Fungi Ascomycota - 2

The Ascomycota - 2

Fungi ├─Chytridomycota └─┬─Zygomycota └─┬─Basidiomycota └─ASCOMYCOTA ├─Taphrinomycotina └─┬─Saccharomycotina └─Pezizomycotina



And a thousand thousand slimy things
Lived on; and so did I.
    -- Samuel Taylor Coleridge
        The Rime of the Ancient Mariner

The Pezizomycotina are the euascomycetes of traditional mycology. The older name is perhaps more descriptive. The other groups of ascomycetes are special cases. The great majority of ascomycetes fall into the Pezizomycotina. Nonetheless, we're not going to say too much about them, since they will have their own page soon enough.

The euascomycetes have traditionally been characterized by the growth of an ascocarp within which the asci develop. The problem here is that Neolecta, a taphrinomycotine very basal acomycete) also has an ascocarp. See image on previous page. This structure is believed to be homologous to the ascocarps of Pezizomycotina. An even more disturbing problem is that euascomycetes have traditionally been classified by reference to details of ascus formation. As usual, the classical phylogenies are not all that far off the mark. Unless we are to adopt the newspeak of the Moleculariat and ignore everything that isn't a DNA sequence, we're going to have to tackle these details. This job involves a good bit of particularly weird and obscure terminology. While the prospect of explaining this stuff fills us with a deep sense of foreboding, we may as well get it over with.

Most of the necessary information can be excavated from the figure, which we have borrowed from Liu & Hall (2004). Their notions of ascomycete evolution are a bit heterodox, but phylogeny is not our concern here. Let us take a walk through this field of fertile fungi simply to see what there is by way of anatomy.

Taphrina we have already encountered amongst the Taphrinomycotina. Taphrina has no ascocarp at all. The asci simply grow out of the normal somatic tissue of the fungus. This is, presumably, the primitive condition. It is simple, but inefficient. The asci have no protection during development. Since the asci are naked, their walls need to be relatively thick, which is inconvenient when it comes time to release the spores. Further, there is no convenient way to send the ascospores on their way into the world.

Aspergillus is presumed by many to represent the primitive condition among pezizomycotes. It is said to have a cleistothecial ascoma (or cleistothecium), because the ascii are completely enclosed. The cleistothecium is formed by fusion of vegetative hyphae. In Aspergillus, the fused hyphae harden into a hard, red pigmented shell at maturity. This is an example of a peridium, a rather vague term that includes any shell or membrane that encloses spores -- for example, the visible outer membrane of a puffball. Hülle cells may be associated with the peridium. These thick-walled, globular cells develop by budding from the tips of specialized hyphae. Hülle cells can envelope the entire developing cleistothecium and may serve as nurse cells. Wu & Miller (1997).

Since the asci are completely enclosed, they are well protected, but the spores can only be released by the decay or mechanical breach of the peridium. As we might expect, the walls of the asci are thin and fragile, designed to fall apart spontaneously, or at the slightest mechanical stress. Such walls are said to be prototunicate.

Peziza has an open or apothecial ascocarp. This makes it much easier to release the spores, but leaves the asci with much less protection during maturation than in cleistothecial forms. In Peziza, the individual asci are thus stronger and unitunicate, meaning that they are formed with a single, relatively stout membrane. In order for the spores to be released, some different mechanism is required. Consequently, Peziza and other forms have evolved asci which are operculate, meaning that they have a sort of lid on the end. As the spores grow, fluid pressure builds up in the ascus until the operculum bursts open and the ascospores are blown out into the environment.

Neurospora develops a perithecial ascocarp. In other words, the ascocarp is closed but for an ostiole pore) at the distal end. The ostiole is blocked by  elongate sterile cells, known generically as paraphyses. "Ingold 1965) studied ascospore discharge in Sordaria. He found that the perithecial necks are positively phototropic. As the asci mature they swell and fill the upper part of the perithecium. One of the asci stretches and pushes through the ostiolar opening while its base remains attached to the perithecial wall. As the ascus tip protrudes, it discharges all of its spores explosively, collapses, and disintegrates, to be followed by each of the other asci in succession. This method of ascospore discharge is probably not confined to Sordaria, but may be the pattern of other members of this family."  BOT 461/561: Lecture#17. Since the perithecium grows out into the medium from the hymenium after nuclear pairing, this is ascohymenialdevelopment. As in other ascohymenial forms, the asci produced are unitunicate.

Pleospora, by contrast, exhibits ascolocular development. Here, the asci develop in a locule, a small hollow in the generalized reproductive tissue of the hymenium a/k/a stroma or ascostroma) [1]. As this is not typical perithecial development, the structure is referred to as a pseudothecium. By contrast to ascohymenial development, only the tip of the otiole is exposed. In Pleospora, the asci develop last -- in contrast to ascohymenial development, in which everything grows up around the developing asci. In Pleospora, the asci develop in a matrix of pseudoparaphyses, long, hair-like cells which grow down from the roof of the locule and often attach to its base. The asci in Pleospora are bitunicate.

And that, we are happy to report, is all the jargon we feel moved to impart on this subject, and doubtless more than the reader wished to absorb.

He went like one that hath been stunned,
And is of sense forlorn :
A sadder and a wiser man,
He rose the morrow morn.

Links: Ascomycetes and anamorphs; BOT 461/561: Lecture#17

[1]  On the difference between the stroma and hymenium: "It may seem obnoxious of me to dredge up this obsolete controversy when no one really cares about any more; but you may run across it in the older literature, and I believe that it is the source of the similar, stupid, and continuing controversy over names for hymenial cells." Illinois Mycological Association

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