Phenotypic dissections of the Blastocladiella emersonii zoospore's developmental choice (original) (raw)

1982, Developmental Biology

Blastocladiella emersonii zoospores can either remain actively motile cells that lack a cell wall and do not grow, or they can encyst, thereby losing cellular motility and gaining a cell wall, in preparation for entering the growth phase. Exclusively post-translational controls appear to mediate the zoospore's choice, in response to environmental cues. Encystment can be elicited by any one of a variety of compounds, but their encystment eliciting effects can be blocked by "zoospore maintenance factor" (ZMF). The minimal concentrations of elicitors just yielding maximal percentage encystment (in the absence of ZMF) vary over 10 orders of magnitude. Cyclic GMP is uniquely effective only in the picomolar range, about 8 orders of magnitude below the effective range for CAMP. ZMF, released into the medium by the organism during zoospore production (W. K. Gottschalk and D. R. Sonneborn, 1981, Exp. MycoL 5, l-14), blocks percentage encystment in a concentration-dependent manner. Encystment initiation is accomplished within l-2 min exposure of zoospores to any given elicitor, in the presence or absence of ZMF, such that an external elicitor is no longer required to complete encystment. The encystment block point mediated by ZMF occurs after initiation, thus leading to retention of the motile zoospore phenotype; the block point is passed, and zoospores encyst, following removal of external ZMF. Certain encystment elicitors (MnS04, cGMP, CAMP, adenosine, divalent cation ionophores) yield bellshaped dose-response (percentage encystment) plots; at appropriately high concentration, each compound mediates an encystment block point that occurs after encystment initiation but before the ZMF block point. In each case, the block is passed, and zoospores encyst, following external removal of the blocking agent. The latter block points, as opposed to the ZMF block point, are inoperative in the NaCl-elicited encystment pathway. The data establish that zoospores possess cellular machinery that can allow them to reassess their developmental choice after the encystment alternative has been initiated.