Figure 1.

The major stages of the life cycle of P. anserina as illustrated by light microphotography, with a corresponding schematic representation shown above. (a) The cycle starts with the germination of an ascospore, after the transit in the digestive tract of an herbivore in the wild. (b) Then, a mycelium, which usually carries two different and sexually compatible nuclei (pseudo-homothallism), called mat+ and mat-, develops and invades the substratum. (c) On this mycelium, male (top; microconidia) and female (bottom; ascogonium) gametes of both mating types differentiate after three days. In the absence of fertilization, ascogonium can develop into protoperithecium by recruiting hyphae proliferating from nearby cells. (d) This structure, in which an envelope protects the ascogonial cell, awaits fertilization. (e,f) This occurs only between mat+ and mat- sexually compatible gametes (heterothallism) and triggers the development completed in four days of a complex fructification (e) or perithecium, in which the dikaryotic mat+/mat- fertilized ascogonium gives rise to dikaryotic ascogenous hyphae (f). (g) These eventually undergo meiosis and differentiate into ascii, mostly with four binucleate mat+/mat- ascospores (pseudo-homothallism), but sometime with three large binucleate ascospores and two smaller uninucleate ones (bottom asci is five-spored). Unlike those issued from large binucleate ascospores, mycelia issued from these smaller ascospores are self-sterile because their nuclei carry only one mating type. (h) When ripe, ascospores are expelled from perithecia and land on nearby vegetation awaiting ingestion by an herbivore. Scale bar: 10 μm in (a-d,f,h); 200 μm in (e,g).