In this article we will discuss about the nature of asci and ascospores.
The members of the Ascomycetes are characterized by the development of asci and ascospores. In the large majority of them the asci are produced in an ascocarp. There are also Ascomycetes whose asci remain naked and are not enclosed in an ascocarp. In general, they remain intermingled with paraphyses forming a well-developed hymenium in the ascocarp.
The asci may also remain scattered at various levels within the ascocarp and may as well be loosely clustered even when they are enclosed in an ascocarp. They are extremely variable in shape being cylindrical to rectangular, globose, subglobose, or ovoid. The asci may be stalked or sessile (Figs. 196 to 198).
Usually the asci are single-chambered but in exceptional cases each one is divided into a many-chambered structure by the development of transverse septa. A mature ascus may be persistent or evanescent depending on whether the ascus wall is persistent or of short existence respectively.
A mature ascus with persistent wall is known as persistent ascus. But when the ascus wall dissolves away soon after the formation of ascospores releasing them in the ascocarp in a mass of mucilage formed by the gelatinization of the dissolved ascus wall, this type of ascus is known as evanescent ascus, as in Eurotium repens.
Nannfeldt (1932) suggested that the asci may be double-walled as well as single- called. But Ghadefaud (1942) said that all asci have two walls—an outer thick wall (ectoascus) and an inner thin wall (endoascus). But it was subsequently demonstrated that both the walls may be extremely variable in texture.
An ascus is designated as unitunicate when both the inner and outer walls are more or less rigid and do not separate during spore ejection (Fig. 196A and B). Again it is known as bitunicate when the inner wall of the ascus is elastic and expands beyond the thick outer wall at the spore liberation (Fig. 196G to F).
Unitunicate Asci are of several kinds:
(i) Operculate—when they open by a lid or operculum at the tip (Fig. 199B) which is pushed open when the ascospores are shot into the air, as in Ascobolus scatigeruis. Operculate asci may or may not be blued in iodine. They usually contain symmetrical ascospores which-are not septate and are usually-large and globose to broad-ellipsoid but often have roughened or warted walls.
(ii) Inoperculate -where there is no operculum and the ascospores escape through an apical pore or ascostome (Fig. 199D & E), as in Bulgaria inquinans. In Others there is a ring-like thickening on the inner surface of the ascus wall at the tip. This is visible as two minute highly refractive dots in optical section and sometimes also stains blue with iodine.
Inoperculate asci have ascospores that are slightly asymmetrical—the upper half of the ascospore is slightly broader than the lower half. The ascospores of inoperculate asci are rather small, narrow and often very long and slender or filiform. They may be septate but do not have warted or otherwise ornamented walls.
(iii) Suboperculate asci are the asci which possess thick apical ring and the apical pore is blocked usually with some substance, pore plug different in its staining properties from the rest of the wall. This pore plug stains blue with iodine, either as a whole or round its edges, as in Xylaria filiformis.
(iv) Bilabiate are the asci which dehisce by means of a slit across the tip (Fig. 199A).
(v) Indehiscent asci liberate their ascospores either by deliquescing or by becoming turgid and bursting.
Most unitunicate asci are accompanied by sterile hyphae, called paraphyses, which occur between them and apparently afford some protection to the developing asci, at least in species with hymenium wide open.
Bitunicate asci possess two distinct, separable walls—outer thick and rigid and inner thin and elastic. A bitunicate ascus is distinctly thick-walled toward the tip and very commonly a well-defined dimple in the inner wall can be seen at the tip of the ascus.
The apex is never coloured blue with Melzer’s chloral-hydrate iodine solution, i.e., it is non-amyloid—and it lacks an operculum or a plugged pore or peculiar ring- like thickenings which are common in unitunicate asci. A bitunicate ascus is usually very short-stalked with the stalk rather sharply delimited from the body of the ascus containing the spores.
There is a tendency for the spores in bitunicate asci to be multicellular and coloured (Fig. 196C to F). Bitunicate asci usually occur in a hymenium within closed ascocarps superficially resembling perithecia but without true paraphyses. Instead the hyphae between the asci are commonly attached to the ascocarp wall at top and bottom and are often branched or form a network among the asci.
These interascal threads are called pseudoparaphyses or paraphysoids and the ascocarp is a pseudothecium. Commonly encountered in Pleospora herbarum.
In general, a mature ascus contains eight ascospores (Fig. 197A to C). But the number of ascospores may be less than eight or may be as many as sixteen, thirty-two, or even more (Fig. 198). Similarly the ascospores are extremely variable in size, shape, colour, nature of wall surface, and other characters.
Ordinarily they are unicellular uninucleate (Fig. 197D to F). Single-celled binucleate ascospores are also not uncommon. There are also cases where the ascospores are two to many- celled (Fig. 198).
The ascospores may be colourless to extremely variable in colour. The spore wall may be smooth or variously sculptured (Figs. 197 & 198). The ascospores may be of any shape from globose to narrow thread-like (Fig. 198). They may be arranged irregularly within the ascus, in a single row—uniseriate or a double row— biseriate, or parallel with one another if they are long and narrow.
Each ascospore may have one or more oil globules—guttules.