Zoospore germination in the water mold, Blastocladiella emersonii: I. Measurement of germination and sequence of subcellular morphological changes

The zoospore of Blastocladiella emersonii contains a highly ordered array of intracellular organelles. Germination of the zoospore is accompanied by a series of abrupt changes in this array. Procedures have been developed whereby the progress of germination can be measured in cell populations. Zoospores are obtained after growth in a defined nutrient medium followed by zoospore formation in buffered CaCl 2. The population is allowed to germinate in a defined inorganic salts medium. Three quantal phenotypes are distinguished: zoospore, round cell, and germling. Virtually every zoospore transforms to a round cell and then to a germling. Under the simple set of conditions adopted as standard, the sequence is rapid and population synchrony is such that nearly complete separations of the three phenotypes can be effected by sampling germinating populations at appropriate times. The beginning population is 100% zoospores; round cells begin to appear after about 5 minutes and constitute 50% and greater than 90% of the population at about 12.5 and 20 minutes, respectively; germlings begin to appear between 17 and 20 minutes and constitute 50% and greater than 90% of the population at about 30 and 40 minutes, respectively. Two different conditions of assay have been developed: in one, germination occurs in stationary liquid culture (dish assay); in the other, germination occurs in stirred liquid culture (spinner flask assay). In the dish assay, only zoospores remain suspendable; round cells and germlings adhere firmly to the dish surface. In the spinner flask assay, all cells remain in suspension. The two assays yield population kinetics of germination that are in close agreement. Moreover, with either assay, germination kinetics are highly reproducible, and performance in either assay constitutes a heritably stable trait. By utilizing the electron microscope and other techniques in conjunction with the population assays of germination, the order of subcellular events accompanying germination has been partially characterized. Early events of round cell formation include: retraction of the flagellar axoneme, appearance of localized channels from intracellular vesicles to the plasma membrane, construction of the initial cell wall, acquisition of cellular adhesiveness, and acquisition of resistance to lytic agents. Later events include breakdown of the nuclear cap and release of ribosomes from it into the cytoplasm, and branching or fragmentation of the single mitochon