Ciliated Protozoa to Zooplankton Biomass in an Acidic, Subtropical Lake

Ciliated protozoa accounted for up to 50% of the mean daily zooplankton biomass in McCloud Lake, a small (5 ha), oligotrophic, acidic (pH 4.7) lake in north-central Florida. Food resources (algae and bacterioplankton) were limiting for crustacean and rotifer zooplankton during much of the year. Myxotrophic ciliates were a dominant component of the planktonic food web. Stentor niger , an uncommon species in the plankton of lakes, dominated the ciliate assemblage and usually comprised >90% of total ciliate biomass. Stentor niger always contained high densities of photosynthetic zoochlorellae and contributed an estimated 30% to the total autotrophic biomass.     

A Note on the Possible Ecological Significance of Chemotaxis in Certain Ciliated Protozoa1

ABSTRACT. A heat-stable chemoattractant has been isolated from bacterial cultures. This component has a molecular weight in the range of 500–1000 daltons, is produced by both Gram-positive and Gram-negative bacteria, and serves equally well as an attractant for both the bacterial feeding Paramecium and for its natural predator, Didinium. Aspects of the ecological relationship between bacterial feeding ciliates and their ciliate predators are briefly discussed with respect to responses of both predator and prey to such a common chemotactic bacterial factor.     

The Contribution of Ciliated Protozoa to Plankton and Benthos Biomass in a European Ricefield

The densities and biomass of ciliates inhabiting the water-sediment interface and the water column of an experimental ricefield were investigated throughout four annual cycles. Ciliate abundance and biomass were higher at the water-sediment interface than in the water column. In both sites, large ciliates (> 10⁵μm³) contributed the higher biomass values, but the highest densities were found in the intermediate size class (10⁴-10⁵μm³). The prorodontid Coleps hirtus dominated the ciliate assemblage and usually comprised > 50% of total ciliate density. Blooms of C. hirtus , occurred in June in the water column and in July at the sediment surface. During the four cycles of rice cultivation, the average daily values for production of the entire ciliate community was 69.6 mgC/m²/d, and the net production efficiency (K₂) was 72.0%. The estimated production values in the present study are high if compared to production measured for ciliates in other freshwater ecosystems.     

Mass production of basal bodies in paraspermiogenesis of Tubificinae (Annelida,Oligochaeta)

The oligochaete annelids, belonging to the subfamily Tubificinae, produce two types of spermatozoa: eusperm (the fertilising ones) and parasperm, protecting and carrying the eusperm. The pathway for the production of the two types is common until the onset of meiosis, but then a regular meiosis produces eusperm, whereas parasperm is generated through a peculiar mechanism of nuclear fragmentation giving rise to an irregular, but very high, number of paraspermatozoa. Since every parasperm has its own flagellum, this entails the necessity of producing a very high number of basal bodies. In the present paper, we describe how basal bodies are generated through a mechanism similar to that producing the basal bodies in ciliated epithelia, but never observed up to now during the genesis of a uniflagellated cell. Basal bodies form in close proximity to a precursor structure called deuterosome, which originates de novo in the cytoplasm from fibrogranular material. The various stages of centriologenesis are positive to anti-centrin antibodies and, observed by electron microscopy, correspond closely to the ones described for ciliated epithelia. However, once formed, the basal bodies migrate to their final position and produce the parasperm flagellum.     

Structural,Mechanical, and Dynamical Variability of the Actin Cortex in Living Cells

In eukaryotic cells, an actin-based cortex lines the inner leaflet of the plasma membrane, endowing the cells with crucial mechanical and functional properties. Unfortunately, it has not been possible to study the structural dynamics of the actin cortex at high lateral resolution in living cells. Here, we performed atomic force microscopy time-lapse imaging and mechanical mapping of actin in the cortex of living cells at high lateral and temporal resolution. Cortical actin filaments adopted discernible arrangements, ranging from large parallel bundles with low connectivity to a tight meshwork of short filaments. Mixing of these architectures resulted in attuned cortex networks with specific connectivity, mechanical responses, and marked differences in their dynamic behavior.     

Ultrastructural studies of male-incubated ovaries of the gypsy moth,Lymantria dispar

Ovaries from Lymantria dispar females were transplanted into an environment lacking the vitellogenin ligand; i.e., the male milieu. Transmission electron micrographs comparing the terminal oocytes of male-grown ovaries and normal ovaries showed that yolk sphere diameters were reduced in the male-grown oocytes. However, there were larger numbers of these small yolk spheres per unit area of cytoplasm, indicating that the coalescence of endosomes into yolk spheres is reduced in the absence of vitellogenin. Although there are larger numbers of yolk spheres in male-grown oocytes, the smaller diameter of yolk spheres resulted in less area being taken up by yolk spheres per unit area of cytoplasm in male-grown oocytes, yielding lowered yolk production. This lowered yolk production is a result at least in part of the lowered number of coated vesicles per unit area of submembrane space and in part of the reduced interfollicular spaces seen in male-grown ovaries.     

Identification and localization of a protein immunologically related to Caltractin (Centrin) in the Myonemes and Membranelles of the Heterotrich ciliate Stentor coeruleus

The contractile properties of the myonemes of Stentor are very similar to caltractin (centrin)-containing fibers of other organisms. We investigated whether the calcium-binding protein caltractin was present in Stentor by using three different antibodies to caltractin or caltractin-related proteins, in conjunction with immunofluorescence microscopy and protein blotting. Immunofluorescence demonstrated that a protein immunologically similar to caltractin is present in the myonemes and in the bases of the membranelles of Stentor. The localization to the myonemes is observed in intact cells, osmotically lysed cells, and isolated cortices. Double-label immunofluorescence with anti-alpha-tubulin and anti-caltractin antibodies showed that the fluorescence in the myonemes was not in the overlying Km fibers. The myonemes in the posterior one-third of the cell appear as thick fibers with no cross-bridging. They become thinner as they approach the anterior end of the cell and show extensive cross-bridging here. Staining in the bases of the membranelles shows a distinct comma-like immunofluorescence pattern similar to that seen with protargol-stained cells and SEM views of the membranellar band reported by others. Western blots demonstrated that the caltractin-like protein in Stentor has an apparent molecular weight of 23 kDa compared with the 20-kDa protein from Chlamydomonas and is a calcium-binding protein.     

Ultrastructural visualization of exocytosis in the pig pineal gland

Summary The pinealocytes of the pig contain conspicuous dense bodies, the nature and role of which are not yet fully elucidated. The aim of this study was to demonstrate whether or not these structures are involved in the secretion process. The tannic acid-Ringer incubation (TARI)-method, which allows a clear-cut ultrastructural study of secretory discharge by exocytosis, has been used. The results indicate that pig pinealocytes release the content of the dense bodies with an amorphous inner structure into the extracellular space via exocytosis and that this secretion is quantitatively important. The secreted material is proteinaceous in nature; this indicates that polypeptides are released by the pineal.     

Ultrastructural tracer studies on the permeability of the Malpighian tubule of the Pill Millipede,Glomeris marginata (Villers)

Summary The electron-dense tracers ferritin, and iron-dextran, and the protein horseradish peroxidase, have been used to investigate the ultrastructural basis of permeability in the upper and lower segments of the Malpighian tubules of Glomeris marginata. All these materials were able to cross the basal lamina and enter the tubule lumen of the upper segment, and it was established that horseradish peroxidase was able to enter the channels which interrupt the apical junctions.In the upper segment, ferritin, iron-dextran, and horseradish peroxidase are all taken up and accumulated within intracellular vesicles. In the lower segment ferritin and iron-dextran enter the cells but become generally distributed over the cyptoplasm, as well as entering membrane-bounded vacuoles. The behaviour of horseradish peroxidase could not be assessed owing to the presence of endogenous peroxidase activity in the cells.After fixation by direct application of glutaraldehyde to the undissected tubules, the extracellular spaces contained large numbers of membrane-bounded vesicles. The significance of these observations is discussed in relation to the physiological activities of the tubules.The authors are indebted to the Science Research Council for financial supportThe authors wish to thank Mrs. Margarita Petri for her technical assistance and advice     

The role of oocyte maturation in the ontogeny of the fertilization site in the hydrozoan Hydractinia echinata

Summary In hydrozoans the sperm will fuse with the egg only at the site of polar body formation. The primary oocyte and maturing oocytes which have produced the first polar body cannot be fertilized even though maturing oocytes which have produced the first polar body attract sperm. These eggs do not acquire the ability to be fertilized until after second polar body formation. If either first or second polar body formation is inhibited or if first and second polar body formation do not take place in close proximity to each other, the fertilization site is not set up. Under normal circumstances the site of polar body formation takes place at the region on the maturing oocyte surface nearest the site where the germinal vesicle resided in the primary oocyte. When maturing oocytes are centrifuged prior to polar body formation, the site of polar body formation is frequently shifted so that it does not correspond to the site where it would be given off under normal circumstances. Under these conditions the shifted site of polar body formation is the only site where the egg can be fertilized, indicating that the fertilization site is selected during oocyte maturation.Oocyte maturation in these hydrozoans is mediated by a hormone released by the somatic cells of gonophores as a consequence of bringing dark adapted gonophores into the light. The hormone acts directly on the oocyte to induce maturation. The oocyte only has to be exposed to the hormone for the first few minutes of the maturation process in order to complete the process of maturation.Dedicated to Professor N.H. Verdonk of the Rijksuniversiteit Utrecht on his 65th birthday