Mechanisms of maternal inheritance of dinoflagellate symbionts in the acoelomorph worm Waminoa litus

Waminoa litus is a zooxanthella-bearing acoel worm that infests corals. It is unique to Bilateria in that it transmits its algal symbionts vertically via eggs irrespective of the heterogeneity of the symbionts. It simultaneously harbors two dinoflagellate genera: Symbiodinium and Amphidinium. In this study, we examined the timing and vertical transmission pathway of algal symbionts in W. litus using light and electron microscopy. The oogenesis of the worm can be divided into three stages: stage I, in which the ovary is absent; stage II, the early vitellogenic zone containing immature oocytes formed in the ovary; and stage III, with both early and late vitellogenic zones in the body. In the early vitellogenic zone at stage II, oocytes are surrounded by accessory-follicle cells (AFCs). Both Symbiodinium and Amphidinium symbionts are not initially observed in the oocytes, but are observed in the AFCs. In the late vitellogenic zone at stage III, oocytes are enveloped by a complete sheath of AFCs; the algal symbionts are taken up by the late vitellogenic oocytes. These observations suggest that AFCs mediate the transfer of the algae from the parent to the oocytes. Ribotype analyses of the Symbiodinium symbionts revealed that they differ from those harbored by coral in the same experimental aquarium. These results indicate that W. litus has an active algal transport pathway and maintains a specific lineage of Symbiodinium via vertical transmission.     

Cellular growth of host and symbiont in a cnidarian-zooxanthellar symbiosis

The hydroid Myrionema ambionense, a fast-growing cnidarian (doubling time = 8 days) found in shallow water on tropical back-reefs, lives in symbiosis with symbiotic dinoflagellates of the genus Symbiodinium (hereafter also referred to as zooxanthellae). The symbionts live in vacuoles near the base of host digestive cells, whereas unhealthy looking zooxanthellae are generally located closer to the apical end of the host cell. Cytokinesis of zooxanthellae occurred at night, with a peak in number of symbionts with division furrows (mitotic index, MI = 12%-20%) observed at dawn. The MI of zooxanthellae decreased to near zero by the middle of the afternoon and remained there until the middle of the next night. Densities of live zooxanthellae living inside of host digestive cells peaked following cytokinesis, whereas densities of unhealthy looking symbionts were highest just before the division peak. Mitosis of host digestive cells was highest in the evening, also preceding the peak in zooxanthellar MI. This is the first study relating phased host cell division to diel zooxanthellar division in marine cnidarians. Food vacuoles were prevalent inside of digestive cells of field-collected hydroids within a few hours after sunset and throughout the night, coinciding with digestion of captured demersal plankton. Laboratory experiments showed that food vacuoles appeared in digestive cell cytoplasm within 2 h of feeding with nauplii of Artemia. The number and size of food vacuoles per digestive cell and the percentage of digestive cells with food vacuoles all decreased 5-7 h following feeding in laboratory experiments, and by mid-day in field-collected hydroids. Light and external food supply were important in maintaining phased division of the symbionts, with a lag in response time to both parameters of 11-36 h. Altering light and feeding during the night did not influence the level of the peak MI the next morning, though in one experiment the absence of light slowed final separation of daughter cells at the end of cytokinesis. In another experiment, hydroids starved for 3-7 d and "pulse-fed" Artemia nauplii for 1 h at the beginning of the dark period showed continued low symbiont division (< 5%) after 11 h, whether maintained in constant light or darkness, implying that most algal division is set more than 24 h prior to actual cytokinesis. Transferred to a 14:10 h light:dark cycle for another 24 h (36 h after feeding), the same hydroids exhibited a "normal" peak MI (ca. 15%) at dawn, but zooxanthellae from hydroids kept in constant darkness still showed a low MI. These results show that mitosis of symbiotic dinoflagellates requires three factors: external food; a minimum period of time following feeding (11-36 h), presumably for digestion; and a period of light following feeding, presumably to provide carbon skeletons necessary for completing cytokinesis.     

The effects of elevated temperature on the photosynthetic efficiency of zooxanthellae in hospite from four different species of reef coral: a novel approach

Bleaching of reef corals is a phenomenon linked to temperature stress which involves loss of the symbiotic algae of the coral, which are known as zooxanthellae, and/or loss of algal pigments. The photosynthetic efficiency of zooxanthellae within the corals Montastrea annularis, Agaricia lamarki, Agaricia agaricites and Siderastrea radians was examined by pulse-amplitude modulation fluorometry (PAM) during exposure to elevated temperatures (30–36°C). Zooxanthellae within M. annularis and A. lamarki were found to be more sensitive to elevated temperature, virtually complete disruption of photosynthesis being noted during exposure to temperatures of 32 and 34°C. The photosynthetic efficiency of zooxanthellae within S. radians and A. agaricites decreased to a lesser extent. Differences in the loss of algal cells on an aerial basis and in the cellular chlorophyll concentration were also found between these species. By combining the non-invasive PAM technique with whole-cell fluorescence of freshly isolated zooxanthellae, we have identified fundamental differences in the physiology of the symbionts within different species of coral. Zooxanthellae within M. annularis appear to be more susceptible to heat-induced damage at or near the reaction centre of Photosystem II, while zooxanthellae living in S. radians remain capable of dissipating excess excitation energy through non-photochemical pathways, thereby protecting the photosystem from damage during heat exposure.     

Regulation and control of intracellular algae (= zooxanthellae) in hard corals

To examine algal (= zooxanthellae) regulation and control, and the factors determining algal densities in hard corals, the zooxanthellae mitotic index and release rates were regularly determined in branch tips from a colony of a staghorn coral, Acropora formosa, recovering from a coral ''bleaching'' event (the stress-related dissociation of the coral–algal symbiosis). Mathematical models based upon density-dependent decreases in the algal division frequency and increases in algal release rates during the post-bleaching recovery period accurately predict the observed recovery period (ca. 20 weeks). The models suggest that (i) the colony recovered its algal population from the division of the remaining zooxanthellae, and (ii) the continual loss of zooxanthellae significantly slowed the recovery of the coral. Possible reasons for the ''paradoxical'' loss of healthy zooxanthellae from the bleached coral are discussed in terms of endodermal processes occurring in the recovering coral and the redistribution of newly formed zooxanthellae to aposymbiotic host cells. At a steady-state algal density of 2.1 x 10⁶ zooxanthellae cm^-2 at the end of the recovery period, the zooxanthellae would have to form a double layer of cells in the coral tissues, consistent with microscopic observations. Neighbouring colonies of A. formosa with inherently higher algal densities possess proportionately smaller zooxanthellae. Results suggest that space availability and the size of the algal symbionts determines the algal densities in the coral colonies. The large increases in the algal densities reported in corals exposed to elevated nutrient concentrations (i.e between a two- and five-fold increase in the algal standing stock) are not consistent with this theory. We suggest that increases of this magnitude are a product of the experimental conditions: reasons for this statement are discussed. We propose that the stability of the coral–algal symbiosis under non-stress conditions, and the constancy of zooxanthellae densities in corals reported across growth form, depth and geographic range, are related to space availability limiting algal densities. However, at these densities, zooxanthellae have attributes consistent with nutrient limitation.     

An Ultrastructural study of oocyte growth within the endoderm and entry into the mesoglea in Actinia fragacea (Cnidaria, Anthozoa)

Sea anemone gametes arise in the endoderm but migrate into the mesoglea at an early stage. In order to observe this process, large individuals of Actinia fragacea were collected from the same intertidal location at regular intervals over a 2-year period, and their gonads were examined by light and electron microscopy. The cellular origin of the oocytes is unclear, but the smallest recognizable oocytes are rounded cells, 6-8 microns in diameter, with relatively large nuclei which may contain synaptinemal complexes. Their cytoplasm contains numerous ribosomes, a flagellar basal-body-rootlet complex, and distinctive dense structures also present in male germ cells but not found in anemone nongerminal cells. During the endodermal phase of growth, the density of the oocyte nucleus increases, a single nucleolus becomes prominent, and mitochondria and glycogen accumulate in the cytoplasm. Most oocytes, but not all, only begin major vitellogenesis after entry into the mesoglea. Most oocytes enter the mesoglea vitellogenesis after entry into the mesoglea. Most oocytes enter the mesoglea before they attain a diameter of 25 microns. The oocytes migrate toward and enter the mesoglea by a process resembling amoeboid movement. During entry, the oocytes are constricted into a characteristic "hourglass" shape and become covered by a basal lamina continuous with that of the gonad epithelium. The last part of the oocyte to enter the mesoglea forms an intimate relationship with the surrounding endodermal cells, which is maintained after entry is complete, and is thought to be important in the establishment of the trophonema.     

Allozyme electrophoresis as a tool for distinguishing different zooxanthellae symbiotic with giant clams

The taxonomy of zooxanthellae in marine invertebrate symbioses is not well understood owing mainly to their lack of reliable morphological differences. Nevertheless, previous work using protein and DNA electrophoreses has set the stage for advancing our taxonomic understanding of cnidarian zooxanthellae. Here we present the use of allozymes as genetic markers for distinguishing algal isolates from tridacnid hosts. Zooxanthellae from seven Tridacna and Hippopus species were isolated and maintained in axenic clonal cultures over many generations. Of 16 enzyme systems, α- and β-esterase (EST), esterase-F (EST-F), glucose phosphate isomerase (GPI), and malate dehydrogenase (MDH) were found suitable polymorphic markers of genetic differences among clonal cultures. Of 39 clonal isolates, 97% were found to be genetically distinguishable. This high extent of genetic variation in zooxanthellae within and between clam species was unexpected, and is difficult to explain based solely on the general notion of asexual reproduction in symbiotic zooxanthellae. Our results are also consistent with the occurrence of sexual reproduction in clam zooxanthellae. The close genetic similarity of the symbionts of Tridacna gigas, the largest and fastest-growing clam species, and the difficulty of initiating their clonal cultures in the given nutrient medium, compared with the symbionts of other clam species, are further indicative of possibly distinct algal symbionts in T. gigas. These findings are discussed in light of current taxonomic understanding of these organisms.     

Histological, histochemical and immunohistochemical study on the growing oocytes of the abyssal teleost Hoplostethus mediterraneus (V)

The oocytes of the abyssal Teleost, Hoplostethus mediterraneus were studied. Four stages of growth were observed and the oocytes of all the stages were surrounded by follicular cells and had several nucleoli in the nucleus. In the oocytes of the II degrees stage, vacuoles without contents, in oocytes of the III degrees stage several vacuoles with a basophilic contents and small yolk globules were identified. General and basic proteins, ribonucleoproteins, acid proteoglycans with -COOH groups were recognized in the cytoplasm, in the nucleoli of oocytes in the II degrees stage and in the vacuolar contents of oocytes in the III degrees stage. In the follicular cells, in the pellucid zone, in the yolk globules, from their beginning, glycoproteins were present. Positivity, for all lectins used, was revealed in the follicular cells and in the four stages of oocytes growth. alpha-D-glucose and alpha-D-mannose binding sites were in the pellucid zone and in the initial yolk globules. In the lather galactose and beta-N-acetyl glucosamine were present too. nNOS and VIP immunopositivity revealed at the periphery of the cytoplasm and at network of nerve fibres between oocytes, suggests NO is involved in a mechanism of regulation of the gametogenesis and of the spawning.     

Light intensity as a factor in the regulation of the density of symbiotic zooxanthellae in Aiptasia tagetes (Coelenterata, Anthozoa)

Experiments designed to investigate the effect of different levels of illumination on the density of symbiotic zooxanthellae in the anemone Aiptasia tagetes (Duch. & Mich.) are described.
The anemones are found to regulate the densities of their zooxanthella populations to fixed levels which are dependent upon ambient light intensity.
Regulation is continuous and results from the interaction of increase in algal numbers, growth of host tissues, population of new host tissues by symbionts and extrusion of zooxanthellae by the host.
The nature of the process by which zooxanthellae are selected for extrusion is discussed and a scheme is outlined indicating the pathways through which the level of the algal population in Aiptasia , and other coelenterates, is believed to be controlled.     

Nutritional role of two algal symbionts in the temperate sea anemone Anthopleura elegantissima brandt

The intertidal sea anemone Anthopleura elegantissima in the Pacific Northwest may host a single type of algal symbiont or two different algal symbionts simultaneously: zooxanthellae (Symbiodinium muscatinei) and zoochlorellae (green algae; Trebouxiophyceae, Chlorophyta). A seasonal comparison of zooxanthellate and zoochlorellate anemones showed stable symbiont population densities in summer and winter, with densities of zoochlorellae about 4 times those of zooxanthellae. Photosynthesis-irradiance curves of freshly isolated symbionts show that the productivity (P(max) cell) of freshly isolated zooxanthellae was about 2.5 times that of zoochlorellae during July; comparable rates were obtained in other months. Models of algal carbon flux show that zoochlorellae may supply the host with more photosynthetic carbon per unit anemone biomass than zooxanthellae supply. Zooxanthellate anemone tissue was 2 per thousand ((13)C) and 5 per thousand ((15)N) enriched and zoochlorellate anemone tissue was 6 per thousand ((13)C) and 8 per thousand ((15)N) enriched over their respective symbionts, suggesting that zoochlorellate anemones receive less nutrition from their symbionts than do zooxanthellate individuals. The disparity between predicted contributions from the algal carbon budgets and the stable isotopic composition suggests that short-term measures of algal contributions may not reflect actual nutritional inputs to the host. Isotopic data support the hypothesis of substantial reliance on external food sources. This additional nutrition may allow both algae to persist in this temperate intertidal anemone in spite of differences in seasonal photosynthetic carbon contributions.     

Phagosome-lysosome fusion inhibited by algal symbionts of Hydra viridis

Certain species of Chlorella live within the digestive cells of the fresh water cnidarian Hydra viridis. When introduced into the hydra gut, these symbiotic algae are phagocytized by digestive cells but avoid host digestion and persist at relatively constant numbers within host cells. In contrast, heat-killed symbionts are rapidly degraded after phagocytosis. Live symbionts appear to persist because host lysosomes fail to fuse with phagosomes containing live symbionts. Neither acid phosphatase nor ferritin was delivered via lysosomes into phagosomes containing live symbionts, whereas these lysosomal markers were found in 50% of the vacuoles containing heat-killed symbionts 1 h after phagocytosis. Treatment of symbiotic algae before phagocytosis with polycationic polypeptides abolishes algal persistence and perturbs the ability of these algae to control the release of photosynthate in vitro. Similarly, inhibition of photosynthesis and hence of the release of photosynthetic products as a result of prolonged darkness and 3-(3,4- dichlorophenyl)-1,1-dimethyl urea (DCMU) treatment also abolishes persistence. Symbiotic algae are not only protected from host digestive attack but are also selectively transported within host cells, moving from the apical site of phagocytosis to a basal position of permanent residence. This process too is disrupted by polycationic polypeptides, DCMU and darkness. Both algal persistence and transport may, therefore, be a function of the release of products from living, photosynthesizing symbionts. Vinblastine treatment of host animals blocked the movement of algae within host cells but did not perturb algal persistence: algal persistence and the transport of algae may be initiated by the same signal, but they are not interdependent processes.     

The adaptive bleaching hypothesis: experimental tests of critical assumptions

Coral bleaching, the loss of color due to loss of symbiotic zooxanthellae or their pigment, appears to be increasing in intensity and geographic extent, perhaps related to increasing sea surface temperatures. The adaptive bleaching hypothesis (ABH) posits that when environmental circumstances change, the loss of one or more kinds of zooxanthellae is rapidly, sometimes unnoticeably, followed by formation of a new symbiotic consortium with different zooxanthellae that are more suited to the new conditions in the host's habitat. Fundamental assumptions of the ABH include (1) different types of zooxanthellae respond differently to environmental conditions, specifically temperature, and (2) bleached adults can secondarily acquire zooxanthellae from the environment. We present simple tests of these assumptions and show that (1) genetically different strains of zooxanthellae exhibit different responses to elevated temperature, (2) bleached adult hosts can acquire algal symbionts with an apparently dose-dependent relationship between the concentration of zooxanthellae and the rate of establishment of the symbiosis, (3) and finally, bleached adult hosts can acquire symbionts from the water column.     

Vitellogenesis in the milkweed bug,Oncopeltus fasciatus Dallas (Hemiptera). A light and electron microscopic investigation

Histochemical and electron microscopic methods have revealed that there are four types of cell inclusions in the late vitellogenic oocytes of Oncopeltus. (a) Type 1 is a vacuole which seems to be contributed from the tropharium via the nutritive tubes. It is suggested that this type consists partly at least of nucleolus-like material (ribonucleoprotein) emitted from the nuclei of the Zone III trophocytes. (b) Type 2 is lipid yolk which in early stage oocytes seems to be produced in the “Balbiani body.” In the vitellogenic oocytes these lipid spheres are apparently imported by the oocyte from the haemolymph either through the follicle cells, or through the extracellular space in the follicular epithelium. (c) Type 3 is carbohydrate/protein yolk where at least part of the protein (“vitellogenic protein”) is taken up from the haemolymph, transported through the extracellular space in the follicular epithelium, and deposited into the oocyte by pinocytosis. (d) Glycogen is deposited from the early phases of vitellogenesis. The tropharium may contribute, besides Type 1 vacuoles, ribosomes, mitochondria, stacks of annulated lamellae, and “food vacuoles” to the oocytes. Specialized cells which line the tropharium and send projections toward the trophic core have been called “peripheral trophocytes.” Contrary to the regular trophocytes, they contain glycogen and an abundance of Golgi complexes.     

The breakdown of symbiotic zooxanthellae in the sea anemone Phyllactis (=Oulactis) flosculifera (Actiniaria)

The sea anemone Phyllactis (=Oulactis) flosculifera Lesueur (Actiniaria) has developed specialized adaptations of a structural, behavioural and chemical nature, which allow the "farming" of its symbiotic zooxanthellae as well as their breakdown and use as a source of nutrition. The presence of an extension of the upper column, the ruff, increases the exposed surface area, and the pattern of expansion and contraction of ruff and tentacles allows the high standing crop of algal symbionts they contain maximum exposure to illumination. A protein extract from the combined ruff, oral disc and tentacles has a destructive effect in vitro on the zooxanthellae of Phyllactis (52%), Aiptasia (37·5%) and Zoanthus (16%). Intracellular degeneration of zooxanthellae is greatest in the phagocytic cells of the trefoil forming the free end of the upper mesentery, but also occurs in other parts of the mesentery. The algal pellet extruded by Phyllactis consists mostly of debris, testifying to the anemone's ability to break down its zooxanthellae. Aiptasia tagetes Duch. & Mich, shows only a simple phototactic response, has no algal-damaging agent and very few degenerate zooxanthellae in its mesenteries, but it extrudes large numbers of its symbionts in all stages of the life history.     

Photoacclimation and the effect of the symbiotic environment on the photosynthetic response of symbiotic dinoflagellates in the tropical marine hydroid Myrionema amboinense

Symbiotic dinoflagellates of the genus Symbiodinium and residing in the tropical hydroid Myrionema amboinense acclimate to low photon flux associated with low light 'shade' environments by increasing the amount of photosynthetic pigments per algal cell. The photosynthetic light intensity (PI) curves suggested that the low-light pigment response involved an increase in the number of photosynthetic units (PSU) in the chloroplast in addition to any increases in PSU size. Comparisons of light-dependent portion of the P-I curves of freshly isolated zooxanthellae (FIZ) with those from symbionts within the intact animal suggest that the host cell environment reduced average light levels reaching the symbiotic algae by more than half. This phenomenon may protect the algae from photobleaching of pigments and/or photoinhibition of photosynthesis at high light intensities present in shallow water habitats. In addition, maximum photosynthesis (P(max)) of symbionts removed from the host cell was higher than that recorded from dinoflagellates in the intact association, suggesting that the availability of carbon dioxide for photosynthesis may be limited in the intact hydroid. Shaded polyps contained fewer zooxanthellae and had less tissue biomass (measured as protein) than unshaded polyps. However symbionts from shaded polyps acclimated to the low light intensities by increasing chlorophyll levels and photosynthetic rates. The higher photosynthetic rates may have resulted from increased availability of carbon dioxide associated with lower symbiont density. Calculations of the contribution of zooxanthellae carbon to the host animal's respiratory demand (CZAR) showed that zooxanthellae from shaded polyps living in the field potentially provide about the same amount of carbon to their host as zooxanthellae from polyps living in the field in unshaded high light intensities.     

On the origin of terpenes in symbiotic associations between marine invertebrates and algae (zooxanthellae). Culture studies and an application of 13C/12C isotope ratio mass spectrometry

13C/12C ratios of sets of compounds, algal sterols and terpenes, isolated from dinoflagellate symbiont (zooxanthellae)-bearing soft corals and gorgonians were determined. In most cases, a significant difference was found between the delta 13C values of the terpenes and of the algal sterols from the same set, the algal sterols containing less 13C than the terpenes. These results can only be explained if terpenes are synthesized by the host. Cultured zooxanthellae, isolated from symbiotic associations, do not make terpenes. Algal sterols of the various sets do not all have the same delta 13C value: average values range from -18.2 to -24.3%. A consistent difference of about 7% in the delta 13C values of sterols of cultured symbionts isolated from two of the gorgonians was found. This has potential applications for the taxonomy of zooxanthellae, most of which are believed by some specialists to be one discrete species.     

STRUCTURE AND REPRODUCTION OF THE ALGAL SYMBIONTS OF HYDRA VIRIDIS1

The algal symbionts of Hydra viridis are found within vacuoles of the gastrodermal digestive cells of the host. Electron microscopy reveals that the symbionts possess cell walls, and that their reproductive cycle follows the general pattern of free-living Chlorella. Nuclear and chloroplast divisions arc followed by formation of new cell walls, the Golgi apparatus being quite active during cell wall synthesis. Autospores are released when the parent wall ruptures. The autospores are then usually segregated into separate animal vacuoles. Remnants of the ruptured parent wall persist in the vacuoles for an indefinite period. The ruptured parent walls curl at the breakage clefts, forming double-layered scroll-like structures. The fate of these wall remnants has not been firmly established. Long-term starvation of the animals does not result in a detectable change in the structure of the symbionts, and they continue to divide and to store carbohydrate as starch grains.     

Dissociation and horizontal transmission of codispersing lichen symbionts in the genus Lepraria (Lecanorales: Stereocaulaceae)

Lichenized fungi of the genus Lepraria lack ascomata and conidiomata, and symbionts codisperse by soredia. Here, it is determined whether algal symbionts associated with Lepraria are monophyletic, and whether fungal and algal phylogenies are congruent, both of which are indicative of a long-term, continuous association between symbionts. The internal transcribed spacer (ITS) and part of the actin type I locus were sequenced from algae associated with Lepraria, and the fungal ITS and mitochondrial small subunit (mtSSU) were sequenced from fungal symbionts. Phylogenetic analyses tested for monophyly of algal symbionts and congruence between algal and fungal phylogenies. Algae associated with Lepraria were not monophyletic, and identical algae associated with different Lepraria individuals and species. Algal and fungal phylogenies were not congruent, suggesting a lack of strict codiversification. This study suggests that associations between symbionts are not strictly maintained over evolutionary time. The ability to switch partners may provide benefits similar to genetic recombination, which may have helped this lineage persist.     

Somatic tissue–male germ cell barrier in Hydra viridis (hydrozoa,coelenterata)

In Hydra viridis, cordons of male germ cells lie in gonadal compartments, which are enlarged spaces between the elongated and “spongy” epidermal cells. The germ cells are surrounded by these cells, except for small areas where the interstitial cells and spermatogonia are in direct contact with the mesoglea. Cells from both epidermis and gastrodermis project cytoplasm into the mesoglea, where they contact each other and form trans-mesogleal bridges. The latter exhibit gap junctions, which are particularly abundant at the spermary region. Here, the mesoglea is thinner then elsewhere in the body. Both epithelia are joined by septate junctions toward their apical ends, which are totally impermeable to horseradish peroxidase (HRP). HRP gained entry to the cells of both epithelia by pinocytosis. Incorporation into the cells was high at the basal disk, in the tentacles, and in the mesoglea in the lower part of the body stalk. The tracer was never found within the gonadal space of the testis during spermatogenesis. In mature spermaries during spermiation, tracer-filled intracellular vacuoles fused with the gonadal spaces as the thin cytoplasmic columns of the epidermal cells ruptured; HRP thus gained access to the germ cells. During spermatogenesis, germ cells of Hydra viridis are in a closed compartment. The barrier that controls the access of metabolites to the germ cells is formed by epidermal cells, thinned-out mesoglea, and numerous transmesogleal interepithelial bridges. The presumed role of the barrier is the control of the environment (1) where interstitial cells are differentiating into spermatogonia and meiosis occurs and (2) in which ripe spermatozoa are kept immotile until spermiation.     

Production and fates of unique organelles (transosomes) in ovarian follicles of Gallus domesticus under various conditions. II

Production and fates of transosomes (sacs of ribosomes made in the follicular cells of an ovarian follicle and subsequently passed to the cytoplasms of the oocyte) have been studied by electron microscopy in ovaries of young chicks, a testosterone-treated hen, aged hens which had ceased laying eggs and a "non-layer" mutant. Study was also made of "primitive yolk" (vacuoles present in both follicular cells and ooplasms of small follicles of normally laying hens). It was found that both transosomes and vacuoles of primitive yolk were present in small oocytes of young chicks, and "non-layer" mutants. However, the transosomes deep within the ooplasms were present within lysosomal vesicles in both of these instances and the vacuoles containing primitive yolk were patently abnormal in the "non-layer" mutant. Very few transosomes or primitive yolk vacuoles were present within the ooplasms of follicles from a testosterone-treated hen or from those of aged hens which were no longer laying. In both of these latter cases such bodies were present in the follicular cells. However, many transosomes were seen to be in the process of being lysed within the cytoplasms of these follicular cells.     

First evidence of maternal transmission of algal endosymbionts at an oocyte stage in a triploblastic host, with observations on reproduction in Waminoa brickneri (Acoelomorpha)

Abstract. Examination of sexual reproduction in a symbiotic acoelomorph worm, Waminoa brickneri from Eilat (Red Sea), presents the first definitive evidence for maternal transmission of dinoflagellate algal symbionts in a triploblastic organism. Sexually mature worms were removed from the stony coral Plesiastrea laxa and raised in the laboratory. Eggs were detected 18 d after the collection of the worms and hatched 4 d later. Histological sections performed on sexually mature worms showed an ovary with oocytes containing two distinct types of algal endosymbionts within their ooplasm. Transmission electron microscopy corroborated the presence of algal symbionts within the developing embryos. Our findings regarding maternal transmission of symbionts shed new light on the diversity of modes of algal symbiont acquisition known in triploblastic organisms.