The Effect of Water Potential on Zygospore Formation in Syzygites Megalocarpus

Abstract

Syzygites megalocarpus Ehrenberg, a homothallic member of the Mucorales, is commonly found in nature on decaying mushrooms. The fungus has a distinguished history in that it was the first in which sexual reproduction was documented, being described by Ehrenberg in 1820 (Ainsworth, 1976). Since this early observation, many other investigators have studied the fungus, especially with regard to the teleomorphic state and the conditions necessary for its formation (TABLE I). Past studies have implicated carbohydrate concentration (Wegner and Lilly, 1966), low temperature (Hesseltine, 1957), temperature and light (Davis, 1967), and numbers of asexual spores and carbohydrate concentration (Poff, 1965) as factors affecting zygospore formation. The results of these studies are contradictory and do not present evidence that clearly demonstrates a single factor which regulates zygospore formation. We studied the effect of water potential on asexual and sexual reproduction in Syzygites megalocarpus. Preliminary observations suggested that the amount of water available to the fungus could determine the nature of its reproductive cycle, sufficient available water inducing the fungus to reproduce asexually while a paucity of water induced the formation of resistant zygospores. An isolate of the fungus was obtained from a decaying mushroom collected in Kingston, Rhode Island. Stock cultures were maintained at 19 C on glucoseyeast-peptone (10:1:2 g/l) agar. To test the effect of water potential on reproduction, four osmotica were added to the standard GYP medium. Included were two mineral salts, KCl and NaCl, in concentrations of 0.1, 0.2, 0.4, 0.6 and 0.8 M per liter of medium, and two carbohydrates, sorbitol and sucrose, added in 0.2, 0.4, 0.8, 1.2, and 1.6 M concentrations. The water potential of media with each osmoticum ranged from -2 bars to -30 bars. If carbohydrate concentration alone were responsible for inducing sexual reproduction, then only media containing sucrose and sorbitol as osmotica would stimulate zygospore formation. On the other hand, if the regulating factor was water potential, then it could be expected that zygospores would be present in all the media of similar osmotic potential. GYP medium without added osmoticum was used as a control. Inoculum was prepared by flooding a wk-old colony of S. megalocarpus with sterile, distilled water. Spores were dislodged from sporangiophores with a flamed dissecting needle to give a dense spore suspension. Two drops (0.1 ml) of this spore suspension were placed in the center of each plate of agar containing the various concentrations of osmotica, as well as the control plates. The cultures were incubated at 19 C with 12 h of artificial light (21,500 lux) and 12 h of darkness. The cultures were checked daily for one wk and the results noted. Four replicates