An ultrastructural study of ascospore delimitation and maturation in Ascodesmis nigricana using freeze substitution fixation

Abstract

Fungi belonging to the class Ascomycetes are characterized by the production of sexually derived spores termed ascospores inside a microscopic, sac-like structure termed an ascus. In the Euascomycetidae, typically uninucleate spore initials are delimited within the ascus as the result of the invagination of membranes that arise from the so-called "ascus vesicle", a discontinuous cylinder of two closely spaced unit membranes that develops around the extreme periphery of the ascus. To date, there are conflicting data regarding the origin of the ascus vesicle. We therefore decided to address the problem using freeze substitution fixation. The advantages of this procedure over conventional chemical fixation protocols for preservation of ultrastructural details in fungi have been well-documented. In this study small pieces of dialysis membrane bearing developing ascocarps ofAscodesmis nigricanswere plunged into liquid propane and processed for TEM according to the procedures of Hoch and Mints et al.