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Ultrastructure of aplanospore production and germination and the role of calcium in germination of aplanospores of Vaucheria longicaulis variety macounii Blum (Chrysophyta, Tribophyceae) Fitch, Robert Scott


Vaucheria longicaulis var. macounii (Blum) is the most common of the four brackish water species of Vaucher ia known to occur in British Columbia and northern Washington. This variety is easy to maintain in culture with minimum nutritional requirements. It survives culture conditions for extended periods of time (up to 2 years or more) without losing its reproductive capabilities. Aplanosporogenesis is easily induced by transferring plants to fresh medium with almost every vegetative filament producing one apical aplanosporangium containing a single aplanospore. Aplanosporogenesis has been examined in sections prepared for light and electron microscopies. As the vegetative filament tip expands, signalling the beginning of aplanosporogenesis, the large central vacuole is displaced from the tip. An inner wall is secreted within the existing cell wall via exocytosis of numerous fibrillar-containing vesicles. Septation of the aplanosporangium from the vegetative filament is accomplished by the centripetal infurrowing of this newly secreted inner wall at the base of the enlarged filament tip. Each aplanosporangium produces a single, multinucleated walled aplanospore, with unique organelle morphologies and associations. The importance of these organelles in the mechanism of aplanospore release and germination are discussed. Germination of aplanospores is initiated at sites characterized by low optical density and leads to the formation of filaments. The most prominent ultrastructural features characterizing germination are the rapid expansion of the central vacuole, the accumulation of dictyosome-derived vesicles at the tip of the germinating filament in association with cell wall expansion, and the increase in the number and redistribution of microtubules. The possible function of unique organelle associations, such as those occurring among the mitochondria-endoplasmic reticulum-dictyosome association, are discussed as well. In addition, quantitative changes in the volume density of some subcellular compartments and organelles are evaluated using morphometric analysis. Using the fluorescent probe chlorotetracycline as an indicator of intracellular Ca²⁺, the role(s) of calcium on aplanospore germination and filament growth are studied. These studies are supplemented with the use of drugs to disrupt the availability and distribution of extra- and intracellular Ca²⁺. The results are then analyzed in terms of the complexities of the mechanisms involved in the germination and growth of Vaucheria aplanospores.

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