Pycnidial Development from Ascospores of Fenestella princeps

Formation of pycnidia directly from ascospores is described in Fenestella princeps. Ascospore germination began with swelling of ascospore cells and production of numerous germ tubes. Following continued ascospore enlargement, internal cells underwent repeated divisions. Ascospore interiors became filled with numerous small, thin-walled cells surrounded by a peridial wall composed of larger, dark-walled cells. Eventually an irregular cavity lined with phialides formed within each developing conidioma, and masses of conidia were extruded from the papillate pycnidial ostiole. The ability of ascospores of this fungus to form pycnidia directly can be interpreted as an example of heterochronic evolution in fungi. Evolutionary implications of these observations are discussed.