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In this article we will discuss about:- 1. Introduction to Ascomycetes 2. Occurrence and Economic Importance of Ascomycetes 3. Somatic Structures 4. Phylogeny.
Introduction to Ascomycetes:
The Ascomycetes and the Basidiomycetes are commonly known as higher fungi. They include fungi which possess much complexity of structure. The Ascomycetes, or sac fungi, as they are commonly known, are characterized by the development of spores, the ascospores in a sac-like structure called ascus.
The ascus is distinguished from the sporangium of the Phycomycetes in the origin and method of formation of spores. A more obvious difference is that the sporangium contains an indefinite number of spores and that too produced without any sexual act.
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Whilst in the Ascomycetes, the ascus invariably contains 8 spores produced by karyogamy and meiosis. In a few exceptional cases the number is a higher multiple of 8. In a few primitive members of the Ascomycetes the number of spores in an ascus is less than 8. The Ascomycetes constitute the largest of the three classes of sexually reproducing fungi.
As may be expected in such a large group there is considerable diversity in form and structure associated with their wide range of distribution and mode of nutrition. At one end are the unicellular organisms commonly known as yeasts, and, at the other, members having their bodies composed of branched hyphae which are always septate and they produce large and elaborate fruiting structures known as ascocarps.
Occurrence and Economic Importance of Ascomycetes:
The fungi included in this group occur throughout all seasons of the year. Some are saprophytes (growing largely oh decayed wood—lignicolous or on soil—terricolous) and others parasites. Again there are others which are coprophilous. A few are hypogean, growing under soil level.
Their ascocarps are underground, they are closed, and their spores remain imprisoned until some animal digs them up for food and scatters the ascospores while devouring the ascocarps. The Ascomycetes have the highest economic importance among fungi.
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A large number of Ascomycetes are utilized profitably for the welfare of the society in the brewery, bakery, cheese-making, organic acid manufacturing industries, preparation of medicine, etc. The parasitic Ascomycetes induce various diseases of plants and animals.
Some of the plant pathogenic Ascomycetes are virulent enough to wipe off cultivation of plants of economic importance in the Western countries. Whereas, the Ascomycetes inducing diseases of animals including human beings are also quite well-known.
Again some of the edible Ascomycetes form delicacy of dishes particularly in the Western countries. But the Ascomycetes causing destruction of food grains in storage, clothes, leather, and similar other essential articles are also not uncommon.
Somatic Structures of Ascomycetes:
In the Ascomycetes the somatic body is generally composed of septate hyphae having a tendency to develop into an aggregate mass of mycelium (Fig. 187) with a few exceptions where the vegetative body is unicellular uninucleate. In others, a series of cells remain adhering end to end to form a chain, is known as pseudomycelxum.
Again, some other Ascomycetes are unicellular under certain conditions, but mycelial under others. Cells of the hyphae are usually uninucleate. But there are also members whose hyphae possess multinucleate cells.
In some Ascomycetes the hyphae become compactly aggregated to form a sclerotium, in which stage they tide over unfavourable climatic conditions. A short branch of one or two cells known as hyphopodium (pl. hyphopodia) may be developed on epiphytic mycelium. Hyphopodia may be mucronate (Fig. 187B) with pointed end or capitate (Fig. 187G) having end rounded. They are of great taxonomic importance.
Phylogeny of Ascomycetes:
There are two widely divergent lines of thought regarding the phylogeny of the Ascomycetes. The one proposed by Sachs in 1875 suggests Floridean ancestry of the Ascomycetes.
Sachs pointed out similarities between the Ascomycetes and the red algae in the following aspects:
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1. Spermatia produced in both the groups perform identical function.
2. The development of receptive organ called trichogyne by the female reproductive structure.
3. Elaborate sexual process in both the groups.
4. Ascocarp and cystocarp are identical organs.
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5. Continuity of cell to cell by cytoplasmic connection in the thallus.
6. Growth in length of the somatic body of both the groups by the elongation and division of terminal cells.
This line of thought failed to consider the physiology of the two groups which is so different to suggest that one cannot evolve from the other. The resemblances are due to parallel development.
The other line of evolution proposed by De Bary in 1887, indicated the possibility of the derivation of the Ascomycetes from the Phycomycetes.
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The arguments in favour of his hypothesis are:
1. Asci developed directly from the zygote.
2. Evolution of conidium from sporangium.
These evidences are not strong enough to eliminate the possibility of a separate flagellate ancestry of the Ascomycetes.
