John D. Dodge

THE MORPHOLOGY AND FINE STRUCTURE OF CERATIUM HIRUNDINELLA (DINOPHYCEAE)1

Some aspects of thecal morphology in Ceratium hirundinella are redescribed by means of scanning electron microscopy. The internal anatomy of the cell has been clearly revealed for the first time in serial sections examined by transmission electron microscopy. An unusual sulcus, termed the sulcul aperture, has been found to open into a large ventral chamber which extends toward the center of the cell. The 2 flagella are inserted at one side of the ventral chamber. The ultrastructure of the flagellar canals and root systems arc described in detail. A microtubular strand has been found to run from near the nucleus to near the flagellar bases but not to conned with either. The fine structure of the theca, trichocysts, nucleus, and chloroplasts is basically similar to that of other dinoflagellates. Food vacuoles are reported. These contain material thought to be undergoing digestion. It is suggested that the food particles enter the cell by way of the ventral chamber, which is lined only by membranes, unlike the rest of the cell, which is covered by thick plates. The new findings are discussed in relation to the earlier work on the species and to the fine structure of oilier dinoflagellates.

A dinoflagellate with both a mesocaryotic and a eucaryotic nucleus. I. Fine structure of the nuclei.

SummaryGlenodinium foliaceum Stein (Pyrrophyta) has a single mesocaryotic nucleus which contains numerous typically dinoflagellate chromosomes and one or more nucleoli with a structure similar to that of nucleoli in higher organisms. In addition this organism possesses another nucleus-like organelle which is here termed the eucaryotic nucleus. This is a polymorphic body which varies in shape from ovoid to a branched filamentous form. As with the mesocaryotic nucleus it is surrounded by a perforated envelope. The organelle contains granular material and usually several nucleoli which again appear to have the typical form of nucleoli. No other dinoflagellate is known in which two nuclei of differing types are found. The function and significance of the presence of the two nuclei is discussed.

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

SummaryLight 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.

The fine structure of Woloszynskia micra Sp.Nov., a new marine dinoflagellate

The morphology and fine structure of a new dinoflagellate species are described. Superficially this organism appears to be a Gymnodinium but electron microscopy has clearly revealed the presence of a complex theca which is indicative of the genus Woloszynskia. The fine structure of the typical dinoflagellate organelles, trichocysts, pusule and nucleus with permanently condensed chromosomes are detailed. The normal mitochondria and chloroplasts with threethylakoid lamellae and intra-chloroplast pyrenoids are illustrated. W. micra was isolated from the English Channel near Plymouth.

The ultrastructure of Chroomonas mesostigmatica Butcher (Cryptophyceae)

SummaryThe detailed fine structure of a pigmented marine member of the Cryptophyceae is reported for the first time. The flagella are similar to those of other cryptomonads which have been studied but the root system is probably less complicated. The ejectosomes (trichocysts) have the typical structure but are present in fewer numbers than in many other members of the class. The chloroplasts contain numerous lamellae each composed of two rather dense thylakoids. A single large pyrenoid is attached to a chloroplast and on a short spur beyond this is the centrally placed eyespot. The chloroplasts, pyrenoid and eyespot are all enclosed by an endoplasmic reticulum sheath and starch grains are laid down between this sheath and the chloroplasts. The interphase nucleus is irregular in shape but has the typical eucaryotic appearance. At the anterior end of the cell are numerous coated vesicles which may have an excretory function. The fine structure of Chroomonas is discussed in relation to the ultrastructure of other algae.ZusammenfassungZum ersten Male wird der Feinbau einer Cryptomonadacee des Meers in seinen Einzelheiten geschildert. Die Geißeln gleichen denen der anderen, bisher untersuchten Cryptomonaden; nur die Art der Einfügung ist vielleicht etwas einfacher. Die Ejectosomen (Trichocysten) haben die übliche Struktur, sind aber weniger zahlreich als bei den anderen Mitgliedern der Familie und Klasse. Die Chromatophoren enthalten zahlreiche Lamellen, deren jede aus zwei ziemlich dichten Thylakoiden zusammengesetzt ist. Ein einzelnes pyrenoid sitzt an den Chromatophoren. Ein zentraler Augenfleck ist an einem kurzen Fortsatz derselben befestigt. Chromatophoren, Pyrenoid und Augenfleck sind gemeinsam von einer endoplasmatischen netzigen Hülle (endoplasmatisches Reticulum) umhüllt. Zwischen dieser Hülle und den Chromatophoren werden Stärkekörner abgelagert. Der Zellkern ist in der Interphase unregelmäßig gestaltet. Er hat das übliche Aussehen der eukaryotischen Lebewesen. Am Vorderende sind zahlreiche umhüllte Bläschen, die wohl Abscheidungsorganellen sind. Der Feinbau von Chroomonas wird mit dem anderer Algen verglichen.

The encystment of a freshwater dinoflagellate: A light and electron-microscopical study

The process of encystment, or resting spore formation, in a freshwater dinoflagellate (Woloszynskia tylota nov. comb.) has been studied with both light and electron microscopy. The main features of the process are as follows: (i) the replacement of the theca by a thin, amorphous outer wall, which gradually thickens by the deposition of material on its inner face; (ii) the appearance of a layer of closely-packed lipid droplets at the cytoplasmic margin of the mature cyst, resembling a granular ‘inner wall’ in the light microscope; (iii) the reduction in size or disappearance of cytoplasmic structures such as chloroplasts, Golgi bodies and pusule; and (iv) the enlargement of a central ‘accumulation body’ and cytoplasmic vacuoles containing crystals. Comparisons are made with light-microscope studies of encystment of other dinoflagellates, with ultrastructural studies of non-motile division stages, with zooxanthellae and with fossil dinoflagellate cysts or hystrichospheres.

The ultrastructure of the dinoflagellate pusule: A unique osmo-regulatory organelle

SummaryThe ultrastructure of the pusule apparatus has been investigated in some 40 dinoflagellates from both feshwater and marine environments. The morphology of the organelle is varied and seven more or less distinct types have been identified. These can be grouped into two categories: a)Pusules with pusule vesicles: vesicles opening directly into the flagellar canal; vesicles opening into a collecting chamber which branches from the flagellar canal; pusule with internal collecting chamber surrounded by vesicles; complex tubular pusule with vesicles.b)Pusules constructed of tubules or sack only: simple tubular pusule which may have a collecting chamber; tubular pusule with invaginations of the walls; sack pusule with invaginated walls. A characteristic feature of the structure of the vesicles, when present, and of the tubules and sacks is the two-membrane nature of the 14–20 nm thick wall. This is constructed of plasma-membrane, invaginated from the flagellar canal, to which is appressed a vacuolar membrane. The vesicles and tubules are always surrounded by extensions of the cell vacuolar system. Various hypotheses concerning the function of the pusule are discussed and it is concluded that it is most likely an excretory or osmo-regulatory organelle.

Evidence for a double-helically coiled toroidal chromonema in the dinoflagellate chromosome

The structure of the dinoflagellate chromosome is examined using spread whole mounts and serial sectioning. Evidence is presented that the chromosome is composed of a toroidal bundle of DNA strands which is coiled plectonemically into a tight double helix. This structure is apparent in interphase whole-mount chromosomes and by careful analysis of serial sectioning can be shown in all mitotic stages. It is particularly apparent in the arms of replicating “Y” shaped chromosomes which often uncoil slightly (either naturally or as a result of the fixation used). A model is presented which is based on these results and recent genetic and biochemical data.

The ultrastructure of mitosis inChroomonas salina (Cryptophyceae)

SummaryA detailed account of the ultrastructure of mitosis in a member of theCryptophyceae is given for the first time. The initial indication of mitosis is the duplication of the flagellar bases. The nucleus migrates towards the anterior of the cell and its envelope and nucleolus break down. The chromatin which at interphase is in the form of scattered clumps, condenses into a solid mass through which run narrow tunnels. Each tunnel allows the passage of one to four microtubules. At metaphase the dense plate of chromatin is situated on the equator and the spindle has a rectangular shape. Individual chromosomes cannot be recognized and no morphologically differentiated kinetochores have been observed. The flagella remain functional, their bases stay at the anterior side of the nucleus and do not move to the poles. At anaphase two plates of chromatin separate and these move apart until they come to lie against the ER sheath surrounding the chloroplasts. The new nuclear envelope starts to form on the opposite side of the daughter nucleus. Cytokinesis may commence early in mitosis and consists of a constriction of the parent cell, starting from the posterior end, followed by separation of the two daughters. The present work supports earlier views that only one chromosome is evident during the nuclear division of these organisms. The mitosis is completely different from that of theDinophyceae with which theCryptophyceae were formerly linked.

Thecal fine-structure in the dinoflagellate genera Prorocentrum and Exuviaella

The structure of the thecae, as seen in the electron microscope, are described for two species of Prorocentrum and five of Exuviaella . For the first time it is possible to distinguish between true pores and the spines and thin areas of wall which have previously been described as pores or poroids. Several distinct features such as the spines covering the valves, the ridged valve margin and special flagella pore structures which are found in only two of the Exuviaella species suggest that it may be necessary to create a new genus.