Single Globule Research Articles

Solvent effect on equilibrium organization of confined polymers.

The equilibrium organization of two polymers in strong confinement has been extensively investigated in the scope of simple models in the recent past, after it was suggested that the entropic repulsion between polymers could be a mechanism for chromosome separation in living cells. Despite many efforts, how solvent quality can manipulate separation or compaction for the formation of domains in a two-polymer model system within symmetric or asymmetric confinement still remains elusive. This paper presents some important findings for a two-polymer system within box-like (symmetric) and triangle-like (asymmetric) confinement with equal areas, in terms of structural transitions such as from a single globule to individual globules, non-monotonic change in polymer size, variation in the free-energy barrier with separation, etc. By using the exact enumeration method on a two-dimensional self-avoiding walk model of polymers, our results reveal that the polymers have a greater preference to form individual globules rather than a 'micelle'-like single globule in triangular confinement compared to box-like confinement. Additionally, our results suggest that asymmetric confinement can act as an ideal tool to trap polymers in poor solvents due to the large free-energy barrier.

Diffusion dynamics of a single collapsed homopolymer globule at the solid–liquid interface

Contradictive to the conventional wisdom that a collapsed polymer globule in poor solvents adsorb on surfaces in a way analogous to the spreading of a liquid droplet, here we have shown via single molecule measurements that a single poly(N-isoporpylacrylamide) (PNIPAM) globule can jump from one spot to another as an elastic nonadhesive ball even on a hydrophobic polystyrene surface. The molecular weight dependence of the effective surface diffusion coefficient measured for the adsorbed globule suggested that it exhibited mostly a similar globular conformation to that in the bulk solution. Both the displacement and waiting time distributions of the adsorbed globules were found to follow a power-law decay rather than an exponential process, suggesting a broad distribution of binding energies due to the difference in degree of globule deformation. These effects together reflect a character of the viscoelasticity even in a single-chain globule in dilute solutions. Our findings also demonstrate that it is not the single-chain globule but the inter-globule aggregates at high concentration that lead to irreversible adsorption on the surface, which provides novel dynamics and mechanisms of how a thermosensitive polymer adsorbs on the hydrophobic surface above its lower critical solution temperature.

Pearl-Necklace-Like Local Ordering Drives Polypeptide Collapse.

The collapse of the polypeptide backbone is an integral part of protein folding. Using polyglycine as a probe, we explore the nonequilibrium pathways of protein collapse in water. We find that the collapse depends on the competition between hydration effects and intrapeptide interactions. Once intrapeptide van der Waal interactions dominate, the chain collapses along a nonequilibrium pathway characterized by formation of pearl-necklace-like local clusters as intermediates that eventually coagulate into a single globule. By describing this coarsening through the contact probability as a function of distance along the chain, we extract a time-dependent length scale that grows in a linear fashion. The collapse dynamics is characterized by a dynamical critical exponent z ≈ 0.5 that is much smaller than the values of z = 1–2 reported for nonbiological polymers. This difference in the exponents is explained by the instantaneous formation of intrachain hydrogen bonds and local ordering that may be correlated with the observed fast folding times of proteins.

Functional ultrastructure and cytochemistry of vitellogenesis and mature vitellocytes of the digenean Cainocreadium labracis (Dujardin, 1845), parasite of Dicentrarchus labrax (L., 1758)

Vitellogenesis and vitellocytes of Cainocreadium labracis were studied by transmission electron microscopy (TEM) and TEM cytochemistry. Four developmental stages were distinguished during vitellogenesis: (I) stem cell of high nucleo-cytoplasmic ratio; (II) early differentiation with chief activity focused on the beginning of protein synthesis and shell globule formation; (III) advanced differentiation with rapid intensification of protein synthesis, progressive fusion of single shell globules into large globule clusters, and formation of unsaturated lipid droplets surrounded by β-glycogen particles; and (IV) mature vitellocyte. Early vitellogenesis with vitellocyte maturation consists of: (1) increase in cell volume; (2) increased development of large, parallel cisternae of GER with production of proteinaceous granules; (3) development of small Golgi complexes that package granules; and (4) within vacuoles, progressive enlargement of proteinaceous granules into shell globule clusters formed during vitellogenesis. Three types of inclusions accumulate in large amounts in mature vitelline cells: (1) shell globule clusters, important component in the formation of egg shell; (2) numerous unsaturated lipid droplets. Though fewer, there are also diphasic droplets consisting of saturated and unsaturated lipids in the same droplet, and (3) a relatively small amount of β-glycogen particles, usually surround a few groups of lipid droplets. The β-glycogen and lipid droplets are nutritive reserves for embryogenesis. General pattern and functional ultrastructure of vitellogenesis greatly resemble those observed in some lower cestodes, such as bothriocephalideans and diphyllobothrideans. Variations and differences in the amount of lipids and of glycogen during vitellogenesis in lower cestodes and other trematodes are compared and discussed.

Explicit description of complexation between oppositely charged polyelectrolytes as an advantage of the random phase approximation over the scaling approach.

A polyelectrolyte complex (PEC) of oppositely charged linear chains is considered within the Random Phase Approximation (RPA). We study the salt-free case and use the continuous model assuming a homogeneous distribution of the charges throughout the polyions. The RPA correction to the PEC free energy is renormalized via subtraction of polyion self-energy in order to find the correlation free energy of the complex. An analogous procedure is usually carried out in the case of the Debye-Hückel (DH) plasma (a gas of point-like ions), where the infinite self-energy of point-like charges is subtracted from the diverging RPA correction. The only distinction is that in the PEC both the RPA correction and chain self-energy of connected like charges are convergent. This renormalization allows us to demonstrate that the correlation free energy of the PEC is negative, as could be expected, while the scaling approach postulates rather than proving the negative sign of the energy of interactions between the blobs. We also demonstrate that the increasing concentration of oppositely charged polyions in the solution first results in the formation of neutral globules of the PEC consisting of two polyions as soon as the concentration reaches a certain threshold value, cgl, whereas solution macroscopic phase separation (precipitation of globules) occurs at a much higher concentration, ccoac, ccoac ≫ cgl. Partitioning of polyions between different states is calculated and analytical dependencies of cgl and ccoac on the polyion length, degree of ionization and solvent polarity are found.

INDUCCION DEL DESOVE, DESARROLLO EMBRIONARIO Y LARVAL DE Chaetodipterusfaber (BROUSSONET) (PISCES, EPHIPPIDAE) EN LA ISLA MARGARITA, VENEZUELA

The paguara Chaetodipterus faber (Broussonet, 1782) is a species of economic importance in the South Caribbean. Juveniles were collected near sandy beaches of Margarita Island (Venezuela) and maintained in pompano cage cultures until they reached maturation of their gonads. Adults were selected and spawning was induced by means of gonadotropin hormones. The eggs of Chaetodipterus faber are spherical, transparent and bouyant, with a diameter of 0.9 - 1.1 mm and a single globule oil of 0.25 - 0.35 mm diameter. Hatching of C. faber eggs took place within 26 h at temperatures of 24.5 - 28.5°C (31-32 h at 20-23.5°C) and salinities between 37.5 - 38.5°/oo. Newly hatched larvae vary in length from 1.9 to 2.1 mm and have a yolk sac of 1.1. - 1.3 mm in total length which is deploted after two or three days. The larval phase was completed 25 days after hatching. Juveniles 45 days old have 17.17 ± 1.75 mm in total length and weigh 0.21 + 0.07 g. Development of eggs and larvae are described.

Mechanisms of Clostridium tyrobutyricum removal through natural creaming of milk: A microscopy study

Clostridium tyrobutyricum is the main spoilage agent of late blowing defect (LBD) in Grana Padano and Parmigiano-Reggiano cheeses; LBD is characterized by openings and holes and is sometimes accompanied by cracks and an undesirable flavor. Even a very few spores remaining in the cheese curd may cause LBD; thus, it is essential to eradicate them during milk natural creaming. By this process, most of the bacteria, somatic cells, and spores rise to the top of the milk, together with the fat globules, and are removed with the cream. Previous studies suggested that milk immunoglobulins mediate the interactions between fat globules and bacteria that occur upon creaming but no direct evidence for this has been found. Moreover, other physical chemical interactions could be involved; for example, physical entrapment of spores among globule clusters. To maximize the efficiency of the natural creaming step in removing Cl. tyrobutyricum, it is essential to understand the nature of spore–globule interactions. With this aim, raw milk was contaminated with spores of Cl. tyrobutyricum before going to creaming overnight at 8°C, after which spore and bacteria removal was >90%. The obtained cream was analyzed by light interference contrast and fluorescence microscopy and by transmission electron microscopy (TEM). Results showed that most of the vegetative cells and spores, which were stained with malachite green before addition to milk, adhered tightly to the surface of single fat globules, the membranes of which appeared heterogeneous when stained with the fluorescent dye DilC18(3)-DS. Using the same dye, we observed transient and persistent interactions among globules, with formation of clusters of different sizes and partial coalescence of adhering membranes. Transmission electron microscopy examination of replicates of freeze-fractured cream allowed us to observe tight adhesion of spores to fat globules. Ultrathin sections revealed that this adhesion is mediated by an amorphous, slightly electron-opaque material, sometimes granular in appearance. Bacteria also adhered to different fat globules, linking them together, which suggests that adhesion was strong enough to maintain a stable contact. Although we cannot exclude physical entrapment of bacteria among fat globule clusters, we show for the first time that most of the bacteria are adhered to fat globules by an electron-opaque material whose nature has yet to be determined. Immunoglobulins are certainly the best candidates for adhesion but other compounds may be involved.

Cluster coarsening during polymer collapse: Finite-size scaling analysis

We study the kinetics of the collapse of a single flexible polymer when it is quenched from a good solvent to a poor solvent. Results obtained from Monte Carlo simulations show that the collapse occurs through a sequence of events with the formation, growth and subsequent coalescence of clusters of monomers to a single compact globule. Particular emphasis is given in this work to the cluster growth during the collapse, analyzed via the application of finite-size scaling techniques. The growth exponent obtained in our analysis is suggestive of the universal Lifshitz-Slyozov mechanism of cluster growth. The methods used in this work could be of more general validity and applicable to other phenomena such as protein folding.

Effect of Induced Self-Organization in Mixtures of Amphiphilic Macromolecules with Different Stiffness

Effect of induced self-organization and liquid-crystal ordering of disordered globules of flexible amphiphilic macromolecules was observed in 1:1 mixtures of flexible and stiff helical macromolecules of similar composition. Mixtures of fully flexible macromolecules and helical macromolecules with various stiffness of backbone were studied by means of molecular dynamics simulation. It was found that if the local helical structure of the backbone is stiff enough to induce formation of aggregates with helical packing of backbone in poor solvent, the helical and flexible macromolecules segregate upon worsening of solvent quality. In dilute solution the flexible macromolecules are compacted into single globules with hydrophobic core and hydrophilic shell. The macromolecules with stiff local helical structure form aggregates of few helical chains. The aggregates have a core composed by intertwined backbones which is surrounded by a hydrophilic outer shell consisting of side groups. Increase of volume fraction o...

Local Probiotic Therapy for Vaginal Candida albicans Infections.

The high rate of vaginal Candida albicans recurrence is attributed to azole resistance rates as high as 15%. The aim of this study was to determine the clinical and microbiological efficacy of standard azole therapy for treatment of vaginal C. albicans infection alone and in combination with local probiotic as well as the effects on vaginal microbiota. This study included 436 women with vaginal candidiasis randomly assigned to two treatment groups. The first group, with 207 patients (12 dropouts), was administered 150 mg fluconazole and a single vaginal globule of fenticonazole (600 mg) on the same day. The second group of 209 patients (8 dropouts) followed the same treatment schedule; however, ten applications of a vaginal probiotic containing Lactobacillus acidophilus, L. rhamnosus, Streptococcus thermophilus, and L. delbrueckii subsp. bulgaricus were also administered beginning the fifth day after azole treatment. Microbiological analysis of the therapy efficacy in the first treatment group showed C. albicans resistance in over 30% of patients. Clinical complaints persisted after treatment administration in 79.7% (n = 165) of women in this group. Clinical complaints in the second group decreased to 31.1% (n = 65) and microbiological efficacy also improved among investigated parameters, from 93.7% (n = 193) to 95.2% (n = 198). The local application of probiotics after administration of combined azoles for treatment of vaginal C. albicans infections increases therapy efficacy and could prevent relapse.

Vitellogenesis in Orientocreadium batrachoides Tubangui, 1931 (Digenea: Allocreadiidae) from Clarias gariepinus, an ultrastructural study

Vitellogenesis in Orientocreadium batrachoides, an intestinal parasite of the catfish, Clarias gariepinus was investigated by transmission electron microscope. Four stages of vitellocyte development have been distinguished during vitellogenesis: (I) stem cell stage of the gonial type (immature vitellocyte), (II) early and (III) advanced stages of maturing vitellocytes. Both early and advanced stages of maturing vitellocytes involved the initiation of protein synthetic activity, onset of shell globule formation and progressive fusion of single shell globules into large shell globule cluster in their cytoplasm and (IV) mature vitellocyte. Vitellocyte maturation is characterized by: (1) an increase in cell volume, (2) extensive development of parallel cisternae of granular endoplasmic reticulum, (3) development of Golgi complexes and (4) a continuous development of shell globules that fused into shell globule clusters. Mature vitelline cells are characterized by shell globule clusters, which play an important role in egg shell formation, lipid droplets and β-glycogen that accumulated in large amounts in their cytoplasm. Single “lamellar” granules are present in the cytoplasm of the mature vitellocytes.

Slow knot formation by suppressed self-reptation in a collapsed polymer chain

Chain-expansion processes from knotted globules have been measured for poly(methyl methacrylate) (PMMA) in the mixed solvent tert-butyl alcohol (TBA) + water (2.5 vol %) by static light scattering. The solution was quenched from the Θ temperature of 41.5 °C to 37.0 °C, aged there for a time period t(p,) and then returned rapidly to the Θ temperature. The chain-expansion process was determined as a time evolution of the expansion factor α(2) after the temperature increase. The measurement was carried out by changing the aging time t(p) from 240 to 7200 min, and the molecular weight from M(w) = 4.0 × 10(6) to 1.5 × 10(7), by taking advantage of the extremely slow chain aggregation in the solution. The chain-expansion process obtained for M(w) = 1.22 × 10(7) became slow with increasing t(p), which revealed the knot formation in single globules. The characteristic time of the chain expansion from globules aged for t(p) = 7200 min was found to depend on the molecular weight as M(w)(2.7). This exponent, which is close to 3, demonstrated a disentanglement process due to self-reptation. The present data were compared with the previous data of the chain expansion from compact globules aged at 25.0 °C. The comparison made at M(w) = 1.22 × 10(7) and at the same values of t(p) revealed that the chain expansion from the globules aged at 25.0 °C was much faster than that from the globules at 37.0 °C, indicating a lower knot density in the more compact globules. It was conjectured that the knot formation due to self-reptation would be suppressed in a compact globule because an entire conformational change required by knot formation would become difficult to occur in the confined space of high segment concentration, particularly for a long polymer chain. The chain collapse of PMMA in the mixed solvent has been observed to occur extremely slowly at the later stage. This slow process was explained by the suppressed self-reptation.

Early development of New Zealand hapuku Polyprion oxygeneios eggs and larvae

This study describes for the first time the normal development of New Zealand hapuku Polyprion oxygeneios embryos and larvae reared from fertilization to 11 days post-hatch (dph) at a constant temperature. Fertilized eggs were obtained from natural spawnings from communally reared captive wild broodstock. Eggs averaged 2 mm in diameter and had single or multiple oil globules. Embryos developed following the main fish embryological stages and required an average of 1859·50 degree hours post-fertilization (dhpf) to hatch. The newly hatched larvae (4·86 mm mean total length, L(T) ) were undifferentiated, with unpigmented eyes, a single and simple alimentary tube and a finfold that covered the entire body. Larvae relied on the energy from the yolk-sac reserves until 11 dph (7·33 mm mean L(T) ), when yolk-sac reabsorption was almost completed. Some of the major developmental stages from hatching to yolk-sac reabsorption were eye pigmentation (5 dph), upper jaw formation (7 dph), lower jaw formation (8 dph) and mouth opening (8-9 dph). By 9 dph, the digestive system consisted of pancreas, liver, primordial stomach, anterior and posterior gut; therefore, P. oxygeneios larvae would be capable of feeding on live prey. The developmental, morphological and histological data described constitutes essential baseline information on P. oxygeneios biology and normal development.

The Gas Phase Structure of Coulombically Stretched Polyethylene Glycol Ions

Prior ion-mobility mass-spectrometry (IMS-MS) studies of polyethylene glycol (PEG) ions have identified only two out of many sharply different observed structures: Linear shapes with several individually solvated singly charged cations at high charge states z (beads on a string), and single multiply charged globules at low z. The present study is devoted to assign all other existing structures of PEG ions, for the first time reaching masses of 100 kDa and charge states up to z = 10. There are at most z different structures at charge state z. All involve a single globule carrying n charges, tied to one or several appendices bearing z - n separate charges in a beads-on-a-string configuration. All sharp shape transitions observed at decreasing ion mass involve ejection of one elementary charge (sometimes two) from the shrinking globule into the growing linear appendage. This picture is supported by molecular dynamics simulations and approximate calculations of electrical mobilities for computed structures.

Ovarian structure and oogenesis of the oviparous goodeids Crenichthys baileyi (Gilbert, 1893) and Empetrichthys latos Miller, 1948 (teleostei, Cyprinodontiformes)

The cyprinodontiform family Goodeidae comprises two biogeographically disjunct subfamilies: the viviparous Goodeinae endemic to the Mexican Plateau, and the oviparous Empetrichthyinae, known only from relict taxa in Nevada and California. Ovarian characteristics of two oviparous species of goodeid, Crenichthys baileyi and Empetrichthys latos, studied using museum collections, are compared with those of viviparous species of goodeids. Both subfamilies have a single, cystovarian ovary. The ovary in the viviparous Goodeinae has an internal septum that divides the ovarian lumen into two compartments, and it may possess oogonia. There is no ovarian septum in the oviparous C. baileyi and E. latos. Oogenesis is similar in both subfamilies with regard to the proliferation of oogonia, initiation of meiosis, primary growth and development of an oocyte during secondary growth in which fluid yolk progressively fuses into a single globule. Notably, eggs of C. baileyi and E. latos are approximately double the size of those of the viviparous Goodeinae in which embryos develop inside the ovarian lumen and are nourished, in part, by nutrients transferred from the maternal tissues, a mode of embryo development called matrotrophy. Egg envelopes of the two subfamilies differ in that those of C. baileyi and E. latos have a relatively thick zona pellucida, attachment fibrils or filaments that develop between the follicle cells during oogenesis, and a micropyle observed only in E. latos. In contrast, viviparous goodeid eggs have a relatively thin zona pellucida, but lack adhesive fibrils, and a micropyle was not observed. These reproductive characters are compared with those of species of the eastern North American Fundulus, a representative oviparous cyprinodontiform. One newlyrecognized shared, derived character, a single, median ovoid ovary with no obvious external evidence of fusion, supports monophyly of the Goodeidae. Differences among the goodeid subfamilies and Fundulus are interpreted relative to the oviparous versus viviparous modes of reproduction.

Effect of confinement on the collapsing mechanism of a flexible polymer chain

In this paper, Brownian dynamics simulation (BDS) studies are executed to demonstrate the distinctive influences of the extent of confinement on the collapsing mechanism and kinetics of a flexible hydrophobic polymer chain in a poor solvent. The collapsing behavior is quantified by the time of collapse, which below a critical dimension of the confinement (h(c)), encounters a drastic reduction with a further strengthening in the degree of confinement. For dimensions greater than this critical one, the collapse occurs through the well-known hydrodynamic interaction (HI) controlled multiple-globule-mediated mechanisms. However, for channel dimensions less than this critical one, the collapse mechanism is drastically altered. Under such circumstances, the collapse gets predominantly controlled by the confinement effects (with negligible contribution of the HIs) and occurs via the formation of a single central globule. This central globule rapidly engulfs the noncondensed polymer segments, and in the process largely hastens up the collapsing event. Under such circumstances, the collapse time is found to decrease linearly with decrements in the channel height. On the contrary, for channel heights greater than h(c), the multiple-globule-mediated collapse is characterized by a collapse time that shows an exponential dependence on the channel height, rapidly attaining a state in which the confinement effect becomes inconsequential and HIs dictate the entire collapsing behavior. We further propose detailed arguments based on physical reasoning as well as free energy estimations to conclusively support the qualitative and quantitative nature of influences of the confinement on the polymer collapse.

Study of putative microfossils in space dust from the stratosphere

AbstractInterplanetary dust particles (IDPs) were recovered from the stratosphere by a cryosampler flown below a balloon flying at altitudes of 20–41 km. The present study uses high-resolution scanning electron microscopy (SEM) and ultraviolet-visible (UV-Vis) spectrophotometry to examine fresh samples collected at 38–41 km. The SEM observations confirm the presence of 7–32 μm sized clusters of coccoidal (0.4–1.3 μm in diameter) and rod-shaped (0.6–2.5 μm in length) objects as components of the IDP complex. Many single globules (1.6–9.0 μm in diameter) are also observed, some of which exhibit a rough surface with filamentous features of variable lengths. The spectrophotometry of the particles in aggregate reveals a prominent peak centred at 216 nm, which is remarkably similar to that of diatoms and close to the UV astronomical feature of 217.5 nm that has been identified as the spectral characteristic of aromatic hydrocarbons. The evidence presented here suggests that the stratospheric particles are IDPs comprising an assortment of materials among which are included microfossil-like features in variable sizes and forms, such as coccoids, rods and filaments.

A new look at sporoderm ontogeny in Persea americana and the hidden side of development

The phenomenon of self-assembly, widespread in both the living and the non-living world, is a key mechanism in sporoderm pattern formation. Observations in developmental palynology appear in a new light if they are regarded as aspects of a sequence of micellar colloidal mesophases at genomically controlled initial parameters. The exine of Persea is reduced to ornamentation (spines and gemmae with underlying skin-like ectexine); there is no endexine. Development of Persea exine was analysed based on the idea that ornamentation of pollen occurs largely by self-assembly. Flower buds were collected from trees grown in greenhouses over 11 years in order to examine all the main developmental stages, including the very short tetrad period. After fixing, sections were examined using transmission electron microscopy. The locations of future spines are determined by lipid droplets in invaginations of the microspore plasma membrane. The addition of new sporopollenin monomers into these invaginations leads to the appearance of chimeric polymersomes, which, after splitting into two individual assemblies, give rise to both liquid-crystal conical 'skeletons' of spines and spherical micelles. After autopolymerization of sporopollenin, spines emerge around their skeletons, nested into clusters of globules. These clusters and single globules between spines appear on a base of spherical micelles. The intine also develops on the base of micellar mesophases. Colloidal chemistry helps to provide a more general understanding of the processes and explains recurrent features of pollen walls from remote taxa.

Comparative morphology, ultrastructure and functions of the excretory organ (postventricular midgut) in the Parasitengona (Acariformes)

The excretory organ of terrestrial mites Leptotrombidium orientale (Schluger, 1948), Euschoengastia rotundata (Schluger, 1955), and Hirsutiella zachvatkini (Schluger, 1948) (Trombiculidae); Platytrombidium fasciatum(Koch, 1836) and Camerotrombidium pexatum (Koch, 1837) (Microtrombidiidae) as well as of water mites Teutonia cometes (Koch, 1837) (Teutoniidae) and Piona carnea (Koch, 1836) (Pionidae) in different developmental stages was studied using transmission electron microscopy and on semi-thin sections. Irrespectively of the species studied and developmental stages, the excretory organ is represented by a simple thin-walled blind sac that occupies an axial position. The excretory organ is not divided into obvious morphologically or functionally different parts. Posteriorly, the excretory organ transforms into the short cuticular-lined excretory duct, the proper hindgut, derivative of proctodeum. The excretory duct terminates by the axially orientated fissured opening (excretory pore, uropore) that anatomically corresponds to the anus, located on the ventral body wall. In larvae P. carnea, the excretory duct and the excretory pore are lacking. The lumen of the excretory organ is variously filled with yellowish double-refractive crystals of excretory wastes (probably guanine) and sometimes also contains single electron-dense globule. The walls of the excretory organ are formed of the single-layered endodermal epithelium, composed of the uniform cells greatly variable in their shape and size. The basal plasma membrane forms round digital indentations frequently containing corresponding projections of the underlying connective tissue cells penetrating through the basal lamina. Clear vacuoles resulting from these indentations immerse into the cells and migrate through the cytoplasm to the apical plasma membrane where they are discharged from the cells. The cells may also contain round or irregularly shaped electron-dense bodies resembling secondary lysosomes or residual bodies. These inclusions may come close to the apical plasma membrane and may be release from the cells. The apical plasma membrane forms irregular microvilli as well as larger irregular projections also bearing microvilli. Golgi bodies and profiles of granular endoplasmic reticulum occur only rarely. Passive transport of residual precursors, vacuolar transport as well as excretion of the electron-dense material provides the accumulation of wastes in the lumen of the organ.

Phylogenetic position of the genus Mesochytrium (Chytridiomycota) based on zoospore ultrastructure and sequences from the 18S and 28S rRNA gene

Ultrastructural features and a molecular phylogeny of Mesochytrium penetrans (strain X-10 CALU) based on 18S and 28S rRNA gene sequences were investigated for the first time. The parasite is strongly specific for the green alga Chlorococcum minutum (Chlorococcales) and did not grow on another 29 strains of algae from the CALU collection. Most attention was paid to the zoospore ultrastructure. Ribosomes are dispersed through the cytoplasm. The nucleus and microbodylipid globule complex (MLC) is surrounded by rough endoplasmic reticulum. The MLC is composed of a single mitochondrion and a single lipid globule partially covered with a microbody and a fenestrated cisterna, which is most posterior. The non-flagellated centriole is shorter than the kinetosome and lies at an angle of approximately 30° to the latter, the two being connected with a broad, dense fibrillar bridge. The flagellar transition zone contains a spiral fiber. The non-flagellated centriole is surrounded by a thin fiber, and has a veil. The features that distinguish Mesochytrium are the partial penetration of the sporangium into the host cell, and a zoospore with unique ultrastructural configuration. In our phylogeny, M. penetrans was nested within Lobulomycetales and the Polychytrium clade, but its phylogenetic position is tenuous because of marginal support, and thus its family and order status are considered incertae sedis. Zoospores of M. penetrans are most similar to those of Synchytrium macrosporum and Rozella allomycis, which belong to different clades. These ultrastructural similarities may be a result of the relative simplicity of zoospore morphology, and do not reflect a phylogenetic relationship between these genera.