Zoospore Structure and Colony Formation in Pediastrum Spp. and Hydrodictyon reticulatum (L.) Langerheim

The process of daughter colony formation in Pediastrum biradiatumMeyen, P. tetras (Ehren-berg) Ralfs, P. duplex Meyen, and Hydrodictyonreticulatum (L.) Lagerheim has been investigated using lightmicroscopy and electron microscopy of thin sections. The precisedisposition of the organelles confers upon the zoospores ofthese algae a special symmetry of a type unusual for the Chlorophyta.Apart from this, the detailed ultrastructure of the zoosporeand young vegetative cells resembles that of other green algaepreviously investigated. A common sequence of events has beenfound to occur during colony formation in both Pediastrum andHydrodictyon, with a correlation between zoospore symmetry andthat of the colony produced. The changes involved in the transitionfrom a motile to a non-motile cell are considered, and appearto have features in common with this process in some other greenalgae. The early stages in wall deposition involve the simultaneousformation of two distinct layers and serve to stick togetherthe cells which by this time have collectively assumed the basicmorphological characteristics of the mature coenobia.     

Division of Chloroplasts in a Green Alga, Pediastrum duplex

The division of chloroplasts in a green alga, Pediastrum duplex,was studied by electron microscopy. Cells were treated for observationwith the freeze-substitution method. Fibrils, or fibrous belts,which we had observed previously at the dividing constrictionsof chloroplasts in Trebouxia potteri were not visible in Pediastrum,even though the method of preparation was the same for bothsets of samples. Microtubules (MTs) and the septum seem notto participate directly in the division of the single chloroplastin Pediastrum cells. Many thin fibrils, 7–20 nm in diameter,attached to, or protruding from, the surface of the dividingconstriction were seen. These fibres were less densely distributedat the constrictions of non-dividing chloroplasts. It is suggestedthat these fibrils are involved in the divison of chloroplastsin Pediastrum duplex. Cell wall, chloroplast division, freeze-substitution, intermediate fibres, Pediastrum duplex, transmission electron microscopy     

Determination of polarity and bilateral asymmetry in palleal and vascular buds of the ascidian Botryllus schlosseri.

Reversal of the bilateral asymmetry of the zooids was induced in a series of colonies of Botryllus schlosseri. Palleal buds from colonies with normal or reversed bilateral asymmetry were isolated in the early stages from the parental zooids and cultured in the vascularized tunic of the same colony or of another colony with opposite asymmetry. Vascular budding was induced in colonies with either type of asymmetry.The bud polarity was shown to depend on the vascularization; the test vessel entering the isolated palleal bud always causes the entrance point to become the posterior end of the developing zooid. On the contrary, the bilateral asymmetric type is predetermined in the bud primordium; the isolated palleal buds develop the type of asymmetry of their parents, even when grafted in the test of a colony with opposite asymmetry. Since the same was also true of the vascular buds, it is concluded that the information for the kind of bilateral asymmetry to be developed is conveyed by the epidermal envelope of the bud. The epidermis of the parental zooids influences the palleal buds, whereas the wall of the test vessels, epidermal extrusions of the zooids, influences the vascular buds.     

An autolysin dissolves the inner cell wall layer of the algaPediastrum (Hydrodictyaceae)

Summary An autolysin produced by young colonies ofPediastrum frees them from the vesicle in which they are formed within 12 hours of release of zoospores from the parent cell. The polysaccharide vesicle is derived from the inner wall layer of the parent cell. Refrigeration delays vesicle disintegration; boiling stops it completely. A purified, lyophilized extract of the vesicle fluid added to boiled vesicled colonies removes the vesicle in 2 hours with the release of reducing sugars and polysaccharides.Biogel P2 and P10 chromatography of the products following incubation of the enzyme preparation and wall showed no more than 1% oligosaccharides; the remaining carbohydrates had a molecular weight of several thousand daltons. Analyses of isolated vesicle wall material (70–85% of the dry weight) showed mannose accounting for approximately 50% of the dry weight, with none of the other neutral sugars present (fucose, xylose, galactose and glucose) representing more than 3%. Uronic acids account for 20–25% of the wall weight, and proteins less than 2%. Pediastrum colonies are thus freed from the vesicles in which they are formed by the action of an autolysin they produce. The autolysin acts on the vesicle wall material to generate reducing sugars and cause it to disintegrate into its constituent polysaccharides.     

Pattern formation and the fine structure of the developing cell wall in colonies of Pediastrum boryanum

A single-layered disc of peripheral pronged cells and central prongless cells impart the typical gear shape to colonies of Pediastrum, while the walls of each cell have a characteristic reticulate triangular pattern. The two-layered wall forms in the cells during colony formation following zoospore aggregation and adhesion. The uniformly thin outer layer reflects contours resulting from differential thickening in the reticulate pattern of the inner, thicker, more fibrillar and granular wall layer. The reticulate pattern thus imparted to the outer wall layer persists in empty zoosporangia following the release of zoospores. Columns of electron-dense material extend through the outer wall layer except at the ridges and centers of the reticulum. Following mitosis and cleavage, the resulting zoospores are extruded within a vesicle membrane consisting of the inner wall layer. Separation of this membrane from the parent cell occurs in material of the inner layer adjacent to the outer wall. Vesicles containing swarming zoospores also contain a granular material which appears to become associated with the aggregating and adhering cells of new colonies. Microtubules occur in zoospores prior to adherence but are absent during wall deposition.     

MITOSIS,CYTOKINESIS, AND COLONY FORMATION IN THE GREEN ALGA SORASTRUM1

Synchronous mitotic divisions produce multi-nucleate cells of Sorastrum. Perinuclear envelopes of endoplasmic reticulum and a virtually intact nuclear envelope enclose mitotic nuclei. Cytoplasmic cleavage, which shirts before the last round of Synchronous mitoses, gives rise to uninucleate fragments which differentiate to form zoospores. These zoospores are released into a spherical vesicle, presumably derived from the inner layer of the parental cell wall, in which they swarm actively before aggregating as a spherical colony. The roughly conical shaped zoospores apparently adhere laterally before withdrawing their flagella and extending horns and a stipe, which, following wall deposition, interconnects the cells at the center of the colony. The probable role of the microtubules, which underlie the plasmalemma of aggregating cells, in determining the shape of both the cells and the colony itself is discussed.     

Multizooidal Budding in Parasmittina trispinosa (Johnston) (Bryozoa,Cheilostomata)

Two principally different wall types occur in the bryozoan colony: Exterior walls delimiting the super-individual, the colony, against its surroundings and interior walls dividing the body cavity of the colony thus defined into units which develop into sub-individuals, the zooids. In the gymnolaemate bryozoans generally, whether uniserial or multiserial, the longitudinal zooid walls are exterior, the transverse (proximal and distal) zooid walls interior ones. The radiating zooid rows grow apically to form “tubes” each surrounded by exterior walls but subdivided by interior (transverse) walls. The stenolaemate bryozoans show a contrasting mode of growth in which the colony swells in the distal direction to form one confluent cavity surrounded by an exterior wall but internally subdivided into zooids by interior walls. In the otherwise typical gymnolaemate Parasmittina trispinosa the growing edge is composed of a series of “giant buds” each surrounded by exterior walls on its lateral, frontal, basal and distal sides and forming an undifferentiated chamber usually 2–3 times as broad and 3 or more times as long as the final zooid. Its lumen is subdivided by interior walls into zooids 2–3, occasionally 4, in breadth. This type of zooid formation is therefore similar to the “common bud” or, better-named, “multizooidal budding” characteristic of the stenoleamates but has certainly evolved independently as a special modification of the usual gymnolaemate budding.     

Body wall morphology of Pentapora foliacea (Ellis and Solander) (Bryozoa,Cheilostomata)

The ascophoran Pentapora foliacea was studied from epoxy sections of skeletal and soft (hard-soft) tissues. The basal wall is double, indicating the colony grew as two independent layers, back to back. The structure of the vertical walls and interzooidal communication organs indicates that zooids were budded in the usual way as in most encrusting cheilostomes. Secondary layers of the frontal wall are of acicular aragonite. The ovicell develops as a flattened cuticular bladder in early ontogeny; the aragonitic layer of the frontal wall later engulfs it. A median vesicle, an evagination of the vestibular wall, is present but the eggs may be supplied with sufficient yolk to nurture the embryo. The overall ovicell structure is similar to that of hyperstomial ovicells in other cheilostomes.     

Developmental pattern of colonies in the polystyelid ascidian,Polyandrocarpa misakiensis

Summary

The growth pattern of zooids formed asexually by budding was studied in the colonial ascidian, Polyandrocarpa misakiensis. Each colony started from a blas- tozooid (the first generation) on the glass plate in two series of experiments. To evaluate the growth of colonies, lineage of all the zooids of three successive generations was traced on photographs which were taken once a week. The zooids of the first generation produced many buds from any basal margin of the zooidal body, and those of the second generation produced a small number of buds mainly from anterior parts of the zooidal body. The zooids of the second generation produced by early budding of mother zooids were clearly more prolific than those produced by late budding. Circular colonies which developed around a zooid of the first generation consisted of stratified zones of successive generations. Each zone was composed of two subzones; the outer one mainly containing early-produced zooids, and the inner one mainly containing late-produced zooids. The zooids in the marginal area of colony are early-produced ones from generation to generation. The seawater temperature may influence the growth of zooids and/or the frequency of budding.     

A morphological study of nonrandom senescence in a colonial urochordate

Botryllus schlosseri is a clonally modular ascidian, in which individuals (zooids) have a finite life span that is intimately associated with a weekly budding process called blastogenesis. Every blastogenic cycle concludes with a synchronized phase of regression called takeover, during which all zooids in a colony die, primarily by apoptosis, and are replaced by a new generation of asexually derived zooids. We have previously documented that, in addition to this cyclical death phase, entire colonies undergo senescence during which all asexually derived individuals in a colony, buds and zooids, die in concert. In addition, when a specific parent colony (genet) is experimentally separated into a number of clonal replicates (ramets), ramets frequently undergo senescence simultaneously, indicating that mortality can manifest itself in nonrandom fashion. Here, we document a morphological portrait of senescence in laboratory-maintained colonies from Monterey Bay, California, that exhibit nonrandom mortality. Nonrandom senescence proceeded according to a series of characteristic changes within the colony over a period of about one week. These changes included systemic constriction and congestion of the vasculature accompanied by massive accumulation of pigment cells in the zooid body wall (mantle), blood vessels, and ampullae; gradual shrinkage of individual zooids; loss of colonial architecture, and ultimately death. At the ultrastructural level, individual cells exhibited changes typical of ischemic cell death, culminating in necrotic cell lysis rather than apoptosis. Collectively, these observations indicate that senescence is accompanied by unique morphological changes that occur systemically, and which are distinct from those occurring during takeover. We discuss our findings in relation to current experimental models of aging and the possible role of a humoral factor in bringing about the onset of senescence.     

The effect of colchicine on colony formation in the algae Hydrodictyon,Pediastrum and Sorastrum

Summary Uninucleate, biflagellate zoospores of Hydrodictyon, Pediastrum and Sorastrum, derived from multinucleate parental cells, aggregate and adhere to form distinctively patterned colonies; earlier work has shown that microtubules underlie the plasmalemma of these zoospores and are also conspicuous in the developing horns of aggregating cells of Pediastrum and Sorastrum. Colchicine applied to parental cells inhibited cytoplasmic cleavage and production of uninucleate zoospores. When zoospores were treated with colchicine, their peripheral microtubules disappeared; the spores failed to aggregate in ordered arrays and did not develop horns. The microtubules therefore appear to play an important role in determining the arrangement of cells in developing colonies by affecting the shape of the cells at the time of their aggregation.     

Cell shape in the algaPediastrum (Hydrodictyaceae; Chlorophyta)

Summary Species ofPediastrum, a genus in which the colonies assemble from aggregating zoospores, differ in the number and form of prongs on peripheral cells and the amount of space between cells of the colony; cell shape appears to be genetically based. Peripheral cells of theP. boryanum colony, for example, have two prongs per cell;P. simplex has one prong per cell. Prong extension is suppressed in the interior cells ofP. boryanum, but prong sites have been reported in scanning electron micrographs of the cell walls. A mutant unicellular strain in which cells of the colony separate after attaining typical form reveals several prong sites (6 or more) in each cell. Multiple suppressed prong sites are evident inP. simplex cells as well. Polyeders, 4- and 5-pronged unicells, occur in the life cycle ofP. simplex. Based on these observations and a recent report byMarchant (1979) of a microtubule organizing center associated with the prongs, it is suggested that several microtubule organizing centers are to be found in zoospores ofPediastrum species and may be related to species differences in cell shape.Research supported in part by Argonne Center for Educational Affairs, U.S. Department of Energy, under contract No. W-31-109-ENG-38.     

Spore discharge in Cordana musae (Zimm.) H?hnel and Zygosporium oscheoides Mont.

In Cordana musae and Zygosporium oscheovides violent spore dischargeoccurs under conditions of rapidly decreasing vapour-pressure;discharge is not dependent on light. Mechanisms of dischargeare postulated. In C. musae, on drying, water evaporates frominside the conidium and conidiophore, causing a negative pressureto develop in the solutions inside these structures and thewalls to be drawn inwards under tension. The sudden appearanceof a gas bubble in one or both of these structures releasesthe tension, and the resultant jolt causes the conidium to beshot away. In Z. oscheoides similar tension and wall distortiondevelop in a specialized conidiophore cell, the vesicle, onwhich the conidia are poised. On appearance of a gas phase,the vesicle wall springs back to its original form and the conidiaare shot away. The vesicle is likened to a single annulus cellin the fern sporangium. The similarity between discharge inC. musae, Z. oscheoides, and Deightoniella torulosa is brieflydiscussed.     

Early Colony Development in Aetea (Bryozoa)

External structures on the erect parts of zooids of Aetea havebeen demonstrated to be brood chambers by observation of release,settlement and metamorphosis of larvae from the chambers. Theancestrula is smaller than, but very similar to succeeding zooidsin the primary zone of astogenetic change, which do not showtubular connections. Sections through brood chambers and zooidsshow that part of the brood chamber wall may be slightly calcified.Brood chambers appear to be products of the external zooid walland not diverticula derived from the tentacle sheath.     

Cyclical generation and degeneration of organs in a colonial urochordate involves crosstalk between old and new: a model for development and regeneration

Botryllus schlosseri is a colonial marine urochordate in which all adult organisms (called zooids) in a colony die synchronously by apoptosis (programmed cell death) in cyclical fashion. During this death phase called takeover, cell corpses within the dying organism are engulfed by circulating phagocytic cells. The "old" zooids and their organs are resorbed within 24-36 h (programmed cell removal). This process coincides temporally with the growth of asexually derived primary buds, that harbor a small number of undifferentiated cells, into mature zooids containing functional organs and tissues with the same body plan as adult zooids from which they budded. Within these colonies, all zooids share a ramifying network of extracorporeal blood vessels embedded in a gelatinous tunic. The underlying mechanisms regulating programmed cell death and programmed cell removal in this organism are unknown. In this study, we extirpated buds or zooids from B. schlosseri colonies in order to investigate the interplay that exists between buds, zooids, and the vascular system during takeover. Our findings indicate that, in the complete absence of buds (budectomy), organs from adult zooids underwent programmed cell death but were markedly impaired in their ability to be resorbed despite engulfment of zooid-derived cell corpses by phagocytes. However, when buds were removed from only half of the flower-shaped systems of zooids in a colony (hemibudectomy), the budectomized zooids were completely resorbed within 36-48 h following onset of programmed cell death. Furthermore, if hemibudectomies were carried out by using small colonies, leaving only a single functional bud, zooids from the old generation were also resorbed, albeit delayed to 48-60 h following onset of programmed cell death. This bud eventually reached functional maturity, but grew significantly larger in size than any control zooid, and exhibited hyperplasia. This finding strongly suggested that components of the dying zooid viscera could be reutilized by the developing buds, possibly as part of a colony-wide recycling mechanism. In order to test this hypothesis, zooids were surgically removed (zooidectomy) at the onset of takeover, and bud growth was quantitatively determined. In these zooidectomized colonies, bud growth was severely curtailed. In most solitary, long-lived animals, organs and tissues are maintained by processes of continual death and removal of aging cells counterbalanced by regeneration with stem and progenitor cells. In the colonial tunicate B. schlosseri, the same kinds of processes ensure the longevity of the colony (an animal) by cycles of death and regeneration of its constituent zooids (also animals).     

Formal Genetics of Ascidians

SYNOPSIS. Our knowledge of ascidian genetics is reviewed. Thepaper is primarily concerned with the author's past and currentwork on the colonial species Bolryllus schlosseri. Five Mendelianloci account for most of its impressive polychromatism. Breedingexperiments have substantiated the hypothesis of a single multialleliclocus for each of three enzymes (MDH, SOD, PGI) suggested byelectrophoretic patterns. The nuclei of three linkage groupshave been revealed. Self—fertilization entails a severeinbreeding depression. A specific self, nonself recognition,expressed by fusion or repulsion of contacting colonies, occursin this species also. At variance with Botryllus primigenus,fusible colonies of B. schlosseri are completely interfertile.This has allowed a more direct genetic analysis of the phenomenon,confirming the alleged control by a single multiallelic locus.In order to fuse, the confronted colonies must share at leastone allele. Young buds grafted in the tunic after removal ofall the zooids develop a new colony at the host's expense onlyif donor and host are fusible. This means that fusibility andhistocompatibility are strictly correlated. Chimerical colonies,obtained either in this way or following the resorption of oneof two fused colonies, are now being investigated for theirrecognition specificity and electrophoretic pattern. Preliminarydata indicate that both can be durably altered, suggesting thatthe allogeneic cell populations are persistent and renewing.     

Opaline gland ultrastructure in Aplysia californica (Gastropoda: Anaspidea)

Secretions released from the ink and opaline glands of Aplysiacalifornica protect this shell-less mollusc from predators inseveral ways; the most recently discovered, phagomimicry, stimulatesthe feeding behaviours of the predator, distracting it fromthe sea hare. The structure of the ink gland has been reported,but little is known about the opaline gland. This paper comparesthe structure of the opaline gland of A. californica with thatof its ink gland, as well as two additional vesicle types foundin the epidermis. The opaline gland consists of single largecells, the vesicle cells, each with an enlarged nucleus, themaximum size of both exceeding that of respective structuresin the ink gland. Opaline vesicles, like ink vesicles, are enclosedby an external layer of muscle. Opaline vesicles, unlike inkvesicles, are not immersed in additional cells, but are freewithin the haemolymph and are, therefore, the probable sitefor the synthesis of their protein contents. The necks of individualopaline vesicles are fused into a central canal, but short necksconnecting each vesicle to the central canal remain; these arefilled with epithelial cells, but lack a muscular release valvelike that in the long necks of ink vesicles. Mucous cells containcircular arrays and are structurally distinct from opaline vesicles;mucous cells, though enlarged, are smaller than opaline or inkvesicle cells; they lack an external layer of muscle and a multicellularneck and, therefore, more closely match another vesicle typein the skin of A. californica, the white vesicle, which is involvedwith excess calcium excretion. (Received 22 September 2006; accepted 20 March 2007)     

Unconventional signalling in tunicates

The zooids in colonial tunicates do not appear to be directly interconnected by nerves, but this has not prevented the evolution of coordinated behaviour in several groups. In Botryllus and other colonial styelid asci‐dians the endothelium lining the blood vessels is excitable and transmits action potentials from cell to cell via gap junctions. These signals mediate protective contractions of the zooids and synchronize contractions of the vascular ampullae. In didemnid ascidians such as Diplosoma a network of myocytes in the tunic serves to transmit excitation and to cause contractions of the cloacal apertures. Individual zooids of Pyrosoma protect themselves by closing their siphons and arresting their branchial cilia when stimulated. At the same time a flash of light is emitted. Neighbouring zooids sense the flash with their photoreceptors and respond in turn with protective responses and light emission. Protective responses thus spread by photic signalling and propagate from zooid to zooid through the colony in a saltatory manner. In chains of Salpafusifortnis, changes in the direction and/or speed of swimming are transmitted from zooid to zooid via adhesion plaques. When a zooid is stimulated, its body‐wall epithelium conducts action potentials to the plaque connecting it to the next zooid, exciting receptor neurons in that zooid. These receptors have sensory processes that bridge the gap between the two zooids. The sensory neurons so excited in the second zooid conduct impulses to the brain where they alter the motor output pattern, and at the same time generate epithelial action potentials that travel to the next zooid in line, where the same thing happens.

It is not clear why these unconventional signalling methods have evolved but the tunic may be an inhospitable environment for nerves, making conventional nervous links impossible.     

The effects of ambient flow velocity,colony size,and upstream colonies on the feeding success of Bryozoa. II. Conopeum reticulum (Linnaeus), an encrusting species

The effects of ambient flow velocity, colony size, and the presence of an actively-feeding colony upstream on the feeding success of the encrusting bryozoan Conopeum reticulum (Linnaeus) were studied. Zooids from both large and small colonies showed a reduction in feeding as flow velocity increased, however, the reduction in feeding was less for zooids from large colonies except at very fast ambient flow velocities. The greater pumping capacity of large colonies may result in a relatively greater per zooid feeding success from moving water. The presence of an actively-feeding colony upstream was found to enhance the feeding of zooids on downstream colonies. Diversion of flowing water by actively-feeding colonies upstream may account for the observed enhancement of feeding by zooids on colonies downstream.The results from this study on an encrusting species are compared with results from a previous study on feeding from flow by an arborescent bryozoan, and the feeding performances of these two colony types are related to their respective flow microhabitats.     

Not like Botryllus: indiscriminate post-metamorphic fusion in a compound ascidian

Fusion trials between metamorphs of the aplousobranch compound ascidian Diplosoma listerianum indicated that chimera formation was not dependent on relatedness. Similar, high rates of union were observed between full siblings, half siblings, unrelated individuals from the same population, and individuals from two geographically distant localities. This is in contrast to the well-studied ascidian genus Botryllus, in which a highly polymorphic allorecognition system governing the fusion–non-fusion reaction (colony specificity) largely limits fusion to close relatives. Fusion in Botryllus establishes a vascular chimera throughout which stem cells may circulate, promoting cell lineage competition between the fusion partners. The restriction of fusion to close kin in Botryllus is thought to reduce the inclusive fitness costs of competitive interactions between cell lineages within the chimera. In contrast to Botryllus, modules (zooids) of a D. listerianum colony are not interlinked by blood vessels, seemingly precluding the exchange of stem cells. The apparent absence of strict colony specificity in D. listerianum is thus in keeping with the predictions of the Botryllus model for the maintenance of allorecognition polymorphism. However, colony specificity has been reported in other species of aplousobranch ascidian that also lack a common vascular system. In these, the threat of migrating blood-borne stem cells cannot be responsible for the presence of colony specificity. One possibility, requiring experimental investigation, is that stem cells could perhaps migrate between zooids by another route, such as through the matrix of the colonial tunic. Even in the absence of stem cell exchange, cheating on the costs of colony maintenance and defence could also produce selective forces favouring colony specificity. In compound ascidians, this could involve unequal contribution to extrazooidal structures, principally the tunic and related tissues. This consideration seems potentially relevant to the lack of discrimination during fusion in D. listerianum, since extrazooidal somatic investment in this species appears minimal, severely limiting the scope for this other form of cheating. The various possible modes of exploitative interaction between fused colonies are not mutually exclusive, and offer fundamentally similar explanations for colony specificity. If none of them can be shown to occur in non-botryllid species possessing colony specificity, the generality of the Botryllus model may require re-evaluation.