These were my mycology notes during my masters. Credit goes to Dr. Paul Cannon Of CABI Bioscience who taught me and provided notes on Ascomycotina.

Ascomycotina

Introduction

The Ascomycotina is the largest subdivision of the fungi, and if one includes their anamorphs it contains perhaps three quarters of all known true fungi (Hawksworth et al., 1983). As the number of all fungi has been variously estimated at anywhere between 100000 and 1600000 (Hawksworth, 1991a), it is clear that despite its image even in most informed non-specialists’ eyes as hardly meriting attention, the group is of the first importance.

As might be expected in such a monumental group, there is enormous diversity of structure and ecology. At one end of the scale are most of the unicellular organisms commonly known as the yeasts, and at the other, species with extensive mycelium and large and elaborate fruiting structures, such as the truffles and morels. The group is of enormous importance to plant pathology, and representatives may be found in almost every imaginable ecological niche. When one considers that most of the Fungi Imperfect! (hyphomycetes and coelomycetes) are either biologically linked to ascomycetes or are clearly related to them, the importance of the group becomes even more clear.

In the Ascomycotina, meiotic spores (ascospores) develop inside special cells, the asci. These either rupture or break down to release the ascospores in order that they may be dispersed. The structure of the asci are particularly important in classification. A peculiar feature of the meiotic process in ascomycetes results in the fact that in most species the number of ascospores produced within asci is strictly controlled, and in the vast majority of cases this number is eight. The asci are commonly accompanied by sterile hyphae, broadly termed paraphyses. Most members of the Ascomycotina have asci which are protected by a globose, flask-shaped or discoid conglomeration of cells, the ascoma. Where the asci are surrounded by wall material, ascospores may be released through a small opening in the ascoma wall, the ostiole, or by its general breakdown. A discoid fruit body is termed an apothecium, a flask-shaped one a perithecium and a globose, closed one a cleistothecium. These terms are losing prominence as intermediates are identified and classifications rely upon them less as basic divisions. The protective covering may be rudimentary, composed only of a web of hyphae, or may be very well-developed, with multiple and varied layers of thin-or thick-walled cells. These are often dark brown or black due to the deposition of melanin, which is probably formed primarily to protect the fungus against ultra violet radiation. The ascomata (plural of “ascoma”) may be formed into compound fruit bodies, stromata.

In many cases, ascomycetes are readily identified as such by using simple squash mounts of cultured material. In some groups of the Ascomycotina, it is not always easy to perform this basic level of identification as the asci deliquesce (the walls break down irregularly or apparently simply disappear) to release the ascospores. Particularly when examining old cultures of such fungi as Chaetomium and Microascus, asci may be hard to detect. In these cases, though, ascospores are commonly seen in clusters of eight even though the wall surrounding them is no longer present.