Populations of Blastocladiella emersonii zoospores can undergo synchronous germination, nuclear division, and development. Three characteristic responses have been noted for ultraviolet- or gamma-irradiated zoospores, namely: development to (1) a normal, surviving sporangium of about 60 microns in diameter with emission of zoospores and colony formation; (2) a giant sporangium of about 200 microns in diameter, sometimes giving off zoospores after a lag; and (3) a slower growing germling with the usual rhizoids, followed by an abrupt cessation of growth at a diameter of about 30 microns, with no zoospore emission. This "block" indicated in (3) occurs at about the same point with either UV or gamma rays over a considerable range of dose. During the earlier growth of this "blocked" germling, nuclear division does not occur normally. The UV damage is photoreactivable. Survival of colony-forming ability has been determined for germlings irradiated at various stages of their nuclear division up to the four nucleus stage. They are the most sensitive just after nuclear division and most resistant just before. Of several hypotheses to explain this variation, differential repair of damage seems to be the most plausible at present.

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