Ascospore ontogeny and discharge in megalosporous Trypetheliaceae and Graphidaceae (Ascomycota: Dothideomycetes and Lecanoromycetes) suggest phylogenetic relationships and ecological constraints

We studied ascospore dispersal and ontogeny in several species of Trypetheliaceae and one species of Graphidaceae with megalosporous (very large) ascospores, including Aptrootia terricola (Aptroot) Lucking, Umana & Chaves, Architrypethelium nitens (Fee) Aptroot, A. seminudum (Mont.) Aptroot, Astrothelium diplocarpoides Mull. Arg., Laurera gigantospora (Mull. Arg.) Zahlbr., and Ocellularia subpraestans Hale. Although mature ascospores are very different among species of Trypetheliaceae, all species studied share plesiomorphic ontogenetic features such as the formation of a single central euseptum and subsequent development of a variable number of transverse distosepta with diamond-shaped lumina (astrothelioid ascospore type). Even the dark brown, richly muriform ascospores of Aptrootia terricola go through an astrothelioid stage. This suggests that ascospore types may be more closely related than suggested by their mature morphology, and could explain why related taxa can develop markedly different ascospore types. We discuss the implications for systematic classification of Trypetheliaceae in the light of recent molecular studies, and also speculate about the ecological importance of large ascospores, especially with muriform septation.