Page 138 - Algae

Anatomy 121

Dinophyta

The dinoflagellate nucleus, known as dinokaryon, is bounded in the usual eukaryotic fashion by a
nuclear envelope penetrated by pores. However, it possesses a number of unusual features, including
high amounts of DNA per cell (five to ten times the most common eukaryotic levels, up to a
maximum of 200 pg in Gonyaulax polyedra). It is relatively large, often occupying about one half
of the volume of the cell. Nuclear shapes are variable, ranging from spheroid to U-, V-, or Y-
shaped configurations. In most dinoflagellates the chromosomes remain continuously condensed
and visible, by both light and electron microscopy, during interphase and mitosis. Chromosome
counts range from 12 to around 400, but may be variable within a species. A prominent nucleolus
is also persistent. Each chromosome consists of a tightly super-coiled structure of DNA double
helix. The permanent condensed chromosome shows a swirled, fibrillar appearance due to the
naked DNA double helices (i.e., no histones are present). The 3–6 nm fibrils are packed in a
highly ordered state, up to six level of coiling. A small amount of basic protein is present in a few
species, such as Oxyrrhis marina, but none of it corresponds in amino acid content to the histones
normally present in eukaryotic chromosomes. A further peculiarity of dinoflagellate chromosomes
is the high amount of calcium and other divalent metals, such as iron, nickel, or copper, which may
play a role in chromosomal organization. In some dinoflagellate, such as Noctiluca and Blastodi-
nium, the chromosomes undergo an expanded, decondensed change during interphase and are
termed noctikaryotic.
Dinoflagellate mitosis is also unusual. At the time of division, chromosomes divide longitud-
inally, the split starting at one end of the chromosome and moving along the entire length. The
nucleus is invaded by cytoplasmic channels that pass from one pole to the other. Microtubules
are present in these channels. The nuclear envelope and the nucleolus are persistent throughout
nuclear division. The chromosomes upon dividing assume a V- or Y-configuration, and the
apices of such chromosomes are closely associated with the nuclear envelope surrounding a cyto-
plasmic channel. This association suggests that the cytoplasmic channels serve as mechanism for
the movement of chromosomes.
The nuclear envelope persists during mitosis (closed mitosis), as it does in other algae, for
example, Euglenophyta and Raphidophyceae. However, with the exception of O. marina, where
mitotic spindle is intranuclear, the chromosomes do not appear fibrillar, and the mitosis strongly
resembles that of Euglena, the mitotic spindle is always extranuclear. The spindle microtubules
pass through furrows and tunnels that form in the nucleus at prophase. No obvious spindle pole
bodies other than concentric aggregation of Golgi bodies are present. Some microtubules contact
the nuclear envelope, lining the tunnels at points where the chromosomes also contact. The chromo-
somes have differentiated dense regions inserted into the envelope.
Dinoflagellate cells undergo binary fission, and each daughter cell can retain half the parent
cell wall, which splits along a predetermined fission line. This mode of reproduction is called
“desmoschisis,” and examples are Ceratium and many Gonyaulax species. In other dinoflagellates,
daughter cells do not share the parent cell wall; this mode of division is called “eleutheroschisis.”
Binary fission may take place either inside the mother cell (many freshwater Peridinium species), or
the protoplast may leave the mother cell wall before division through a hole or slit. In thecate
species (many marine Protoperidinium species) the protoplast escapes, after special thecal plates
are dislocated, through a hole. In several species the protoplast has no flagella and deforms its
cell wall during the escape from the mother theca; however, this is not a typical ameboid stage.
After cell division, each daughter cell produces a new cell wall, or a new theca in thecate cells.

Euglenophyta
In these algae the interphase nucleus lies in the central or posterior region of the cell; it is spherical
in most spindle-shaped species, and ovoid or long and narrow in elongate cells. In the largest
species the nucleus is at least 30 15 mm in dimensions, but in small species the spherical