Dinoflagellata

From Palaeos

Eukarya
  |--+--Fungi
  |  `--Metazoa
  |--Amoebozoa
  `--+--Rhizaria
     |--Metamonada
     `--o--Plantae
        `--o--Chromista
           `--Alveolata
                |--Ciliophora
                `--Miozoa
                     |--Dinozoa
                     |    `--Dinoflagellata
                     `--Apicomplexa

Ceratium, a dinoflagellate (Dinophyceae, Peridiniales, Ceratiaceae

[edit] Summary

This page briefly describes the morphology, origins and taxonomy of the dinoflagellates.

Keywords: Dinoflagellata, Pyrrhophyta, dinoflagellate, theca, cyst

[edit] Introduction

Dinoflagellates are alveolates: single celled organisms (protists) which are neither animals nor plants, though for nomenclatural purposes they are treated as if they were plants. They are found in most aquatic environments and form a major part of the modern plankton.

"Living dinoflagellates may be autotrophs, phagotrophs, symbionts or parasites. Photosynthetic species (autotrophs) account for about half the number of living dinoflagellate genera. Some species have more than one nutritional strategy; for example, species of Protoodinium are both parasitic and photosynthetic. Free living dinoflagellates are a major component of the marine phytoplankton and thus important primary producers. Some toxic marine species cause paralytic shellfish poisoning (Shimuzu 1987; Taylor 1987), particularly when forming red tides. Symbiodinium and its allies ("zooxanthellae") are photosynthetic symbionts of other protists and invertebrates, notably corals, and play a major role in reef and other marine ecosystems. Dinoflagellates, although most common in marine environments, also inhabit fresh water environments (Pollingher 1987), snow, and the interstices of wet sand" (Fensome et al. 1996, p. 108).

"Many genera are sensitive to such conditions as water salinity and nutrients, and some genera are characteristic of latitudinal oceanic temperature zones; hence, the geographic distributions of dinoflagellates can be important indicators of environmental conditions (Dale 1996), not only for present day environments but also for ancient ones. Fossilized dinoflagellate cysts are widespread in Mesozoic-Cenozoic sedimentary rocks" (Moldowan & Talyzina 1998, p. 1168).

[edit] Life Cycle

"Among protists, life cycles may be:

haplontic, in which the vegetative (i.e. actively feeding and asexually reproducing) cells are haploid, the zygote being the only diploid cell in the life cycle;

diplontic, in which the vegetative cells are diploid, the gametes being the only haploid cells in the life cycle; or

diplohaplontic, in which there is an alternation of diploid and haploid vegetative generations.

With rare exceptions, dinoflagellates are known, or believed, to have haplontic life cycles.

"The life cycle of most dinoflagellate species involves relatively simple asexual division of one cell into two daughter cells, the process commonly including a shedding of part or all of the parent cell wall. However, more complex life cycles occur, especially among parasitic and symbiotic species, and many free-living dinoflagellates are known to produce cysts (Text-Fig. 4). A cyst is any nonmotile cell possessing a cell wall (see next section). Some cysts have walls composed of cellulose and are not preservable as fossils; others are fossilizable, having walls composed of a complex organic polymer similar to sporopollenin (see Brooks et al. 1971), termed dinosporin (Fensome et al. 1993b). Cysts can be categorized in terms of their function. Among living dinoflagellates, three functional types of cyst are prominent (Dale 1983; Taylor 1990):

resting cysts: Resting cysts represent a dormant stage in which normal life processes are greatly reduced. Dinoflagellate resting cysts have, so far, been found to result from sexual fusion; they are thus zygotic resting cysts, termed hypnozygotes. Walls of resting cysts are commonly strengthened by a sporopollenin-like material (dinosporin) and may comprise several layers. Most fossil dinoflagellates are probably hypnozygotes, although this is not directly demonstrable for extinct species.

temporary cysts: A motile dinoflagellate cell with a well developed pellicle may, under adverse conditions, shed its flagella and outer wall (including plates, where present) and form a temporary cyst surrounded by the pellicle (see next page).

vegetative cysts: Vegetative cysts are nonmotile cells surrounded by a continuous wall, probably the pellicle. These cells are metabolically and/or reproductively active, in contrast to resting and temporary cysts. In some dinoflagellates, especially parasitic and symbiotic taxa such as Blastodinium and Symbiodinium, the principal life cycle stage is represented by vegetative cysts. Pyrocystis is an example of a free-living dinoflagellate that passes most of its life cycle as a vegetative cyst.

The sexual process, which can result in a hypnozygote, is known for only one percent of living dinoflagellates (Pfiester & Anderson 1987). However, it may be more widespread than currently observed. As Pfiester& Anderson pointed out, the sexual process has probably been overlooked in many species because: 1) gametes resemble normal cells; 2) fusion is slow and readily confused with division; 3) fusion occurs at night in photosynthetic species; and 4) warty zygotes have been misinterpreted as aberrant cells."

(After Fensome et al. 1996, pp. 108-109.)

[edit] Habit

"Although generally motile and biflagellate (Text-Fig. 3A-], L, R-U), dinoflagellates may also occur as coccoid cells (Text-Fig. 3N), amoeboid cells, multinucleate cells, tentacle bearing cells (Text-Fig. 3Q), and filamentous and ribbon-like colonies of cells (Text-Fig. 3K, M). Coccoid cells (including most cysts) are nonmotile, thus lacking flagella, and have a continuous wall. Amoeboid cells (e.g. in Stylodinium) may all represent parasitic life cycle stages. Polykrikos is unique among dinoflagellates in having multinucleate cells, each cell bearing several sets of flagella and flagellar furrows. Cells of Noctiluca are also unusual in having a single, small, inconspicuous flagellum and a prominent, food procuring tentacle (Text-Fig. 3Q); these cells contain extensive vacuoles separated by strands of cytoplasm, and are best described as buoyancy regulating, rather than motile. The non-parasitic Dinoclonium and Dinothrix (Text-Fig. 3M) and the tapeworm-like parasitic Haplozoon (Text-Fig. 3K) exist as filamentous and ribbon-like multicellular forms, respectively, during prominent parts of their life cycles" (Fensome et al. 1996, p. 107).

[edit] Encystment

... purpose of encystment ... Evitt p. 13

[edit] Interpretation of Fossil Cysts

Evitt p. 13b

... on the other hand ...

Fensome's bit about nannoceratopsians etc. (p. 155)

"Evitt (1981) cautioned against a literal interpretation of the dinoflagellate fossil record on the basis that few living dinoflagellates produce fossilizable cysts. He concluded that fossil dinoflagellates have only a limited relevance in elucidating the pattern of dinoflagellate phylogeny. However, if there were no dinoflagellate fossils, we would be unaware of the Nannoceratopsiales - the “missing link” between the Peridiniphycidae and Dinophysiphycidae; we would not know that peridinialean and gonyaulacalean tabulations have been separate since Jurassic times; we would know nothing of the early Mesozoic Rhaetogonyaulacineae - a precursor of later gonyaulacaleans and possibly also of the Peridiniales; we would not know that Ceratium-like dinoflagellates existed in the Late Jurassic and that Balechina-like ptychodiscaleans (Dinogymnium and its allies) were present in the Late Cretaceous" (Fensome et al. 1996, p. 155).

[edit] Morphology

[edit] Anatomical Features

"Living dinoflagellates exhibit a great diversity in form, habit, and habitat that belies their systematic position near the base of the phylogenetic tree of the eukaryotes. Their primitiveness is shown especially by properties of the nucleus, mitotic apparatus, and chloroplast. The nuclear structure (typically with chromosomes permanently condensed) and the mitotic apparatus (with spindles external to the nuclear membrane) are perhaps the most primitive in any eukaryote. The chloroplast structure and the pigment assortment that includes chlorophyll a and c2, but not c1, suggest that only the red algae may be more primitive. However, the general organization of the dinoflagellate cell and extreme specializations to be found in certain taxa hardly match the usual concept of primitive. As an example of a highly specialized organelle, consider the light-sensitive structure, with eye-like succession of lens, fluid-filled "camera", retinoid, and pigment backing, which occurs in a few species (Francis, 1967; Greuet, 1970). Less spectacular but interesting for their widespread occurrence are the vacuolelike pusules, fiuid-filled bodies which occur two per cell and possibly have an excretory or assimilative function.

"Chloroplasts may be present or absent, and holophytic, phagotrophic, saprophytic, symbiotic, and parasitic nutritional regimes occur. Planktonic forms inhabit the open sea, coastal and estuarine waters, and rivers and lakes-environments which, collectively, encompass extreme ranges in temperature, salinity, and other aspects of water chemistry" (Evitt 1985, p. 7).

"Dinoflagellates are primarily single-celled organisms (variously considered algae, protozoans or, nowadays preferably, protists) that occur typically as motile cells with two flagella (Text-Fig. 1). The transverse flagellum is ribbon-like, encircles the cell, is usually within a transverse furrow known as the cingulum or girdle, and is thrown into many waves. The longitudinal flagellum is whip-like, trails posteriorly, is thrown only into a few waves and, proximally, is usually within a longitudinal furrow known as the sulcus. The flagella, together with the unique forward rotating motion which they impart..." (Fensome et al. 1996, p. 107).

"Most dinoflagellates are distinguished by a dinokaryon, a special eukaryotic nucleus involving, among other distinctive features, fibrillar chromosomes that remain condensed during the mitotic cycle. The dinokaryon and other internal cell structures have been recently reviewed in detail by Taylor (1990) and Fensome, Taylor et al. (1993)" (Fensome et al. 1996, p. 107).

[edit] Armoured and Unarmoured Forms

Evitt p. 14

[edit] Orientation and Terminology

"In terms of orientation of the motile cell, that part towards the direction of movement is anterior, while the trailing part of the cell is posterior. The anterior end is the apex and the posterior end is the antapex. The two flagella usually emanate from a single pore, commonly in the equatorial region of the cell. That side of the cell from which the flagella arise is ventral, the opposite side is dorsal. Left and right sides of the cell are then determined by biological convention, as in humans. Although other shapes occur, many motile dinoflagellates have a more or less streamlined configuration, commonly with a single protrusion or horn at the apex (apical horn) and an antapex that may be broadly rounded, or that may have two, commonly unequal, antapical horns. Motile cells may be spheroidal (e.g. Protoceratium), dorsoventrally compressed (e.g. Ceratium), anteroposteriorly compressed (e.g. Ostreopsis), or laterally compressed (e.g. Dinophysis)" (Fensome et al. 1996, p. 108).

"That part of the cell (whether cyst, thecate motile cell or athecate motile cell) anterior to the cingulum is termed the episome; that part of the cell posterior to the cingulum is termed the hyposome. Equivalent terms specifically for the cyst are epitract (or epicyst) and hypotract (or hypocyst); equivalent terms specifically for a thecate motile cell are epitheca and hypotheca; and equivalent terms for an a thecate cell are epicone and hypocone" (Fensome et al. 1996, p. 108).

[edit] Tabulation

"The complex outer region of dinoflagellate cells (Text-Fig. 2) is termed the amphiesma (see Morrill & Loeblich III 1983) or cortex (Netzel & Dürr 1984). Dinoflagellate motile cells are bounded by the cell membrane (plasmalemma). Beneath the plasmalemma, a single layer of vesicles (amphiesmal vesicles) is almost invariably present. The vesicles may contain cellulosic plates (thecal plates) in taxa that are thus termed thecate (or armored); or the vesicles may lack thecal plates, such taxa being termed athecate (unarmored or naked). In athecate taxa, the amphiesmal vesicles playa structural role. In thecate taxa, thecal plates, one of which occurs in each amphiesmal vesicle, fit tightly together (Text-Fig. 5). Thecal plates vary from being thin and difficult to observe under the light microscope to thick and heavily ornamented. Collectively, the thecal plates of a single cell constitute a theca.

"In some athecate dinoflagellates there is a thin discontinuous layer within the amphiesmal vesicles that resembles the plate precursor layer in thecate species. According to Morrill & Loeblich III (1983), the membrane bounding the amphiesmal vesicles may partially break down and this discontinuous layer develops into a continuous layer, the pellicle. Perhaps more commonly, the pellicle develops as a separate layer internal to the amphiesmal vesicles. The pellicle, however formed, consists primarily of cellulose, sometimes with a dinosporin component. In some athecate genera (e.g. Balechina, Ptychodiscus and Noctiluca), the pellicle forms the principal strengthening layer of the amphiesma, and the cells are termed pelliculate. The pellicle is sometimes present beneath the theca (e.g. of Alexandrium and Scrippsiella) and forms the wall of temporary cysts. The pellicle may also be the layer represented by the wall of fossilizable resting cysts. A dinoflagellate is said to have a cell wall if a cellulosic or otherwise strengthened layer - i.e. a theca or pellicle - is present in the amphiesma. Hence, athecate, nonpelliculate cells lack a cell wall whereas thecate motile cells and pelliculate motile and nonmotile cells (including fossil resting cysts) possess a cell wall.

"Conventionally, the term tabulation has been used to refer to the arrangement of thecal plates. However, as thecal plates occur within amphiesmal vesicles, and since there is a morphological continuum between taxa that have thecal plates and those that do not, tabulation can also be conceived of as the arrangement of amphiesmal vesicles, with or without thecal plates. Although each thecal plate occurs within an amphiesmal vesicle (Text-Fig. 2, 6), the plates adjoin one another tightly along linear plate sutures (Text-Fig. 5), usually with the margin of one plate overlapping the margin of the adjacent plate. It is generally assumed that thecal plates are composed of cellulose. Most plates are penetrated by trichocyst pores (see Dodge 1987) which may lie in pits (areolae). The plates may be ornamented, for example, by a reticulum (Text-Fig. 5) or by striae.

"Cell growth, and hence increasing surface area, is accommodated by secondary growth of the plates at one or more of the plate margins. The growth bands thus produced are usually striated at right angles to the adjacent suture (Text- Fig. 5) and have been termed "intercalary bands". However, the term" growth band" avoids confusion with the unrelated term "intercalary plate". Growth bands lack trichocyst pores. Dinoflagellate tabulations can be grouped into six types (Text-Fig. 7), each of which is discussed below."

(After Fensome et al. 1996, pp. 110-111.)

[edit] Tabulation Notation Systems

Most popular of the tabulation notation systems is Kofoid's, which is a strictly descriptive notation system. There are others, such as the Evitt-Taylor and Edwards systems. Each has some advantages but they share a common failing in attempting to codify presumed plate homologies within the notation itself. While there may yet come a day when these homologies are so well-understood that they acquire a near-factual status, for the present they remain interpretive and interpretation has no place in a descriptive notation.

Continued on next page:

[edit] Phylogeny and Evolution

[edit] References

See end of Phylogeny page.

Credits

(c) Chris Clowes 2003 - Peripatus Home Page