Microtubule organization during zoosporogenesis inAllomyces macrogynus

Summary The microtubule cytoskeleton and cytoplasmic organization ofAllomyces macrogynus during zoosporogenesis was studied using light and electron microscopy. Indirect immunofluorescence methods revealed that the microtubule cytoskeleton progressed through three distinct stages of cytoplasmic distribution during zoospore development. During the first 10 minutes of zoosporogenesis, nuclei were strictly located in the periphery of the cytoplasm, and their associated centrosomes were positioned immediately adjacent to the plasma membrane. Microtubules emanated from centrosomes into the surrounding cytoplasm. Within 20 to 30 min after the induction of zoosporangial cleavage, nuclei migrated to new positions throughout the sporangial cytoplasm and microtubule arrays were primarily organized at and emanated from nuclear surfaces. During the final stage of zoosporogenesis, nuclear envelope-associated microtubules were not observed. Instead, primary organization of cytoplasmic microtubules returned to centrosomes (i.e., basal bodies) and flagella formation was evident. The MPM-2 antibody, which recognizes phosphorylated epitopes of several proteins associated with microtubule nucleation, stained centrosome regions throughout zoosporogenesis but did not stain nuclear envelopes.Abbreviations BSA bovine serum albumin - DAPI 4,6-diamino-2-phenylindole - dH₂O deionized water - DMSO dimethyl sulfoxide - DS dilute salts solution - G/5 0.1% glucose medium - LN₂ liquid nitrogen - LSCM laser scanning confocal microscopy - MTOC microtubule-organizing center - PBS phosphate buffered saline - PCM pericentriolar matrix - TEM transmission electron microscopy - VELM videoenhanced light microscopy     

Zoosporogenesis in Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew

During sporulation of Pseudoperonospora cubensis on cucumber leaves ( Cucumis saliva ) zoosporangia are formed on the dichotomously branched sporangiophore. The mature zoosporangium has a preformed discharge papilla and the cytoplasm is uncleaved. The zoosporangium wall is decorated and the outer layer of the wall is electron opaque in ultrathin sections. As the zoosporangium is able to survive freezing (- 18°C) for prolonged periods of time (3–4 months) the zoosporangium may serve as the "resting" structure which survives overwintering in Northern latitudes in the absence of oospore formation.
Zoospore cleavage can be synchronized by placing freshly harvested zoosporangia in distilled water. Cleavage of the zoosporangial cytoplasm is by means of the fusion of small vesicles apparently derived from dictyosomes which become highly active after zoosporogenesis is induced.
Vesicles with an osmiophilic electron opaque content are the dominant type of vesicle found in the zoosporangia. The content of these vesicles undergoes dynamic changes during zoosporogenesis and during the late stages of sporogenesis the content becomes finely striated as is typical of these vesicles when observed in the zoospore. On the basis of the results presented here it is suggested that zoosporangium formation and zoosporogenesis in P. cubensis could serve as a model system for assays with obligate oomycetous plant pathogens, also in relation to fungicide mode of action studies.     

SPERMIOGENESIS OF THE TELEOST,ORYZIAS LATIPES,WITH SPECIAL REFERENCE TO THE FORMATION OF FLAGELLAR MEMBRANE *

In the early stage of Oryzias spermiogenesis, an axonemal bud appears at the distal end of a centriole characterized by its electron dense accessories. When the axoneme begins to grow in the cytoplasm, small vesicles come to surround it. These vesicles are similar to those produced by the Golgi apparatus which lies close to the growing axoneme. At this stage, the spermatid cell membranes disappear, causing transformation of the mononuclear spermatids into a multinucleated syncytium. As each axoneme elongates in the syncytium, it is enveloped by a cylindrical array of vesicles which are most likely derived from the Golgi apparatus. Shortly after this stage, the syncytium is again partitioned by cell membranes, restoring the existence of mononuclear spermatids. The arrayed vesicles fuse with each other to form two concentric membranes surrounding the axoneme. The inner membrane becomes the flagellar membrane and the outer one, the membrane of a flagellar sheath. These observations lead to the conclusion that the formation of the flagellar membrane is due to the fusion of vesicles surrounding the axoneme which are derived from the Golgi apparatus. In the course of spermiogenesis, no indication of an acrosomal structure is observed.     

Differentiation of Male Gametes in Gracilaria verrucosa (Rhodophyta, Gracilariales)

The development of male gametes (spermacia) in the red alga Gracilaria verrucosa has been studied using methods of transmission electron microscopy. Early spermatangia located along the wall of the conceptacle show an elongated shape in the thin sections. In the central part of the electron-dense cytoplasm of these cells there is a nucleus; numerous fibrous vesicles are arranged in the periphery. During the process of differentiation, the spermatangia become more rounded in shape and a large spermatangial vesicle is developed. The subsequent development of spermatium is accompanied by polarization of the spermatangium and the subsequent excretion of the spermatangial vesicle. The spermatia are oval cells containing a nucleus and fibrous vesicles. The process of differentiation of male gametes in G. verrucosa does not differ from that in five species of the genus Gracilaria, where it has already been studied. However, any conclusions about the degree of similarity between the spermatia in all the studied species can be made only after a detailed comparative analysis of the ultrastructural characteristics of these gametes.     

<Emphasis Type="Italic">Colletotrichum echinochloae</Emphasis>, a new species on Japanese barnyard millet (<Emphasis Type="Italic">Echinochloa utilis</Emphasis>)

Five isolates of a species of Colletotrichum were collected from Japanese barnyard millet (Echinochloa utilis) in Japan. Although the fungus had once been identi-fied as C. graminicola sensu lato, it was clearly different from C. graminicola isolated from maize (Zea mays) in its falcate and short conidia, 18.0–22.2 μm in length, cultural characteristics, and specific pathogenicity to E. utilis. Moreover, molecular phylogenetic analyses using sequences of rDNA-ITS, HMG, and SOD2 indicated a monophyly of the isolates. A new species, Colletotrichum echinochloae, is then proposed based on the morphological, pathological, and molecular characteristics.     

Membranes and calcium sequestration during spermiogenesis in the cotton seed bug (Dysdercus intermedius: Heteroptera)

Summary By use of osmium ferricyanide (OsFeCN) staining the fate of cytoplasmic membranes was followed during spermiogenesis in the cotton seed bug (Dysdercus intermedius). During early spermiogenesis interzonal lamellae of endoplasmic reticulum become aggregated as a stack of membranes traversing the entire cell body from the nucleus to the cytoplasmic bridge connecting neighbouring spermatids. Cisternae of endoplasmic reticulum ensheath the acroblast from which vesicles of different sizes are pinched off into the cytoplasm. The oxalate method was used to show that acroblast and associated vesicles are calcium-sequestering sites in spermatids. Membrane profiles with dense calcium oxalate precipitate derived from the acroblast form an uninterrupted membranous sheath at the apical side of the nucleus where the proacrosome will be attached. With further development of the spermatids, the vesicles derived from the acroblast also participate in forming a calciumsequestering sheath enveloping the axoneme and the mitochondrial nebenkern derivatives.     

Ultrastructural aspects of spermatogenesis in Calyptogena pacifica Dall 1891 (Vesicomyidae; Bivalvia)

The process of spermatogenesis and spermatozoon morphology was characterized from a deep‐sea bivalve, Calyptogena pacifica (Vesicomyidae, Pliocardiinae), a member of the superfamily Glossoidea, using light and electron microscopy. Spermatogenesis in C. pacifica is generally similar to that in shallow‐water bivalves but, the development of spermatogenic cells in this species has also some distinguishing features. First proacrosomal vesicles are observed in early spermatocytes I. Although, early appearance of proacrosomal vesicles is well known for bivalves, in C. pacifica, these vesicles are associated with electron‐dense material, which is located outside the limiting membrane of the proacrosomal vesicles and disappears in late spermatids. Another feature of spermatogenesis in C. pacifica is the localization of the axoneme and flagellum development. Early spermatogenic cells lack typical flagellum, while in spermatogonia, spermatocytes, and early spermatids, the axoneme is observed in the cytoplasm. In late spermatids, the axoneme is located along the nucleus, and the flagellum is oriented anteriorly. During sperm maturation, the bent flagellum is transformed into the typical posteriorly oriented tail. Spermatozoa of C. pacifica are of ect‐aqua sperm type with a bullet‐like head of about 5.8 μm in length and 1.8 μm in width, consisting of a well‐developed dome‐shaped acrosomal complex, an elongated barrel‐shaped nucleus filled with granular chromatin, and a midpiece with mainly four rounded mitochondria. A comparative analysis has shown a number of common traits in C. pacifica and Neotrapezium sublaevigatum.     

Microtubules,organelle movement,and cross-wall formation at the sporangial-rhizoidal interface in the fungus,Chytridium confervae

Summary Chytridium confervae is a eucarpic, monocentric chytrid. We have used light and electron microscopy to study the relationship between the nutrient absorbing rhizoids and the asexually reproductive sporangium during growth. We have also examined the induction of zoosporogenesis by starvation, and subsequent differentiation until zoospore release. During growth the cytoplasm of the rhizoids and the developing sporangium was continuous and similar. At the start of starvation a bundle of fibers that were visible with light microscopy appeared at the junction between the rhizoids and the sporangium. Two hours after initiation of starvation a wall, that was also visible with light microscopy, formed to separate the rhizoids from the sporangium. Electron microscopy revealed a large, ordered array of microtubules in the thallus at the same time that the fibers appeared, and a sharp difference in the density of ribosomes in the cytoplasm of the sporangium and that of the rhizoids that was apparent immediately after starvation. This cytoplasmic difference was preserved by the formation of a cross-wall that was penetrated by plasmodesmata. After the wall was formed the cytoplasm of the rhizoids senesced. Comparison ofC. confervae with other organisms that use arrays of microtubules to move organelles is made and speculation on the role of the microtubules in organelle movement and wall formation inC. confervae is offered.     

Abnormal spore cleavage: Abnormal spores of Allomyces macrogynus

In the aquatic phycomycete Allomyces macrogynus abnormal spore cleavage takes place in the presence of colchicine or benomyl resulting in multinucleate–multiflagellate spores due to failure in the formation of cytoplasmic microtubules after the induction of zoosporogenesis. The 27 cytoplasmic microtubules which normally surround the nucleus and nuclear cap of the mature spore are not formed in the presence of colchicine or benomyl. At high concentrations of colchicine (4–8 mg/ml) the spores do not have a flagellum. Colchicine or benomyl inhibit microtubule formation during zoosporogenesis and also appear to perturb the mobilization of the gamma bodies which are believed to be the source of the vesicles which form the axonemal membrane and cleavage furrows. These observations are discussed in relation to the hypothesis of Heath that cytoplasmic microtubules formed during zoosporogenesis determine cytoplasmic domains which will delimit the spore initials at cleavage. The observations presented here appear to confirm this hypothesis.     

Electron microscopy of infection of nematodes byDactylaria haptotyla

Infection of nematodes byDactylaria haptotyla, a nematode-trapping hyphomycete, was studied by electron microscopy. The cytoplasm of the adhesive knob in the fungus contained a number of electron-dense, membrane-bound vesicles, 0.2–0.5 µm in diam. The vesicles were rarely seen in the stalk cell or vegetative cell cytoplasm. When the adhesive knob came into contact with the nematode's cuticle, it secreted an adhesive which was seen in ultrathin sections between the knob and the cuticle as an amorphous mass. At the same time, electron-dense vesicles in the cytoplasm were reduced in number and many small vacuoles developed. Soon after capture of a nematode, the cell wall of the adhesive knob became obscure at the prospective site of penetration, where a vesicle, 0.7 µm in diam, was found in serial thin sections of the knob's cytoplasm. At the site facing the vesicle, the peripheral part of the nematode's cell exhibited a high electron density. The vesicle, which appeared to be derived from smaller electron-dense vesicles coalesced with each other, released its enzymic contents toward the captured nematodes before penetration by the fungus.     

Ultrastructure of the vegetative organisation and initial stages of silica plate deposition in the soil testate amoebaCorythion dubium

Summary The siliceous body plates ofCorythion dubium are bound by a band of organic cement which is thickest at the lateral margins. The anterior vacuolar cytoplasm is separated by a pigment zone, which forms a dark band in the mid-body region, from the compact posterior region containing a typical vesicular nucleus surrounded by a region of dense endoplasmic reticulum. A pellicular basket of microtubules surrounds the posterior cytoplasm. The large Golgi complex lies between the nucleus and the fundus. Numerous coated and uncoated vesicles from the Golgi cisternae are seen in the peripheral cytoplasm alongside developing plates. These small siliceous plates are enclosed in silicon deposition vesicles lying in surface ruffles of the plasmalemma, often in association with a pair of microtubules. Observations are made on the formation of these vesicles and the early stages of silica deposition. A comparison is drawn between silica deposition inC. dubium and choanoflagellates where there is a similar association between silicon deposition vesicles and microtubules.     

Relationship between temperature optima and secreted protease activities of three Pythium species and pathogenicity toward plant and animal hosts

The in vitro physiological characteristics of three species of Pythium (oomycetes) that utilize different food sources were compared with their ecological activities: P. insidiosum is a pathogen of mammals (including humans), P. graminicola infects the roots of graminaceous hosts, and P. grandisporangium is an enigmatic water mold isolated from mangrove leaves and marine algae. P. insidiosum and P. graminicola showed peak growth rates at 37 °C before complete inhibition of growth at 40 °C; P. grandisporangium grew fastest at 22 °C. Differences between the invasive pressures exerted by the hyphae of these microorganisms were not considered significant in relation to the substrates colonized by these water molds. All three species showed substantial secreted protease activity, producing three or more serine proteases with weights ranging from 24-38 kDa. Fastest growth rates were supported when collagen was supplied as the sole carbon source, and none of the species were able to grow on purified plant cell wall polysaccharides. The growth and nutritional characteristics of P. graminicola and P. grandisporangium bear little obvious relationship to the ecological niches that they inhabit. This highlights the caution necessary in extrapolating from laboratory analyses to the natural environment, and points to the potential importance of ecological opportunity in determining the host range and food source of certain microorganisms.     

福建省直播稻根结线虫病的病原种类鉴定

[目的] 福建省福清市江镜镇与福安市潭溪镇水稻区直播稻苗期分别发生严重的根结线虫病,本研究对其病原物进行形态和分子鉴定,明确病原物种类,以期为福建省直播稻根结线虫病害防控提供理论依据。[方法] 通过根结线虫各虫态形态学特征进行观测;同时对其rDNA-ITS区进行测序,通过贝叶斯法与最大似然法构建了系统发育树来确定种类;利用拟禾本科根结线虫特异性引物Mg-F/Mg-R检测种群。[结果] 根结线虫的雌虫、雄虫和2龄幼虫的形态学特征与拟禾本科根结线虫原始描述种一致;rDNA-ITS区序列长度为576 bp,与GenBank拟禾本科根结线虫种群相似度均达99%以上;系统发育树明确了该根结线虫与拟禾本科根结线虫处于同一分支;特异性引物Mg-F/Mg-R检测进一步明确病原为拟禾本科根结线虫单一种群。[结论] 本研究通过形态与分子特征,明确了在福建直播稻上发现的根结线虫为拟禾本科根结线虫。拟禾本科根结线虫在福建省最早于2011年在政和县小范围水稻田发现,此后未在其他水稻种植区发现。本次在福建直播稻上首次发现大面积的根结线虫为害。随着直播稻的种植面积扩大,拟禾本根结线虫引起的水稻病害可能会成为制约其发展的新问题,应引起足够重视。     

Ultrastructure and potential taxonomic importance of euspermatozoa and paraspermatozoa in the volutid gastropods Zidona dufresnei and Provocator mirabilis (Caenogastropoda, Mollusca)

The ultrastructure of mature spermatozoa is investigated for the first time in the Volutidae, based on the commercially significant South American species Zidona dufresnei (Donovan, 1823) (fresh material) and supplemented with observations on testicular (museum) material of the deep sea New Zealand species Provocator mirabilis (Finlay, 1926). Euspermatozoa of Z. dufresnei (ex sperm duct) consist of: (1) a tall-conical acrosomal vesicle (with short basal invagination, constricted anteriorly) which is flattened anteriorly and associated with an axial rod, centrally perforate basal plate and short accessory membrane; (2) a rod-shaped, solid and highly electron-dense nucleus (with short basal fossa containing centriolar complex and initial portion of a 9 + 2 axoneme); (3) an elongate midpiece consisting of the axoneme sheathed by 5–6 helical mitochondrial elements, each exhibiting a dense U-shaped outer layer; (4) an elongate glycogen piece (axoneme sheathed by nine tracts of putative glycogen granules); (5) a dense annulus at the junction of the midpiece and glycogen piece and (6) a short free tail region (axoneme surrounded only by plasma membrane). Paraspermatozoa of Z. dufresnei are vermiform and dimorphic: the first type contains approximately 14–20 axonemes (arranged peripherally and interspersed with microtubules) and numerous oblong dense vesicles, numerous less dense (round) vesicles, occasional, large lipid-like vesicles, and scattered mitochondria; the second type contains 25–31 axonemes (peripherally arranged, interspersed with microtubules), occasional mitochondria and extensive cytoplasm. Results obtained for P. mirabilis from testis material are essentially as observed in Z. dufresnei, although the euspermatozoan acrosome still has to achieve its compressed transverse profile. Observations on paraspermatozoa were limited by fixation quality of available (testis) tissues, but these cells are similar to the first type of Zidona paraspermatozoa. Although most of the euspermatozoal features are also observed in many neotaenioglossans and neogastropods, the U-shaped outer layer of each mitochondrial element has not previously been reported and may prove a diagnostic feature of the Volutidae, the subfamily Zidoniinae or possibly only the Zidonini (in which Z. dufresnei and P. mirabilis are currently placed).     

An ustilaginomycetous yeast,Pseudozyma graminicola sp. nov., isolated from the leaves of pasture plants

Two strains belonging to a novel anamorphic species, Pseudozyma graminicola, were isolated from the leaves of herbaceous plants in the Moscow region (Russia). This species was genetically distinct from all known Pseudozyma species, based on sequence divergence in the D1/D2 domains of the large subunit rDNA and the ITS region. It is related phylogenetically to species of the genus Sporisorium (Ustilaginaceae, Ustilaginales). Physiological characteristics distinguishing this novel species from the other species of the genus Pseudozyma are presented.     

Light is a crucial signal for zoosporogenesis and gametogenesis in some green microalgae

Patterns of reproduction were investigated in some microalgal species of Chlorophyceae (Botryosphaerella sudetica, Neochloris aquatica, Neochloris vigensis, Bracteacoccus minor). Under continuous light, the microalgae reproduced asexually producing autospores. However, appropriate manipulation of external conditions led to a change in the reproduction pattern towards production of zoospores or gametes. Production of zoospores and gametes was inhibited by light; motile cells emerged when microalgae were cultivated in darkness. The period of dark treatment necessary for zoosporogenesis or gametogenesis differed substantially among species that were tested. Sexual reproduction was observed in Neochloris vigensis and Bracteacoccus minor, whose generative life cycle had not been previously reported. The morphology of motile cells, the mode of sexual reproduction, and the efficiency of both the production of motile stages and mating events, were investigated. In order to gain detailed insights into patterns of reproduction, Botryosphaerella sudetica was selected for investigation under different light treatments. Non-actinic red light applied in the early phase of dark cultivation (up to 2 h) suppressed both zoosporogenesis and gametogenesis. However, after a 3-h dark pre-treatment, red light treatment had no effect on zoosporogenesis or gametogenesis. In contrast, non-actinic blue light did not block zoosporogenesis or gametogenesis, regardless of the time of treatment. The possible role of a red-light photoreceptor in zoosporogenesis and gametogenesis is discussed.     

Fixation methods can produce misleading artifacts in sperm cell ultrastructure of diploid and tetraploid Pacific oysters, Crassostrea gigas

Spermatozoa from diploid and tetraploid Pacific oysters (Crassostrea gigas) were examined after anisotonic fixation. Morphological anomalies, such as membrane rupture, detached tails, and the formation of tail vesicles (typically associated with damage attributable to procedures such as cryopreservation) were observed; the Mantel-Haenszel Chi-square test indicated a strong association between the anomalies and fixative osmolality (P<0.001). The present study also indicated that media in a range of 800 to 1,086 mOsm/kg could be assumed to be functionally isotonic to Pacific oysters, and osmolalities below or above this caused severe cell damage. For example, the maximum volume of flagella obtained after hypotonic fixation was approximately twice the volume of the flagella in isotonic fixation. Sperm cell flagellar volumes after hypertonic fixation (1,110 mOsm/kg) were 32% smaller than those in isotonic fixation, and sperm heads were 25% smaller. Although the damage associated with anisotonic fixation was evident in all parts of the sperm cells, the most vulnerable locations were the plasma membrane and flagellum motor apparatus. The formation of tail vesicles after hypotonic fixation was also examined. Because of water uptake, oyster sperm became swollen in hypotonic fixative, and bending or coiling of the axoneme within the tail vesicles led to the appearance of multiple axonemal structures in cross sections when observed by transmission electron microscopy. This phenomenon might be generally misinterpreted as the presence of double tails. This and other fixation artifacts can lead to the misinterpretation of damage caused by cryopreservation in ultrastructure studies of sperm of aquatic species, especially those in marine species.This work was supported in part by funding from the USDA-SBIR program, 4Cs Breeding Technologies, and the Louisiana Sea Grant College Program.     

Ultrastructure of euspermatozoa and paraspermatozoa in the volutid snail <Emphasis Type="Italic">Adelomelon ancilla</Emphasis> (Mollusca: Caenogastropoda)

The ultrastructure of the euspermatozoa and the paraspermatozoa is investigated in Adelomelon ancilla, through histological section observed by transmission electron microscopy. Euspermatozoa of A. ancilla consists of: (1) a conical acrosomal vesicle (with a short basal invagination, constricted anteriorly) which is flattened at the apex and associated with an axial rod, a centrally perforated basal plate and a short accessory membrane, (2) a rod-shaped, solid and highly electron-dense nucleus (with a short basal fossa containing a centriolar complex and a initial portion of a 9 + 2 axoneme), (3) an elongate midpiece consisting of the axoneme sheathed by 5–6 helical mitochondrial elements each exhibiting a dense U-shaped outer layer, (4) an elongate glycogen piece (where the axoneme is sheathed by nine tracts of glycogen granules), (5) a dense annulus at the junction of the midpiece and glycogen piece, and (6) a short free tail region (where the axoneme is surrounded only by plasma membrane). We observed a parasperm in A. ancilla. This is vermiform in shape and is composed of multiple axonemes and extensive cytoplasm with numerous vesicles, and mitochondria are scattered inside the axonemes. Sperm of A. ancilla is characterized by the euspermatozoa type 2 and the paraspermatozoa morphology belongs to type 5. The U shaped electrodense mitochondrial element in the midpiece of the eusperm and the constriction in the acrosomal vesicle present in A. ancilla are exclusive. We suggest that these characteristics could have taxonomic importance, because these was observed in other volutids and have not been observed in the rest of caenogastropods studies. We consider that the morphology of paraspermatozoa in A. ancilla corresponds to the “lancet” type.     

The formation of the pronuclei and their associated perinuclear cytoplasm; the determination of the phases of the first cleavage cycles in Crepidula fornicata (Mollusca,Gastropoda)

Summary

The behaviour of the male and the female pronuclei in Crepidula fornicata is studied, beginning at the formation of the second polar body. Shortly after the extrusion of the second polar body the female pronucleus is formed, and then the male pronucleus enters the yolk-free cytoplasm near the animal pole. Both pronuclei are enveloped by a typical nuclear membrane, and increase in size until the prophase; a zygote nucleus is not formed (“Ascaris type” of fertilization). In the meantime, the chromatin of both pronuclei is arranged in a meshwork in the centre of the pronuclei.

Shortly after the formation of the second polar body a special cytoplasm, the “perinuclear cytoplasm”, is formed in the vicinity of each of the pronuclei. During the early stages of the first cleavage cycle this cytoplasm is composed of numerous Golgi complexes, small dense Golgi vesicles, smooth endoplasmic reticulum vesicles, mitochondria and rosettes of glycogen-like granules. At later stages, when the pronuclei have met and their plasms coalesced, the number of Golgi elements decreases; at the same time, the small dense Golgi vesicles increase in number and aggregate in clusters.

The phases of the first three cleavage cycles are determined by cytophotometry. The nuclear DNA of the male pronucleus and that in the nuclei of the blastomeres of the 2- and the 4-cell stage is reduplicated between 7 and 33% of the normalized cleavage cycles; the G₂-phase is between 33 and 57%, while the mitotic phase occupies the last part of each cleavage cycle and the first 7% of the next cleavage cycle. There is no G j-phase. Since the female pronucleus lies just beneath the polar bodies, its DNA content could not be measured separately.