Observations on the mitosis and on the chromosome evolution during the lifecycle of Oodinium,a parasitic dinoflagellate

The life cycle of the dinoflagellate Oodinium alternates between an ectoparasitic trophic phase and a phase of multiplication as free-living flagellates. The nucleus of the young ectoparasite has rod-like chromosomes similar to those of free-living dinoflagellates. As growth of the trophont proceeds the nucleus becomes increasingly homogeneous. When Oodinium leaves its host, nuclear reorganization processes occur rapidly; they correspond to a peculiar prophase of the first sporogenetic division. The following division stages are similar. A conspicuous fusorial system appears between two archoplasmic areas which are responsible for daughter-chromosome segregation. The nuclear envelope remains intact while the fusorial microtubules are attached at distinct, kinetochore-like structures onto the nucleus. As the chromosomes become more condensed the kinetochore-like formations disappear.     

An unusual mitotic mechanism in the parasitic protozoan Syndinium sp

Syndinium and related organisms which parasitize a number of invertebrates have been classified with dinoflagellates on the basis of the morphology of their zoospores. We demonstrate here that with respect to chromosome structure and chemistry as well as nuclear division, they differ fundamentally from free-living dinoflagellates. Alkaline fast green staining indicates the presence of basic proteins in Syndinium chromosomes. Chromatin fibers are about 30 Å thick and do not show the arrangement characteristic of dinoflagellate chromosomes. The four V-shaped chromosomes are permanently attached at their apexes to a specific area of the nuclear membrane through a kinetochore-like trilaminar disk inserted into an opening of the membrane. Microtubules connect the outer dense layer of each kinetochore to the bases of the two centrioles located in a pocket-shaped invagination of the nuclear envelope. During division kinetochores duplicate, and each sister kinetochore becomes attached to a different centriole. As the centrioles move apart, apparently pushed by a bundle of elongating microtubules (central spindle), the daughter chromosomes are passively pulled apart. During the process of elongation of the central spindle, the cytoplasmic groove on the nuclear surface which contains the central spindle sinks into the nuclear space and is transformed into a cylindrical cytoplasmic channel. A constriction in the persisting nuclear envelope leads to the formation of two daughter nuclei.     

Fine structure of the macronucleus during the cell division cycle of Euplotes eurystomus,a Ciliate Protozoon

This paper reports new observations obtained from a study of macronuclear fine structure throughout various stages of the cell division cycle of Euplotes. Study of the ultrastructural organization of the macronuclear chromatin indicates that much of the chromatin is organized into continuous masses, portions of which appear to be attached to the nuclear envelope. The macronuclear envelope appears unchanged in the region of a replication band, and apparent attachments of the chromatin to the inner membrane of the nuclear envelope are maintained in the reticular and diffuse zones. Intranuclear helices were never observed in the diffuse zone. During macronuclear division, linear elements (fibrils or microtubules) were observed in close association with both chromatin bodies and nucleoli. The ultrastructural data suggest that the intranuclear linear fibrils have two functions: elongation of the dividing nucleus, and attachment of chromatin bodies and nucleoli to the envelope. The significance of these observations for macronuclear division and chromatin segregation is considered.     

DIVISION IN THE DINOFLAGELLATE GYRODINIUM COHNII (SCHILLER) : A New Type of Nuclear Reproduction

Dinoflagellates are of interest because their chromosomes resemble the nucleoplasm of prokaryotes both chemically and ultrastructurally. We have studied nuclear division in the dinoflagellate Gyrodinium cohnii (Schiller), using cells obtained from cultures undergoing phasic growth. Electron micrographs of serial sections were used to prepare three-dimensional reconstructions of nuclei and chromosomes at various stages of nuclear division. During division, a complex process of invagination of the intact nuclear envelope takes place at one side of the nucleus and results in the formation of parallel cylindrical cytoplasmic channels through the nucleus. These invaginations contain bundles of microtubules, and each of the bundles comes to lie in the cytoplasm of a cylindrical channel. Nuclear constriction occurs perpendicular to these channels without displacement of the microtubules. There are no associations between chromosomes and the cytoplasmic microtubules. In dividing cells most chromosomes become V-shaped, and the apices of the V's make contact with the membrane surrounding cytoplasmic channels. It is proposed that the membrane surrounding cytoplasmic channels in the dividing nucleus may be involved in the separation of daughter chromosomes. Thus, dinoflagellates may resemble prokaryotes in the manner of genophore separation as well as in genophore chemistry and ultrastructure.     

THE FINE STRUCTURE OF MITOSIS AND CELL DIVISION IN THE CHRYSOPHYCEAN ALGA OCHROMONAS DANICA1

At the ultrastructural level, cell division in Ochromonas danica exhibits several unusual features. During interphase, the basal bodies of the 2 flagella replicate and the chloroplast divides by constriction between its 2 lobes. The rhizoplast, which is a fibrous striated root attached to the basal body of the long flagellum, extends under the Golgi body to the surface of the nucleus in interphase cells. During proprophase, the Golgi body replicates, apparently by division, and a daughter rhizoplast, appears. During prophase, the 2 pairs of flagellar basal bodies, each with their accompanying rhizoplast and Golgi body, begin to separate. Three or 4 flagella are already present at this stage. At the same time, there is a proliferation of microtubules outside the nuclear envelope. Gaps then appear in the nuclear envelope, admitting the microtubules into the nucleus, where they form a spindle. A unique feature of mitosis in O. danica is that the 2 rhizoplasts form the poles of the spindle, spindle microtubules inserting directly onto the rhizoplasts. Some of the spindle microtubules extend from pole to pole; others appear to attach to the chromosomes. Kinetochores, however, are not present. The nuclear envelope breaks down, except, in the regions adjacent, to the chloroplasts; chloroplast ER remains intact throughout mitosis. At late anaphase the chromosomes come to lie against part of the chloroplast ER. This segment of the chloroplast ER appears to be incorporated as part of the reforming nuclear envelope, thus reestablishing the characteristic nuclear envelope—chloroplast ER association of the interphase cell.     

From proto-mitosis to mitosis — An alternative hypothesis on the origin and evolution of the mitotic spindle

Based on the assumption that the ancestral proto-eukaryote evolved from an ameboid prokarybte I propose the hypothesis that nuclear division of the proto-eukaryote was effected by the same system of contractile filaments it used for ameboid movement and cytosis. When the nuclear membranes evolved from the cell membrane, contractile filaments remained associated with them. The attachment site of the genome in the nuclear envelope was linked to the cell membrane by specialized contractile filaments. During protomitosis, i.e., nuclear and cell division of the proto-eukaryote, these filaments performed segregation of the chromosomes, whereas others constricted and cleaved the nucleus and the mother cell. When microtubules (MTs) had evolved in the cytoplasm, they also became engaged in nuclear division. Initially, an extranuolear bundle of MTs assisted chromosome segregation by establishing a defined axis. The evolutionary tendency then was towards an increasingly important role for MTs. Spindle pole bodies (SPBs) developed from the chromosomal attachment sites in the nuclear envelope and organized an extranuclear central spindle. The chromosomes remained attached to the SPBs during nuclear division. In a subsequent step the spindle became permanently lodged inside the nucleus. Chromosomes detached from the SPBs and acquired kinetochores and kinetochore-MTs. At first, this spindle segregated chromosomes by elongation, the kinetochore-MTs playing the role of static anchors. Later, spindle elongation was supplemented by poleward movement of the chromosomes. When dissolution of the nuclear envelope at the beginning of mitosis became a permanent feature, the open spindle of higher eukaryotes was born.     

An electron microscope study of nuclear and cell division in a dinoflagellate

Summary Light microscopical observations on the cell division of the small dinoflagellate Woloszynskia micra are correlated for the first time with an electron microscopical study. In prophase, whilst the nucleus enlarges and becomes pearshaped, the chromosomes divide to give pairs of chromatids. This process starts at one end and works to the other giving Y- and V-shaped chromosomes as it occurs. Cytoplasmic invaginations pass through the nucleus and by the end of prophase these are seen to contain a number of microtubules of about 180 Å diameter. There is no connection between the microtubules in the nuclear in vagination and either the flagellar bases or the chromosomes. At anaphase the nucleus expands laterally and the sister chromatids move towards opposite ends. The cell hypocone is now partially divided and the two longitudinal flagella well separate. The nucleus completes its division into two daughter nuclei and for a time portions of the cytoplasmic invaginations remain visible. Cell cleavage is completed by the division of the epicone. The nuclear membrane remains intact throughout division and the nucleolus does not break down.The mitotic division in this organism, which is unusual in comparison with the mitosis of higher organisms, is discussed in the light of other types of mitosis which have been reported and of earlier light microscopical observations on dinoflagellates.     

Ultrastructural Features of Mitosis in Anisonema sp. (Euglenida)1

The fine structure of stages in mitosis in a colorless euglenoid, Anisonema sp., reveals that chromosomes remain condensed throughout the life cycle and are attached to the nuclear envelope at interphase. The onset of mitosis is marked by the anterior migration of the nucleus towards the base of the reservoir and by elongation of the nucleolus. The nuclear envelope persists throughout mitosis. Microtubules are generated in the peripheral nucleoplasm adjacent to the envelope and attach to the chromosomes while they are still associated with the envelope. The region of microtubular contact develops into a distinct layered kinetochore as the developing spindle with attached chromosomes separates from the nuclear envelope and moves into the nucleoplasm. The mature spindle consists of a number of subspindles each containing about 8–10 microtubules and a few associated chromosomes. Both chromosomal and non-chromosomal microtubules are present in each subspindle and extend towards the envelope terminating at or near the nuclear pores. Chromosomal segregation is concomitant with nuclear elongation. By late division, an interzonal spindle develops in the dumbbell-shaped nucleus and nucleolar separation occurs. Continued invagination of the nuclear envelope in the region of the interzonal spindle eventually separates the daughter nuclei. A remnant of the interzonal spindle persists in the cytoplasm until cytokinesis.     

Unusual pattern of mitosis in the free-living flagellateDimastigella mimosa (Kinetoplastida)

Summary The three-dimensional ultrastructural organization of the mitotic apparatus ofDimastigella mimosa was studied by computer-aided, serial-section reconstruction. The nuclear envelope remains intact during nuclear division. During mitosis, chromosomes do not condense, whereas intranuclear microtubules are found in close association with six pairs of kinetochores. No discrete microtubule-organizing centers, except kinetochore pairs, could be found within the nucleus. The intranuclear microtubules form six separate bundles oriented at different angles to each other. Each bundle contains up to 8 tightly packed microtubules which push the daughter kinetochores apart. At late anaphase only, midzones of these bundles align along an extended interzonal spindle within the narrow isthmus between segregating progeny nuclei. The nuclear division inD. mimosa can be described as closed intranuclear mitosis with acentric and separate microtubular bundles and weakly condensed chromosomes.Abbreviation MTOC microtubule-organizing center     

Dinoflagellate Evolution: Speculation and Evidence*†

SYNOPSIS. Nuclear features of dinoflagellates that were used originally to support the Mesocaryota concept are reviewed. The fibrillar diameter of dinoflagellate chromatin, low level of chromosomal basic proteins, membrane attachment of chromosomes and swirl pattern observed in sectioned chromosomes are features that support a prokaryotic affinity. The presence of repeated and highly complex DNA, a S-phase of DNA synthesis in the cell cycle, presence of basic proteins, and the reinterpretation of extranuclear microtubules as a spindle support the contention that dinoflagellates are eukaryotes. This combination of prokaryotic and eukaryotic features suggests that dinoflagellates are a geologically old group and that perhaps they diverged from the higher eukaryotic lineage before evolution of eukaryotic chromatin but after the evolution of repeated DNA. The 2 patterns of carotenoid composition exemplified by the presence of peridinin or fucoxanthin suggest separate origins of dinoflagellate plastids, perhaps by prokaryotic and eukaryotic capture. It is suggested that the species possessing fucoxanthin obtained their plastids by capture of photosynthetic eukaryotes. A new class and order, Syndiniophyceae and Syndiniales, are proposed for the dinoflagellates with low chromosome numbers, V-shaped chromosomes, chromosomes containing a sufficient quantity of basic proteins detectable histochemically, possession of centrioles associated with mitosis, intracellular parasitism as a mode of nutrition, and lack of a cellular covering containing plates. Ultrastructural and paleontologic evidence indicates that the thecate is more primitive than the nonthecate condition. The Prorocentrales are considered to be primitive and their thecal construction is reinterpreted as having epithecal and hypothecal regions surrounding a flagellar pore region containing 7 plates. Acritarchs resemble cysts of modern dinoflagellates in size, structure, and chemical composition except for the absence of a polygonal excystment aperture and lack of any indication of transverse and longitudinal flagellar grooves on the acritarchs. The suggestion that some acritarchs may have dinoflagellate affinities is supported by the occurrence of modern dinoflagellates (Prorocentrales) which lack a theca of numerous polygonal plates and lack transverse and longitudinal flagellar arrangement. The Prorocentrales, as opposed to the more typical Dinophyceae, perhaps represent the type of organism that produced some acritarchs.     

The role of microtubules and microfilaments in the micronucleus ofParamecium bursaria during mitosis

Summary A unique spindle apparatus develops during mitosis in the micronucleus ofParamecium bursaria. During interphase the micronucleus contains short microtubule profiles and clumps of condensed chromatin. Throughout mitosis the nuclear envelope remains intact. During prophase, cup-shaped structures termed microlamellae develop in close association with regions of condensed chromatin. Each micromella consists of an outer sublamella, an inner sublamellae, and ring-shaped structures termed microsepta that join the two sublamellae. Microtubules elongate parallel to the division axis. During metaphase, the microlamellae appear to act as kinetochorelike structures that aid in the alignment of the chromosomes. The microlamellae appear conical and join to a meshwork of microfilaments at their apices. Further toward the polar regions the microfilaments join with microtubules that converge and terminate near the nuclear envelope. During metaphase-anaphase and anaphase the chromosomes are apparently moved by the microfilaments pulling on the kinetochorelike microlamellae. Also during metaphase-anaphase, extranuclear microtubules join the nuclear envelope of the micronucleus to microtubule elements of the cell cortex. By anaphasetelophase, microlamellae and the microfilament meshwork degenerate and microtubules represent the only spindle elements. The evidence of this report supports the hypothesis that microfilaments can participate with microtubules in the movement of chromosomes.This report is part of a Ph.D. Thesis presented by the senior author at Fordham University.     

A UNIQUE MITOTIC VARIATION IN THE MARINE DINOFLAGELLATE OXYRRHIS MARINA (PYRROPHYTA)1

Mitosis is described in the flagellate Oxyrrhis marina Dujardin and is compared in related genera. Dense plaques develop in the nuclear envelope at prophase and give rise to an intranuclear spindle. Some of the microtubules associate with the chromosomes while others extend across the nucleus. The basal bodies migrate toward the poles early in division and retain a position lateral to the nuclear poles throughout mitosis. Microtubules are not present between the nucleus and the basal bodies. The nucleolus is persistent and elongates throughout anaphase and telophase. Chromosomal separation is accomplished by sliding of non-chromosomal microtubules and by elongation of the nuclear envelope rather than by shortening of the spindle microtubules. The nuclear envelope begins to constrict in the center early in anaphase. Continued constriction of the envelope and elongation of the nucleus leads to the formation of a dumbbell-shaped nucleus by late telophase. Mitosis culminates by the constriction of the nucleus into two daughter nuclei. The taxonomic position of Oxyrrhis marina is discussed in light of these findings.     

ULTRASTRUCTURE OF CELL DIVISION AND REPRODUCTIVE DIFFERENTIATION OF MALE PLANTS IN THE FLORIDEOPHYCEAE (RHODOPHYTA): CELL DIVISION IN POLYSIPHONIA1

Cell division in the marine red algae Polysiphonia harveyi Bailey and P. denudata (Dillwyn) Kutzing was studied with the electron microscope. Cells comprising the compact spermatangial branches of male plants were used exclusively because of their small size, large numbers and the ease with which the division planes can be predetermined. Some features characterizing mitosis in Polysiphonia confirm earlier electron microscope observations in Membranoptera, the only other florideophycean algae in which mitosis has been studied in detail. Common to both genera are a closed, fenestrated spindle, perinuclear endoplasmic reticulum, a typical metaphase plate arrangement of chromosomes, conspicuous, layered kinetochores, chromosomal and non-chromosomal microtubules, and nucleus associated organelles (NAOs) known as polar rings (PRs) located singly in large ribosome-free zones of exclusion at division poles in late prophase. However, other features, unreported in Membranoptera, were observed consistently in Polysiphonia. These include the presence of PR pairs in interphase-early prophase cells, the attachment of PRs to the nuclear envelope during all mitotic stages, the migration of a single PR to establish the division axis, a prominent, nuclear envelope protrusion (NEP) at both division poles at late prophase, the prometaphase splitting of PRs into proximal and distal portions, and the reformation of post-mitotic nuclei by the separation of an elongated interzonal nuclear midpiece at telophase. During cytokinesis, cleavage furrows impinge upon a central vacuolar region located between the two nuclei and eventually pit connections are formed in a manner basically similar to that reported for other red algae. Diagrammatic sequences of proposed PR behavior during mitosis are presented which can account for events known to occur during cell division in Polysiphonia. Mitosis is compared with that reported in several other lower plants and it is suggested that features of cell division are useful criteria to aid in the assessment of phylogenetic relationships of red algae.     

Spindle membranes and calcium sequestration during meiosis ofDysdercus intermedius (Heteroptera)

The fate of intracellular membranes stained by the osmium ferricyanide (OsFeCN) procedure was followed from premeiotic interphase to interkinesis inDysdercus intermedius. During diakinesis the centrioles forming primary cilia attach temporarily with their proximal ends to the nuclear envelope which is stretched from pole to pole. Breakdown of the nuclear envelope is preceded by deep indentations with microtubules from growing asters. Vesicles of smooth endoplasmic reticulum which accumulate gradually in the course of prophase contribute to the ensheathment of the chromosomes with membranes. When the nuclear envelope breaks down, the polar parts of the formerly perinuclear membranes follow the ingrowth of the spindle microtubules towards the cell equator where the seven bivalents are arranged in a circle with the X₁X₂ sex chromosomes in the centre. The metaphase I spindle thus contains longitudinally oriented membranes between the poles, membranous envelopes around all chromosomes and radial connections from the autosomes to the sex chromosomes in the centre. At anaphase the homologues leave their common sheath and a microtubular stembody surrounded by membranes appears between the receding dyads. In the interkinetic nucleus the gonosomes are separated from the autosomes by a common membranous sheath which may be instrumental in their joint assignment to only one pole in the second meiotic division. Calcium sequestering sites visualized by oxalate precipitation are the Golgi lamellae and vesicles derived from them that surround the whole spindle body.     

Mitosis of the free-living flagellate Bodo saltans strain Ps+ (Kinetoplastidea, Bodonida)

Mitosis of the free-living flagellate Bodo saltans of the Ps+ strain characterized by the presence of prokaryotic cytobionts in the perinuclear space was studied. Division of B. saltans Ps+ nuclei occurs by the closed intranuclear type of mitosis without condensation of chromosomes. At the initial stages of nuclear division, consecutive anlage of two spatially separated microtubular spindles begins. The spindle containing about 20 microtubules appears first, then, at an angle of 30–40° to it, the second spindle containing half as many microtubules is formed. The microtubules of the first spindle are associated with 4 pairs of kinetochores, the microtubules of the second one—with 2 pairs. The kinetochores of B. saltans Ps+ have a pronounced laminar structure. Both spindles rest with their ends directly on the internal membrane of the nuclear envelope and form 4 well-pronounced poles. The equatorial phase of mitosis in B. saltans Ps+ is not revealed. The divergence of sister kinetochores towards the poles occurs independently in each spindle. At the elongation phase of mitosis, the poles of both spindles are united in pairs to form a single bipolar structure composed of two loose bundles of microtubules. At this stage of nuclear division, the kinetochores reach the poles of the subspindles and cease to be visible. At subsequent nuclear division stages the nucleus acquires a dumbbell shape. During the reorganization phase the sister nuclei are separated. In the perinuclear space of the interphase nuclei of B. saltans Ps+, 1–2 prokaryotic cytobionts are present. In the course of mitosis, these organisms divide intensively, such that their number can reach 20 and more per nucleus. During separation of sister nuclei, the “excessive” cytobionts are released into the cytoplasmic vacuoles formed by external membranes of the nuclear envelope.     

Fine structure of the giant aflagellate spermatozoon in pseudostomum quadrioculatum (leuckart) (platyhelminthes,prolecithophora)

The giant aflagellate spermatozoa of P. quadrioculatum are composed of two different parts: a thicker head piece and a more slender tail piece. In the head there exist a large elongated nucleus and an elongated mitochondrial derivative situated in a groove-like cavity of the nucleus. In mature spermatozoa the nuclear material is arranged in many small membrane bounded areas. Both structures, nucleus and mitochondrial derivative, are spirally coiled. The outer part of the membrane in the mitochondrial derivative forms many loop-like foldings. Both organelles continue to the tail in form of two small, helically coiled ribbons; the nucleus is anchored within the mitochondrial derivative by an electron-opaque process. A sheath of spirally-orientated cortical microtubules starting from the tip of the head runs to the tip of the tail under the cell membrane. In addition, a second sheath of tubules occurs in the tail region, these tubules also run parallel to each other, but in the opposite direction to the microtubules of the outer sheath.The possible relations between the structures observed and the motility of the spermatozoa are discussed; in addition, some phylogenetic comments are attempted.Abbreviations c — cerebrum - com — cortical microtubules - cop — copulatory organ - fm — foldings of the mitochondrial membrane - l — lattice - mid — mitochondrial derivative - mt — microtubules - n — nucleus - ne — nuclear envelope - ph — pharynx - pn — protonephidium - rp — ribbon-like nuclear process - te — testis - tt — testis - tt — tip of the tail - vi — vitellarium - vs — vesicula seminalis     

CHROMOSOMES AND DNA IN SYMBIODINIUM FROM AUSTRALIAN HOSTS1

Chromosome counts are reported for five species of Symbiodinium Freudenthal isolated from the host species Plesiastrea versipora, Aiptasia pulchella, Parerythropodium sp., Zoanthus robustus, and Capnella gaboensis based on light and electron microscope studies. Both techniques resulted in large variations in the apparent numbers of chromosomes per nucleus in Symbiodinium isolated from the same host colony. Adjacent regions of condensed DNA were occasionally linked by thin fibrils of DNA, The results suggest that the regions of condensed nuclear DNA in dinoflagellates may not represent whole chromosomes, and so chromosome counts are considered inappropriate for defining Symbiodinium taxa.     

Centriolar plaque and spindle microtubules in the ascomyceteSordaria humana

Centriolar plaque and spindle microtubules in young asci of an ascomycete,Sordaria humana were studied by electron microscopy. Centriolar plaque is electron opaque and has an amorphous structure. Two dispositions of centriolar plaque were observed, one entirely contiguous to the nuclear envelope in a meiotic division and the other partially joined to the envelope in a mitotic division following meiosis. The spindle was formed inside the nuclear envelope and spindle microtubules terminated at the polar protrusion of the nuclear envelope. Some spindle microtubules seem to connect directly with the centriolar plaque passing through perforations of the nuclear envelope.     

MITOSIS IN THE OCTAFLAGELLATE PRASINOPHYTE,PYRAMIMONAS AMYLIFERA (CHLOROPHYTA)1

Cell division is described in the octaflagellate prasinophyte Pyramimonas amylifera Conrad and is compared in related genera. Basal bodies replicate at preprophase and move toward the poles. Cells remain motile throughout division. The nuclear envelope disperses and chromosomes begin to condense at prophase. Pairs of multilayered kinetochores are evident on the chromosomes of the metaphase plate. Spindle microtubules extending from the region of the basal bodies and rhizoplasts attach to the kinetochores or extend from pole to pole. Numerous vesicles and ribosomes have entered the nuclear region and the incipient cleavage furrow invaginates. The chromosomes move toward the poles at anaphase leaving a broad interzonal spindle between the two chromosomal plates. The nuclear envelope reforms first around the chromatin on the side adjacent to the spindle poles and later on the interzonal side. The cleavage furrow progresses into the interzonal spindle at telophase. By late telophase the nucleoli have reformed and the chromosomes have decondensed. The interzonal spindle has not been observed late in telophase. As the cleavage furrow nears completion the cells begin to twist and contort, ultimately separating the two cells.