A cytological study of morphogenesis in Coccidioides immitis

Coccidioides immitis is still classified as a Deuteromycete because the perfect stage is unknown. Mating experiments have been unsuccessful. Some investigators consider the endosporulating spherule to be a sporangium with sporangiospores, but others describe it as an ascus. The development of a reproducible method for cultivation of the spherule-endospore cycle in vitro provided an opportunity to study cytological changes occuring throughout the arthrospore-hyphae-arthrospore cycle and the arthrospore-spherule-endospore cycle. Our observations of the arthrospore-mycelial cycle confirm an earlier report by another investigator demonstrating a resting stage followed by ring formation, a V-form, and a filament stage. In addition, we describe four pairs of chromosomes and formation of possible spindles and nucleoli. The spherule-endospore cycle is initiated by an arthrospore with a single large nucleus, most probably derived from the multinucleate arthrospores in the inoculum. This uninucleate arthrospore is present for less than 24 hours. During continued incubation, the arthrospore becomes a round cell with four pairs of chromosomes. Subsquently, the cell enlarges to become a spherule and this growth is accompanied by a series of synchronous divisions of the nuclei. Although endospores are formed apparently progressive cleavage, clusters of uninucleate endospores are enclosed in a membranous sac as seen in squashed spherule preparations. The question of a relationship to the phycomycetes or the ascomycetes remains unresolved.