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Abstract
The process of endospore cleavage (sporangiospore formation) within what were originally ballistosporic conidia was examined with light and electron microscopy in two isolates of Basidiobolus ranarum. A pathogenic isolate was obtained from skin ulcers on a frog (Davis isolate) and a nonpathogenic isolate was obtained from toad feces (Toad isolate). Electron microscopy revealed that endospore formation was not the result of successive bipartitioning or progressive cleavage as previously described. Instead, the protoplasm of the ballistosporic conidium was bisected by a wall, thus dividing the conidium into one or more sporangia within which endospores (sporangiospores) were delimited. The walls segregating the conidial cytoplasm into sporangia were continuous with the inner layer of the original conidial wall. As spores were demarcated, some cytoplasmic debris was left in the space between the conidial/sporangial walls and the spore walls. After a period of 16 hours of low light levels, turning on the laboratory lights stimulated the formation and discharge of conidia with a peak in conidial discharge 5–6 hours later.