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PHYCOLOGICAL SOCIETY OF AMERICA

Pseudo-nitzschia Vegetative Growth

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 slide apart.
• 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

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.

 

Colony Formation

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.