Ascomycetes: Origin and Formation | Fungi | Botany

In this article we will discuss about:- 1. Origin of Class Ascomycetes 2. Formation of Ascus and Ascospores 3. Classification.

Origin of Class Ascomycetes:

The Class Ascomycetes, commonly known as sac fungi include such forms as the ‘brewer’s yeasts’ (Saccharomyces), the black mould (Aspergillus niger), the blue moulds of oranges (Penicillium ex­pansum) , the powdery mildews of cultivated crops (Erysiphe), the cup fungi (Peziza, Ascobolus), the common edible morel (Morchella), the ergot (Claviceps), and other fungi of considerable economic importance. A great number of them are parasites on higher plants but most of the species are saprophytes.

The Ascomycetes are characterized by the production of spe­cialized asexual reproductive cells, known as ascospores, produced (usually eight in number) within a mother cell, called an ascus (spore sac), which are typically club-shaped or cylindrical, but may also be globose or pyriform in shape. The asci are often arranged side by side in a definite layer forming a hymenium, but sometimes they occur singly or are arranged irregularly, so that no definite hymenium is formed.

Intermingled with the asci of a well-developed hymenium there also occur sterile haploid branches, usually slightly exceeding the asci in length, known as paraphyses. An ascus deve­lops as a dikaryon cell in which the nuclei fuse and then divide reductionally into eight nuclei, which with some adjacent cytoplasm, are cut off by walls (free cell formation) forming ascospores.

Among the simpler Ascomycetes, as the yeasts, the entire vegetative body (a cell) is directly transformed into an ascus. In addition to ascos­pores, multiplication may also take place by means of conidia and chlamydospores.

In most Ascomycetes the asci usually arise in groups and re­main protected by a common wall of hyphal tissue, known as the peridium, and a definite structure, thus formed, is a fructification, called the ascocarp. When young, the ascocarp is somewhat glo­bose in form and remains completely closed.

At maturity, it may become flask-shaped in outline opening only by a terminal pore, called the ostiole; such fructification is called a perithecium. In some cases, it opens out to form a cup-shaped or saucer-shaped structure, in the concavity of which the asci are arranged in a hymenium.

This type of fructification is known as an apothecium. The other two forms are termed as the perithecium. On the other hand, a mature ascocarp, if it remains closed through­out, is termed a cleistothecium.

Depending on the mode of development of the apothecium, Corner (1929) recognized three types as follows:

(a) Gymnocarpic:

The hymenium is not enclosed by any hyphal covering from the start of its development.

(b) Angiocarpic:

From the very start of development the hy­menium layer of the apothecium becomes invested by a hyphal covering.

(c) Hemiangiocarpic:

At first the hymenium begins its deve­lopment, as in the angiocarpic type, but finally it becomes free by a rupturing of the hyphal covering at the apical region.

Further, those apothecia, in which the asci open by definite apical pores, are known as the operculate apothecia, while those in which the asci have no such regular pores but get irregularly rup­tured at their tips, are termed as the inoperculate apothecia.

The vegetative body of Ascomycetes, with the exception of uni­cellular yeasts, consists of a mycelium which is much-branched and septate, and contains uninucleate or multinucleate cells. The mycelium usually develops within the substratum, but in a few cases part of it may be superficial. In saprophytic forms it is quite extensive, but in many parasitic forms the mycelium is localized at the region of infection.

The parasitic mycelium may be entirely intracellular, or it lives in the intercellular spaces of the host tissue and frequently develops special hyphal branches, which penetrate the living cells and function as absorbing organs, called haustoria.

The problem of sexual reproduction in Ascomycetes is very variable and has received considerable attention during recent years, in general, it may be summarized that a gradual decline in sexuality can be traced in the group and the range includes from a well-developed oogamy, in which both antheridia and oogonia are present and functional, to complete suppression of sex organs, although perhaps there always remains a fusion of nuclei at a certain stage in the life cycle, representing the essentials of sexual reproduction.

In between these two extremes, there are interme­diate forms in which the oogonia are always present but the anthe­ridia are either absent or when present entirely functionless. The sex organs may be uninucleate or multinucleate.

The Ascomycetes may also be homothallic or heterothallic. In the former case, two mycelia are necessary for sexual union and the formation of ascus and ascospores, while in the latter, only single mycelium is required for the same purpose. In the majority of homothallic species, all the ascospore nuclei of a single ascus are genetically the same, so far as the sex factors are concerned; this phenomenon is referred to as the primary homothallism.

On the other hand, the ascospores may be genetically dissimilar with res­pect to the sex factors. Still the mycelia produced from these as­cospores will be homothallic, and such a phenomenon is termed as the secondary homothallism.

Heterothallism also may be of two types:

1. Morphological and

2. Physiological.

In the morphologically heterothallic species, the mycelia belong to two different catego­ries:

(a) The male mycelium producing antheridia or spermatia, and

(b) The female one bearing the ascogonia only.

A physiologically heterothallic mycelium, however, gives rise to both the male and female elements, but itself suffers from self-sterility. It can, never­theless, unite sexually with some other compatible mycelia belong­ing to the same species.

Formation of Ascus and Ascospores:

In the lower Ascomycetes (e.g. Eremascus, Dipodascus Saccharomyces etc.) the ascus is developed either directly from a zygote or parthenogenetically from a gametangium. Usually the two nu­clei of opposite sexes fuse inside the young zygote, which is now regarded as the developing ascus. The ascus then enlarges consider­ably, and the fusion nucleus undergoes reduction division forming usually 4—8 haploid daughter nuclei. From these nuclei, ultimately 4—8 ascospores are produced.

In the higher Ascomycetes (e.g, Pyronema, Peziza, Ascobolus, etc.), however, the ascogonium never develops directly into an ascus after fertilization. From the ascogonium, on the contrary, one to several unbranched or profusely branched, aseptate or septate, filliform ascogenous hyphae are produced.

In an ascogenous hypha either all the cells or only a few cells at its apical portion are binu­cleate the two nuclei being of different sexualities. Usually the terminal cell (of the ascogenous hypha) becomes curved like a hook and is often termed as a crozier.

The two nuclei in the crozier divide conjugately, and the daughter nuclei get themselves oriented in such a fashion, that the arch of the crozier receives one pair of them (of opposite sexes), the extreme tip of the crozier having only one, while the other solitary daughter nucleus is situated in the region below the crozier arch.

Following this, two partition walls are laid down resulting in the formation of three cells. Of these the ultimate cell is uninucleate, the penultimate one is bi- nucleate, and the antepenultimate one is also uninucleate. From the binucleate cell, finally an ascus is developed. The two uninu­cleate ultimate and antepenultimate cells generally unite together to form another binucleate cell. This type of ascus development is commonly found in Pyronema confluens.

The binucleate cell developing into an ascus becomes greatly enlarged and somewhat clavate. Meanwhile, the two nuclei fuse together forming a diploid nucleus. This diploid nucleus then un­dergoes a complete meiotic division followed by a mitotic one, resulting in the formation of eight haploid nuclei. Nuclear division as a rule, stops at this stage, but cases of formation of 32—1,024 nuclei have also been recorded.

Around each of these eight nuclei, some amount of cytoplasm collects, and finally eight ascospores are delimited. The remaining cytoplasm, which is left inside the ascus, after the formation of ascospores, is known as epiplasm.

It was considered previously that the gamete nuclei fused in pairs inside the ascogonium, and consequently, each of the two nuclei in the cells of an ascogenous hypha was diploid. So, when two such nuclei fused in the developing ascus, a tetraploid nucleus was formed.

Subsequently, this tetraploid nucleus had to undergo a double reduction in the number of chromosomes in order to give rise to the haploid ascospore nuclei. This phenomenon was known as brachymeiosis, and was regarded as a unique feature of the ascomycetous fungi. Recent investigators, however, do not support this idea. They suggest that the gamete nuclei do not fuse, but lie in pairs in very proximity.

In this paired condition, the nuclei migrate into the ascogenous hyphae, and the fusion nucleus in the ascus is, therefore, only diploid, and not tetraploid. Consequently, there can be only one reduction in the number of chromosomes.

Besides the normal crozier or hook type, other types of ascus development are also found in the higher Ascomycetes. A brief account of these is given below:

In Geopyxis catinus (Peziza catinus), the tip cell of an ascoge­nous hypha is uninucleate just like the ultimate cell of the crozier in Pyronema confluens, while the cell behind it is binucleate. This binucleate subterminal cell finally pushes out in a lateral direction and grows out into an ascus.

The next type of ascus development, called the Plicaria (Galactinia) type, is exemplified by Plicaria succosa (Galactinia succosa) and Acetabula leucomelas (Peziza leucomelas). In this case, a num­ber of cells towards the tip of an ascogenous hypha possess a dikaryon each. The two nuclei in the terminal cell fuse and this cell ultimately develops into an ascus.

In Pustularia vesiculosa (Peziza vesiculosa) the terminal cell of an ascogenous hypha may form a crozier as in Pyronema confluens. The binucleate penultimate cell, however, grows out into another hypha with several binucleate cells, the terminal one of which forms an ascus following the Plicaria type of formation.

A fourth type of ascus formation, termed the rectascous type by Dangeard, has been found in the lower plectascales. In this case, the cell is binucleate, as in the Plicaria type. Here, however, a number of asci are formed in a chain lying one behind the other; but this has not been properly examined from cytological stand­point.

In the last type, which can be referred to as the Laboulbenia type, true ascogenous hyphae are lacking entirely. Instead, after fertilization, a cell complex appears from the ascogonium, and these cells give rise to asci.

Classification of Class Ascomycetes:

The class Ascomycetes has been divided into two subclasses:

Subclass I. Protoascomycetes or Hemiascomycetes:

Charac­terized by the formation of naked asci directly from the zygote, either singly or in loose clusters; ascogenous hyphae and ascocarp absent.

Subclass II. Euascomycetes:

Characterized by the formation of asci from the ascogonium produced by the ascogenous hyphae, in dense clusters within the ascocarps.

Euascomycetes has been sub-divided into three series:

Series A. Plectomycetes:

Characterized in having a closed ascocrap without any definite opening, the cleistothecium, within which the asci either arise from the floor and stand parallel to one another, or they are irregularly arranged; in some cases a definite fructification is wanting.

It includes three orders:

Order 1. Plectascales:

Simple types without apothecium or perithecium asci are produced either angle, one from an ascogonium and without any covering, or a number of asci irregularly arranged within a protective covering of thick-walled hyphae forming the cleistothecium. Common genera are Saccharomyces, Eurotium (As­pergillus), Penicillium, etc.

Order 2. Erysiphales:

Superficially growing parasites; aerial mycelia colourless or dark-coloured; mostly having a single layer of cylindrical asci, arranged parallel to one another within a cleistothecium with prominent appendages. Common genera are Erysiphae, Sperotheca, etc.

Order 3: Exoascales.

Series B. Discomycetes:

Characterized in having a fruit body, an apothecium, which becomes more or less cup-shaped at matu­rity; the asci intermingled with paraphyses stand in parallel series forming a hymenium which occupies the concavity of the cup.

It includes five orders:

Order 1. Pezizales:

Saprophytes with well-developed mycelia; ascocarp is usually fleshy, but sometimes leathery apothecium (usually cup-shaped); usually bounded by a peridium which is closed at first, but later on pushed open exposing the hymenium. Ascus contain usually 8 (sometimes more or less) oospores and opening by the shedding of an operculum. Common genera are Peziza, Ascobolus, Pyronema, Humaria, etc.

Order 2. Helvellales:

Saprophytes, ascocarp mostly large, fleshy and stalked; and is often convoluted; fertile region distinct from the sterile stalk; spreads over the upper surface; ascus opening by an operculum. Common genera are Helvella, Morchella, etc.

Order 3: Tuberales.

Order 4: Phasidiales.

Order 5: Hysteriales.

Series C. Pyrenomycetes:

Characterized in haying a flask- shaped fructification, the perithecium, opening at the top by an ostiole, and containing within it asci in parallel series.

It includes four orders:

Order 1. Hypocreales:

Parasites or saprophytes; mycelium usually forms a stroma (parenchymatous mass) in which the perithecia are partially or entirely embedded; the perithecial-wall and stroma, when present, are distinct and soft in texture, colourless or light coloured; asci are formed within the perithecium; peridium absent. Common genera are Claviceps, Nectria, etc.

Order 2. Sphaeriales.

Order 3. Dothidiales.

Order 4. Laboulbeniales.