Introduction to Fungi, Third Edition

328 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.7 The life cycle of Neurospora crassa, diagrammatic and not to scale (based on Fincham & Day,1971).The fungus is
heterothallic and a mature ascus (left of diagram) contains four haploidmultinucleate ascospores of mating type a and four of mating
type A. Ascospores germinate to form a branched, incompletely septate mycelium with multinucleate segments. Multinucleate
macroconidia and uninucleate microconidia develop, and both types of conidium can germinate to form a new mycelium.
Protoperithecia, consisting of a coiled ascogonium and a trichogyne, and surrounded by sterile hyphae, also develop on the haploid
mycelium (right of diagram).When microconidia or macroconidia of opposite mating type are transferred to a trichogyne,
plasmogamy (P) occurs and the protoperithecium develops into a perithecium. Ascogenous hyphae containing paired A and a nuclei
grow out from the ascogonium. At the tip of the ascogenous hypha a crozier develops (bottom of diagram) and, within the
penultimate cell, karyogamy (K) occurs between nuclei of the two mating types.Within this diploid cell, which is the ascus initial,
meiosis (M) takes place, followed by mitoses (not shown). Ascospores are cleaved around the nuclei (not shown) and the mature
asci discharge their ascospores through the neck of the perithecium.