Orbilia ultrastructure, character evolution and phylogeny of Pezizomycotina.

Molecular phylogenetic analyses indicate that the monophyletic classes Orbiliomycetes and Pezizomycetes are among the earliest diverging branches of Pezizomycotina, the largest subphylum of the Ascomycota. Although Orbiliomycetes is resolved as the most basal lineage in some analyses, molecular support for the node resolving the relationships between the two classes is low and topologies are unstable. We provide ultrastructural evidence to inform the placement of Orbiliomycetes by studying an Orbilia, a member of the only order (Orbiliales) of the class. The truncate ascus apex in the Orbilia is thin-walled except at the margin, and an irregular wall rupture of the apex permits ascospore discharge. Ascus, ascogenous and non-ascogenous hyphae were simple septate, with septal pores plugged by unelaborated electron-dense, non-membranous occlusions. Globose Woronin bodies were located on both sides of the septum. Nuclear division was characterized by the retention of an intact nuclear envelope, and a two-layered disk-shaped spindle pole body. The less differentiated nature of the spore discharge apparatus and septal pore organization supports an earliest diverging position of Orbiliomycetes within the subphylum, while the closed nuclear division and disk-shaped spindle pole body are interpreted as ancestral state characters for Ascomycota.