Studies of morphogenesis in Dipodascopsis uninucleata (Batra and Millner)

Abstract

In this study of morphogenesis in D. uninucleata, growth was followed on semi-solid and liquid media. The ellipsoid ascospores germinated to form a branched septate mycelium, which is fragmented in shake culture. Sexual reproduction began when the sprout had 4-5 cells. The first ascus was initiated by the formation of isogametxvia between the sprout mother cell and the distal adjacent cell, these fused at their tips and formed an ascus. The ascus grew in size and formed multispored uninucleate ascospores on maturation. The pattern of ascus insertion in the sprout was predictable. Each ascus at initiation was 2 cells away from the adjacent proximal ascus and 3--5 cells behind the distal hyphal tip. Ascus insertion was governed by the metabolic level in the cells. The external nutrient gradient was not involved as the pattern was the same in the batch culture and in cultures on semi-solid media. The ascus unit is supported by 2 cells. The proximal poles of the sprout cells are more active in forming gametangia than the distal poles. 40-60% of the attempts to form asci fail due to only single gametangia being produced or two gametangia failing to fuse. A thick capsule was formed during the swelling phase of ascospore germination. It consists of microfibrils embedded in an amorphous matrix. In a search for morphological mutants no capsuleless mutant was found after treatment with UV or NTG. One sterile mutant was isolated with larger cells and larger nuclei than the wild type and was possibly a polyploid. Attempts to improve the synchrony of germinating ascospores and growing cells using hydroxyurea were unsuccessful. Using different carbon sources and different levels, vegetative growth and sexual reproduction were equally affected, irrespective of the rate of growth. Sprouts reproduced normally on reaching the 4-5 cell stage. Inhibitors of nucleic acid, protein synthesis, TGA cycle and metal chelating agents were added to the cultures at different stages of growth and showed that inhibition of growth could be achieved but at concentrations much higher than was shown in other fungi. If vegetative growth was inhibited ascus formation was equally inhibited. Exogenous cAMP increased the percentage of successful attempts to produce asci and decreased the time required for the culture to reach maturity. High temperature of 35°C increased the percentage of gametangial fusion failure and had no effect on other steps of ascus formation or the vegetative growth.