Factors Affecting the Production of Zygospores by Choanephora Cucurbitarum

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Abstract
SUMMARY The mycelia of the (+) and (-) sexes of C. cucurbitarum isolated from the same source were found to differ slightly in appearance on certain media. In the presence of mixed (+) and (-) mycelia zygospores develop abundantly within a few days on a wide variety of agar and liquid media. The range of favorable temperature was approximately 15–37° C. Light was not required and had no apparent effect, except at 15° C, where it was inhibitory. Zygospores formed in an atmosphere containing as much as ten percent carbon dioxide. Zygospore production coincided with the exhaustion of the available carbon supply in the medium and was induced by subjecting well nourished mycelium to starvation conditions. Some sugars which were poor for mycelial growth favored quick zygospore production. All nitrogen sources tested favored their formation, provided the medium did not become too acid. The favorable pH range was approximately 4.5 to 8.5. A simple culture tube designed for the slow continuous flow of liquid medium over growing mycelium is described. This method was used to maintain constant pH of the medium. In mixed cultures of (+) and (-) mycelia, particularly in liquid media, there is produced a greatly increased amount of beta-carotene, apparently stimulated by hormone-like substances secreted by the mycelium of the opposite sex. The beta-carotene becomes concentrated in the suspensors and is suspected of having some function in sexual reproduction of this fungus. Mature typical zygospores were found in moist, well-decayed pumpkin flowers collected from a garden where both sexes had been found previously.