Pseudo-nitzschia Auxospore Development
What is an auxospore?
A cell is considered an auxospore upon plasmogamy even if the nuclei of the gametes have not yet fused. They can be free in the medium or associated with the gametangial thecae. [Plasmogamy yields a cell with two nuclei, an a cell is considered a zygote when the two nuclei fuse.] A primary organic wall, composed mainly of polysaccharides, is formed around and auxospore.
In Pseudo-nitzschia, growth occurs as bipolar expansion and thus gives rise to its pennate shape. As the auxospore grows and expands, the primary organic wall ruptures equatorially. After rupturing, this wall can be seen as two caps at the poles of the auxospore (see figure below). Bands composed of silica are laid down in a transverse fashion. This occurs at the poles near and within the caps. Thus, growth occurs at the ends unlike vegetative growth which occurs at the center. This network of bands is called a perizonium, and it consists of a wider, central, primary band, as well as slimmer, transverse bands on either side of the center.
After Auxospore Expansion
After the bipolar expansion occurs in the pennate diatom Pseudo-nitzschia, thecae are laid down. When compared to the proceeding generations, this first set of thecae have a modified morphology. This modification is seen because these initial valves are formed within an auxospore wall rather than within a parent frustule. The auxospore wall is far less limiting than the vales and girdle structure of the vegetative cells. This first set of epitheca and hypotheca are shaped within the inner surface of the auxospore casing (perizonium) and have a rounder shape than their vegetative counterparts.
The process in which an auxospore develops into a vegetative cell is seen in the figure to the right. When the auxospore reaches an expanded state (frame 1), the protoplast contracts in order to form the initial epivalve. Following the formation of the epivalve, the protoplasm contracts once again in order to form the hypovalve. During the formation of both valves, the nucleus lies along the side that is contracting.
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copyright Jennifer Shin 1999.