Internal Changes in Hyphae of Rhizopus sexualis (Smith) Callen and Mucor hiemalis Wehm. associated with Zygospore Formation

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Abstract
The distribution of nucleic acids, nuclei, mitochondria, and reserve foods in vegetative hyphae, zygophores, and developing zygospores of Rhizopus sexualis and Mucor hiemalis were examined by differential staining. The extreme tips and growing zones of vegetative hyphae contained a high concentration of RNA and numerous mitochondria. Nuclei were not present at the extreme tip but were numerous just behind it. In older parts of the hyphae the concentration of RNA was low and both nuclei and mitochondria were fewer than in the zone of elongation. Glycogen and lipids were present in all parts of the living hyphae except the extreme tips and were more highly concentrated in the older parts of the hyphae. Young zygophores showed a much lower RNA/DNA ratio than that found in the vegetative hyphal tips. Transfer of colonies from 20° C to temperatures of less than 10° C, which is known to prevent zygospore initiation, caused some but not all recognizable zygophores of R. sexualis, but not those of M. hiemalis, to revert to the RNA/DNA ratio characteristic of vegetative hyphae. Some zygophores of Rhizopus and most of those of Mucor developed into sporangiophores at low temperature, retaining the relatively low RNA/DNA ratio throughout development. It is suggested that a reduction in the RNA/DNA ratio is an early step in the change from the vegetative state to the reproductive one. At first this step is reversible, but soon becomes irreversible by an additional step, the nature of which is unknown. For some time after this the reproductive hyphae are capable of either producing asexual sporangia or of conjugating to produce zygospores. Once conjugation has taken place development either ceases or continues until the spore is fully mature, but it cannot under any circumstances then be reversed. The development and maturation of the zygospore involves a great increase in number of both nuclei and mitochondria and in the concentration of glycogen and lipids.