PHYCOLOGICAL SOCIETY OF AMERICA
Pseudo-nitzschia's cell cycle, like most diatoms, resembles
the cell cycle observed in other eukaryotic organisms. The cell grows
to twice the initial volume, making a second copy of their chromosomes
while also doubling their organelles (mitochondria, plastids, dictyosomes,
etc.). After partitioning this "life material", the parent
cell can divide into two daughter cells.
It is the events that occur after replication that set truly sets Pseudo-nitzschia,
along with other diatoms, apart. In order to become daughter cells,
the parent cell much create new wall elements (two hypothecas) to
complement the conserved epithecas (both valves become epithecas
since they are the oldest valve of the daughter frustule). Due to
the rigid silica valves, diatoms are limited to unidirectional growth
and extend as the girdle allows the hypotheca and epitheca move apart.
There is a mean size decrease over the course of many generations
since the valves and girdle of the daughter cells originate within
the parent cell. Also amongst pennate diatoms, including Pseudo-nitzschia, the
valve structure may change slightly due to a greater length decrease
when compared to the length.
Like other diatoms, the cell cycle of Pseudo-nitzschia is
affected by the external environment. The amount of irradiance from
the sun as well as silicon concentrations affect the cycle. Without
the presence of silicon, for example, the cycle may stop at the start
of the cell cycle following DNA replication, mitosis, and cytokinesis
but prior to valve formation and separation of the daughter cells.
The Cell Cycle
- Interphase: During Interphase, the centrosome serves as
the microtubule center where upon microtubules originate and radiate
outwards. It shifts to the side of the cell (midline of the girdle)
and also pulls the nucleus, through nuclear interaction with the
microtubules, along with it. A spindle precursor is visualized a
small, disc structures near the centrosome and, due to its proximity
to the centrosome, is thought to originate fro it. In Pseudo-nitzschia,
the spindle precursor starts to form soon after the valve formation
(that, in itself, occurs after cell division) of the prior cell division.
- Prophase: The spindle precursor, situated now at the side
of the cell, divides and effectively becomes the polar plates of
the spindle apparatus. Between the two polar plates, a network of
numerous parallel microtubules, called the central spindle, develops.
The nuclear envelope breaks down, and the spindle grabs onto the
nucleus while expanding its network of microtubules. The central
spindle of the microtubule network is where additional microtubules
are added, and the spindle as a whole lengthens. During Prophase,
the centrosome also disappears.
- Late Prophase/Prometaphase/Metaphase: During this state
of the cell cycle, the central spindle can now be visualized as two
parts of a whole interconnected as half spindles through a portion
of overlapping microtubules at the center. Since the nuclear envelope
has broken down, the chromosomes attach to the spindle through kinetochore-microtubule
interactions and line up at the center of the cell.
- Anaphase: The chromosomes, since they are attached to microtubules
that are, in turn, attached to the polar plates, move towards the
spindle poles. The central spindle lengthens as the two half spindles
- Telophase: The half spindles are now entirely separate
from each other, and disassembly of the spindles takes place. The
polar plate and spindle disassociate from each other. The nuclear
envelope forms once again, enclosing the chromosomes. A centrosome
is formed again and it lies close to the daughter nucleus. It is
hypothesized that the centrosome might be formed from the remnants
of the polar plate. A cleavage furrow, consisting of a band of contractile
microfilaments, appears and will act to cut the cell into two.
- Post-Telophase/Cytokinesis: Plasmolysis, a separation of
the protoplast, occurs to a certain extent in this phase, and now
that the half spindles are entirely separate and undergoing disassembly,
cytokinesis can be completed The cleavage furrow further develops,
and it is thought that the plane of cleavage might be determined
by markers found in the cell wall (perhaps a component of the hypocingulum).
During this stage, the daughter cells are separate though still contained
within the parent frustule.
Valve formation takes place after cell cleavage has been completed.
Each valve forms within a silicon deposition vesicle (SDV). These
vesicles start out as a thing, long tube. It should be noted that
there are cytoplasmic structures (like the nucleus, microtubules,
microfilaments, endoplasmic reticulum) that are associated with the
SDV, and it is thought that these structures aid in the formation
of the SDV. The SDV grows lengthwise, and it seems that there is
some sort of mechanism that indicates when the cell should stop (perhaps
a transmembrane protein). Microfilaments towards the sides may aid
in the expansion and growth of the SDV as well as the raphe (which
is also formed at this time). See the Raphe section
for more information about raphe formation!
Valve patterns are conserved from generation to generation, with Pseudo-nitzschia being
able to reproduce that patterns at each division. The spacing between
the ribs and pores are consistent. When the SDV has grown to the
size of the new valves, layers may be added both above and below
the vale. The exact mechanism for how how this is done is not known
(pretty amazing, huh?) After the valves are created, the girdle bands
are added to the hypotheca. The protoplast, having already been partitioned
due to the cleavage furrow, can now settle into their respective
daughter frustules, and two daughter frustules have now been created
from a singular parent.
After the parent cell divides to yield two daughter cells in Pseudo-nitzschia, the
new hypothecae(the new valves of both the daughter cells) lie back
to back. In a filamentous colony, the valves at the end are always
epithecae, and the colony basically expands bidirectionally from the
initial start of the colony (the first division). In Pseudo-nitzschia, the
raphe allows for sliding of the valves over each other, and the colony
can thus be visualized as overlapping valve ends (rather than overlapping
whole valves in non-raphid genera of diatoms).
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copyright Jennifer Shin 1999.