Zoospore differentiation in Blastocladiella emersonii

The process of sporangium formation and zoospore differentiation in Blastocladiella. The inner diagrams illustrate the changing appearance of sporangia during differentiation, as seen in the light microscope. The outer series summarises the intracellular changes observed in electronmicrographs. Development progresses clockwise in this figure and the experiments used synchronous cultures to determine the timing of these events as follows: 1, 15 h 30 min after the start of cultivation (zoospores germinate synchronously on transfer to a medium containing 10 mM MgCl2 + 50 mM KCl; this transfer = time zero); 2, 17 h 30 min; 3, 18 h; 4, 18 h 40 min; 5, 19 h to 19 h 13 min. Intracellular events were timed as follows: A, 15 h 30 min; B, 16 h 30 min to 17h 30 min; C, 16 h 30 min to 17 h 30 min; D, 17 to 18 h; E, 18 h; F, 18 h 10 min; G, 18 h 20 min; H, 18 h 40 min; I, 18 h 50 min; J, 18 h 50 min to 19 h; K, 19 h 15 min. Modified from the original drawing by James S. Lovett in Lessie & Lovett (1968), which illustrates all of these features with electronmicrographs, though it does refer to the Blastocladiella thalli as both ‘fungus’ and ‘plant’, something which would not be done now.

These authors describe the process like this: “Soon after the beginning of flagella formation it is possible to find early stages of ‘cleavage furrow’ formation ...This process … involves the fusion of many small vesicles … cleavage vesicle fusion results in progressive expansion of the primary cleavage furrows and it appears that this activity is simultaneously initiated at many points. Occasionally vesicles can be found in somewhat linear arrangements over a short distance. They more often occur in less orderly clusters and fuse in irregular ring-shaped patterns lying roughly in the plane of the developing cleavage furrow. The frequent occurrence of cytoplasmic peninsulas surrounded by U-shaped areas of cleavage vesicle suggests that many of the rings may in fact be short cylinders; if so, the closure and interconnection of the rings may be irregular and only gradually assume the form of a regular furrow. … The cleavage furrows also fuse with the earlier formed vesicles surrounding the flagella with the result that these finally lie within the cleavage furrows and outside of the uninucleate blocks of cytoplasm delineated by the [newly formed] membrane system”. From Lessie & Lovett, 1968. Reproduced with permission of the Botanical Society of America.