Growth and Reproduction
Growth in diatoms occurs unidirectionally and results in a mean size reduction at each division. The unidirectional growth is a result of the siliceous nature of the frustule, for it cannot expand directly within the frustule and therefore must expand along the girdle. There is a reduction in mean size because each new valve is produced within the frustule of the parent.
Here is the general mechanism for growth in diatoms:
(1) The valves (the epitheca and hypotheca) start shifting apart due to the construction of new valve elements back to back within the parent frustule. In order to allow this movement, the hypocingulum is constantly being added to and is not fully constructed until just before the cell actually divides.
(2) When the cell has fully completed mitosis (and the elements for the new valves are now enclosed within the parent frustule), the cell can divide. The hypovalve/hypocingulum of the parent frustule now becomes the epivalve/epicingulum of the daughter cell, and the parent epivalve/epicingulum becomes the epivalve/epicingulum of the other daughter cell.
Due to the mean size reduction at every cell division, the diatom cell needs to have a way in which to regain its size when it gets too small. This is done by the formation of an auxospore through sexual reproduction. The vegetatively reproducing cells can differentiate into spermatocytes and oocytes. Sexual reproduction can occur in the form of either oogamy (usually found in centric diatoms) or isogamy (usually seen in pennate diatoms). The auxospore is a cell that develops and grows, eventually proceeding to produce a new frustule. Auxospore formation allows the diatom to restore its size, and vegetative growth can continue once again.
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copyright Jennifer Shin 1999.