Ultrastructure of the dimorphic ascospores in Mamillisphaeria dimorphospora

The ultrastructure of the dimorphic ascospores of Mamillisphaeria dimorphos pora are illustrated. The small brown ascospores differ from the larger hya line ascospores in having a thick melanin layer in the ascospore wall. They are also 2-3 times smaller and do not germinate on PDA. The significance o f these observations with respect to dispersal of aquatic fungi is discusse d. The large hyaline ascospores may be morphologically adapted to dispersal in streams, while the smaller brown ascospores are thought to be adapted t o aerial dispersal. The reasons for this are discussed. The method by which two spore types can be produced in separate asci in one ascoma is hypothes ised.