新船叶藓的核型分析

新船叶藓Neodolichomitra robusta(Broth．)Nog．为东亚特有种,其配子体茎尖细胞有丝分裂中期的染色体数为n＝5,核型为K(n)＝5＝4V+1J或K(n)＝5＝4 m+1 sm,在核型分类中属于“2A”型。该研究结果为国内外首次报道。     

Head regeneration and polarity reversal inHydra attenuata can occur in the absence of DNA synthesis

Summary Regeneration in hydra is considered to be morphallactic because it can occur in the absence of cell division. Whether DNA synthesis is required for regeneration or other repatterning events is not known. The question was investigated by blocking DNA synthesis with hydroxyurea and examining several developmental processes. Head regeneration, reversal of regeneration polarity and battery cell differentiation all took place in the absence of DNA synthesis. Hence, morphallactic regulation in hydra is independent of both DNA synthesis and mitosis.     

Bud formation inHydra: Inhibition by an endogenous morphogen

Summary From crude extracts ofHydra tissue a substance has been purified which prevents or retards the asexual reproduction by budding. The molecular weight is in the range of 300 to 1000 daltons. Inhibition of bud formation can be observed with concentrations equivalent to the extract from one hydra per 4 ml, that is, to a more than 10,000-fold dilution of the initial crude extract of a hydra. The purified inhibitor is active at a concentration of less than 10^–8 M.Most of the inhibitor present inHydra is bound to cells. Within the cells the substance is mainly bound to particulate structures which sediment at 10,000 g. Its concentration is highest in the hypostomal region and decreases in the direction of the tentacles and peduncle. A second, lower, peak has been found in the basal disc. Treatment of the animals with a toxic agent (nitrogen mustard) which depletes the animal of interstitial cells, nematocytes and nematoblasts excludes the possibility that the inhibitor is present to any great extent in these cells. In conjunction with cell separation experiments by centrifugation of fixed cells in suspension, these results indicate that nerve cells are the most likely sites of storage of the inhibiting substance, although epithelial cells are not excluded as sources for the inhibitor.     

Analysis of early stages of budding inHydra by means of an endogenous inhibitor

Summary Buds originate inHydra attenuata at a position 1/3 of the body length from the basal disc. The position with respect to the vertical axes is determined first and the position of the bud on the circumference of this budding region is specified later.Bud formation in hydra is reversibly prevented by pre-treatment with an inhibitor purified from hydra tissue (Berking, 1977). Some hours after the end of the treatment with the inhibitor, bud formation is resumed. From the starting or restarting point of development after the inhibitory treatment to the visible beginning of bud formation, 4 intermediary stages were distinguished on the basis of different responses to a second treatment with inhibitor. The pre0treatment is followed immediately by a period of maximal sensitivity to the inhibitor, which varies in length. At the conclusion of this phase the time interval required for the visible appearance of buds is fixed (12 h). In this and the following phase another application of inhibitor can cancel the entire preparatory process from the pre-treatment onwards. A transition to near complete resistance to inhibitor is the basis for defining a third phase. In a fourth phase, immediately before the evagination of the bud starts, the proesence of the inhibitor will again hinder the development. Upon removal of the inhibitor the suppressed buds will appear.     

Ultrastructural and experimental investigations of sperm-egg interactions in fertilization ofHydra carnea

Summary Fertilization in the freshwater hydrozoanHydra carnea has been examined by light, scanning and transmission electron microscopy. Sperm penetrate the jelly coat which covers the entire egg surface only at the site of the emission of the polar bodies. The egg surface exhibits a small depression, the so called fertilization pit at this site. Sperm-egg fusion takes place only at the bottom of the fertilization pit.Hydra sperm lack a structurally distinct acrosome and in most of the observed cases, fusion was initiated by contact between the membrane of the lateral part of the sperm head and the egg surfacce. Neither microvilli nor a fertilization cone are formed at the site of gamete fusion. The process of membrane fusion takes only a few seconds and within 1 to 2 min sperm head and midpiece are incorporated in the egg.Electron dense material is released by the egg upon insemination but cortical granule exocytosis does not occur and a fertilization envelope is not formed. The possible polyspermy-preventing mechanisms in hydrozoans are discussed. Hydra eggs can be cut into halves whereupon the egg membranes reseal at the cut edges and the fragments assume a spherical shape. Fragments containing the female pronucleus can be inseminated and exhibit normal cleavage and development. The observation that in such isolated parts the jelly coat will not fuse along the cut edges was used to determine its role in site-specific gamete fusion. These experiments indicate that site-specificity of gamete fusion can be attributed to special membrane properties at the fertilization pit.     

The spatial distribution of cations in nematocytes of Hydra vulgaris

The spatial distribution of cations was assayed qualitatively and quantitatively in tentacular nematocytes of Hydra vulgaris in a scanning transmission electron microscope by means of x-ray microanalysis performed on 100 nm thick freeze-dried cryosections. The matrix of undischarged cysts (stenoteles, desmonemes and isorhizas) was found to contain mainly K⁺. In isolated nematocysts of Hydra the intracapsular potassium can be readily substituted by practically any other mono- and divalent cation (Na⁺, NH₄ ⁺, Mn²⁺, Co²⁺, Mg²⁺, Ca²⁺, Fe²⁺) all, except Fe²⁺, without impairing the ability of the cyst to respond to the chemical triggering with dithioerythritol or proteases. Monovalent cations increase the osmotically generated intracapsular pressure compared to divalent ions.     

Properties of the foot inhibitor from hydra

Summary A substance was isolated from crude extracts of hydra that inhibits foot regeneration. This substance, the foot inhibitor, has a molecular weight of 500 daltons. It is a hydrophilic molecule, slightly basic in character and it has no peptide bonds. The pruified substance acts specifically and at concentrations lower than 10^–7 M. At this low concentration only foot and not head regeneration is inhibited. Hydra are sensitive to purified foot inhibitor between the second and eight hour after initiation of foot regeneration by cutting. In normal animals the foot inhibitor is most likely produced by nerve cells. A substance with similar biological and physico-chemical properties is found in other coelenterates.     

Notch-signalling is required for head regeneration and tentacle patterning in Hydra

Local self-activation and long ranging inhibition provide a mechanism for setting up organising regions as signalling centres for the development of structures in the surrounding tissue. The adult hydra hypostome functions as head organiser. After hydra head removal it is newly formed and complete heads can be regenerated. The molecular components of this organising region involve Wnt-signalling and β-catenin. However, it is not known how correct patterning of hypostome and tentacles are achieved in the hydra head and whether other signals in addition to HyWnt3 are needed for re-establishing the new organiser after head removal. Here we show that Notch-signalling is required for re-establishing the organiser during regeneration and that this is due to its role in restricting tentacle activation. Blocking Notch-signalling leads to the formation of irregular head structures characterised by excess tentacle tissue and aberrant expression of genes that mark the tentacle boundaries. This indicates a role for Notch-signalling in defining the tentacle pattern in the hydra head. Moreover, lateral inhibition by HvNotch and its target HyHes are required for head regeneration and without this the formation of the β-catenin/Wnt dependent head organiser is impaired. Work on prebilaterian model organisms has shown that the Wnt-pathway is important for setting up signalling centres for axial patterning in early multicellular animals. Our data suggest that the integration of Wnt-signalling with Notch-Delta activity was also involved in the evolution of defined body plans in animals.     

Effects of Altered Light and Feeding Regimes on the Zooplankton Feeding Capability of Hydra viridis

The effects of light regime, feeding regime and tentacle number on the zooplankton feeding capability of Hydra viridis were tested in the laboratory. Feeding was measured by exposing Hydra to a known volume of Artemia salina nauplii and recording the number captured and ingested. In all cases there was a correlation between the number of Artemia captured and the number ingested. H. viridis with 7 tentacles captured and ingested more Artemia than Hydra with 6 tentacles. However, changes in light and/or feeding regimes did not alter the number of tentacles/Hydra. Varying light and feeding regimes altered the number of Zoochlorellae/cell and Hydra growth rate. There was no effect on the number of Artemia captured or ingested and no effect on the percent ingestion of captured Artemia. These data suggest that, under these conditions, zooplankton feeding by H. viridis is independent of nutritional history.