Page 140 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 123

The end of the anaphase is marked by a sudden flow to the poles of the central region of the
elongated endosome, and the persistent nuclear envelope seals around the groups of chromatids
and the daughter endosomes to form the telophase nuclei. Once telophase is established, with
separate daughter nuclei, one of the two flagella in each daughter reservoir grows to emerge as a
locomotory flagellum. A cleavage line is initiated between the now distinct daughter canals and
progress helically backward to separate the daughter cells. High chromosome numbers are the
rule for species of Euglenophyta, indicating a possible polyploidy.

Chlorophyta
Nuclear and cell division has been intensively and extensively studied in the green algae at light-
microscopic and ultrastructural level, and several different patterns have been recognized. Two
basic patterns have merged from these studies regarding nuclear and cell division in green algae:

. Intranuclear mitosis, with the nuclear envelope closed at metaphase or open at the poles,
interrupted by the microtubules of the spindle; other microtubules, transverse to the longi-
tudinal axis of the spindle, are present at telophase and are called the phycoplast; the latter
functions somehow in cytokinesis, by furrowing or by cell-plate formation; the daughter
nuclei at telophase are in close proximity
. The spindle and nuclear envelope are open and a phycoplast is absent; at telophase the
microtubules of the intranuclear spindle are persistent and a phragmoplast-like structure
is organized as cytokinesis by furrowing proceeds

Cylindrocapsa is an example of the first type; the parietal chloroplast divides before mitosis. Two
pairs of centrioles are already present at the beginning of the interphase. In early prophase, the
nuclear envelope is surrounded by one or two layers of endoplasmic reticulum, which is rough,
that is, covered by ribosomes. Perinuclear microtubules appear around the nucleus and in late pro-
phase they proliferate within the nucleus to form a tilted mitotic spindle between the pairs of cen-
trioles lying at the spindle poles. At metaphase the nuclear envelope is still intact and surrounded by
the endoplasmic reticulum, so that mitosis is closed. The fully condensed chromosomes become
aligned to form a distinct metaphase plate and have plate-like layered kinetochores. The non-
persistent telophase spindle soon degenerates, though a few microtubules can still be found
around the reformed nuclear enevlopes. The pairs of centrioles migrate around the telophase
nuclei, away from the former spindle poles and towards the center of the equatorial plane, where
they remain until after cytokinesis.
Cisternae of endoplasmic reticulum proliferate in the narrow zone of cytoplasm present
between the two daughter nuclei at the center of the cell. They bleb off smooth endoplasmic reti-
culum vesicles, which become aligned in the equatorial plane to form a cell plate of smooth ves-
icles. These vesicles coalesce to form a transverse system separating the daughter cells. Here the
vesicles accumulate within a phycoplast, that is, a plate of microtubules lying in the future plan
of division. After completion of the transverse septum and the resultant separation of the daughter
cells, a new cell wall is secreted around each daughter protoplast by exocytosis of Golgi-derived
vesicles containing wall material. Each daughter cell thus gains a complete new wall; in the case
of Cylindrocapsa, daughter cells remain united to form filaments, because the parental walls are
persistent. In the case of other green algae with this type of mitosis and cytokinesis, daughter
cells are liberated from the parent cell wall as non-flagellate autospores, or as zoospores with
centrioles ready to form flagella.
Coleochaete possesses the second type of nuclear and cell division; the chloroplast begins to
cleave at prophase; the single centriolar pair present during the interphase replicates at prophase
and each of the two pairs takes up a position at one pole of the future spindle. Microtubules then
form between the centriolar pairs, outside the envelope of the elongate prophase nucleus. The