ASCOMYCETES: CHARACTERISTICS, NUTRITION, HABITAT, REPRODUCTION - BIOLOGY - 2024

The ascomycetes or fungi Ascomycota are forming the phyla Ascomycota within the sub-kingdom dikarya. It contains approximately 33,000 species distributed in various habitats throughout the planet.

The main characteristic of ascomycetes is the presence of ascospores (sexual spores) enclosed in small sacs called asci. They can be unicellular (yeast) or multicellular, forming a body (mycelium) composed of filamentous structures (hyphae).

Diversity of Ascomycetes

The hyphae are septate and present small cellular organelles called Worenin bodies. The set of hyphae generates a pseudo-tissue called plectrenchyma.

General characteristics

Reproduction

The reproduction of ascomycetes can be asexual or sexual. Generally, the asexual state (anamorph) predominates over the sexual state (telomorph).

Asexual reproduction can be by chlamydospore formation, fission, budding, fragmentation or production of conidia. In sexual reproduction, the processes of fusion of cytoplasms (plasmogamy), fusion of nuclei (karyogamy) and meiosis occur for the formation of ascospores.

Nutrition

These fungi are heterotrophic, with saprophytic, parasitic, symbiont, and carnivorous species. Saprophytes can degrade almost any carbon substrate.

Symbionts form associations with algae (lichens), with the leaves and roots of plants (endophytes) or with their roots (mycorrhizae) as well as with various arthropods.

Diseases

Parasitic species are abundant and are responsible for various diseases in plants such as wilt produced by species of the genus Fusarium.

They can also cause diseases in humans such as pneumonia (Pneumocystis carinii) or candidiasis (Candida albicans). The carnivore group is restricted to the order Orbiliomycetes, generally capturing nematodes.

Subphiles

The monophyly of ascomycetes has been proven in phylogenetic studies, being a sister group to the Basidiomycetes. It has traditionally been subdivided into three sub-phyla: Taphrinomycotina, Saccharomycotina and Pezizomycotina that are differentiated by the arrangement of their cells and sexual structures.

Taphrinomycotina appears to be paraphyletic, while the other groups are monophyletic. Pezizomycotina contains the largest number of species, divided into thirteen classes and fifty-four orders.

In Saccharomycotina there are most of the yeast species such as Saccharomyces cerevisiae, used in the fermentation of bread and beer among others.

Morphology

Ascomycetes can be unicellular or multicellular. They present a cell wall composed of glucans and chitin. In the cell wall of yeast (unicellular species) there is a greater amount of glucans.

Multicellular species are composed of filamentous structures formed by several cells, known as hyphae, which together constitute the vegetative body of the fungus (mycelium).

Yeasts can form short filaments when new cells are generated, called psedomicels. Some species present both forms of growth (dimorphic).

In ascomycetes the hyphae are septate, presenting a pore between the septa through which the cytoplasm can move from one cell to another and, sometimes, to the nuclei. Worenin bodies are double membrane microbodies, which are located near the pore and are believed to help prevent cytoplasm movement between cells.

Hyphae can intertwine to form a tissue-like structure called plectrenchyma, which is known as prosenchyma when one hypha can be differentiated from another, and pseudoparenchyma when they cannot be individualized.

The characteristic shared by all ascomycetes is the presence of ascospores (sex spores) that form from specialized structures called asci.

Phylogeny and taxonomy

The Ascomycetes constitute a monophyletic group that is brother of the Basidiomycetes, forming the Dikarya sub-kingdom. This phyllum has traditionally been divided into three sub-phyllum: Taphrinomycotina, Saccharomycotina, and Pezizomycotina.

Taphrinomycotina is considered paraphyletic and has been divided into five classes, including yeast, multicellular, and dimorphic species.

Most yeasts are found in Saccharomycotina. They do not have much morphological diversity, although in some cases pseudomycelia are formed.

Pezizomycotina is the largest group with thirteen classes and includes saprophytic, symbiont, parasitic, and carnivorous species. The morphology of the reproductive structures is highly variable and various groups of Pezizomycotina are recognized by the type of disgust.

Nutrition

Ascomycetes are heterotrophs and obtain their food in different ways from both living and dead organisms.

Saprophytic species

Saprophytic species decompose various compounds as carbon sources, such as wood, the body of some arthropods, and some species are capable of decomposing even fuel or wall paint.

Yeasts

In the case of yeasts, they have the ability to carry out alcoholic fermentation, which has given rise to different products for human consumption such as bread, beer or wine, among others.

Symbiote groups

Symbiont groups are related to other organisms and form different associations. Lichens are associations of algae or cyanobacteria with various species of ascomycetes.

In this association, the fungi obtain their food from the photosynthetic process of the algae and provides the same protection against desiccation and a greater water absorption capacity.

Mycorrhizae

Mycorrhizae are associations of various groups of fungi, including various species of ascomycetes, with the roots of plants. The hyphae of the fungus spread in the soil and absorb water and minerals that are used by the plant, while it provides sugars produced by photosynthesis.

Within the mycorrhizal groups, the species of the genus Tuber that make up truffles stand out due to their economic value, highly appreciated for the flavor and aroma they provide to food.

Endophytic ascomycetes

Endophytic ascomycetes are those that during their life cycle develop in living plant tissues. These fungi apparently provide the plant protection against herbivory and pathogen attack.

Ants of the genus Apterostigma have a symbiotic relationship with fungi of the genus Phialophora (black yeasts) that develop on the thorax of the ant.

Parasite groups

Parasitic groups within ascomycetes are abundant. These are the cause of various diseases in plants and animals.

In animals, Candida albicans, which causes candidiasis, Pneumocystis carinii, the causative agent of pneumonia, and Trichophyton rubrum, responsible for athlete's foot, stand out. In plants, Fusarium oxysporum causes wilting and necrosis in various crops, causing significant economic losses.

The Orbiliomycetes order is made up of species considered carnivorous, which present adhesive traps that capture nematodes that later degrade to obtain the nutrients contained in their body.

Habitat

Ascomycetes are cosmopolitan and can be found growing in various habitats. They can develop in both freshwater and marine aquatic environments, mainly as parasites of algae or corals.

In terrestrial environments they can be distributed from temperate to tropical zones, being able to be present in extreme environments.

For example, Coccidioides immitis grows in desert areas of Mexico and the United States and is the cause of a lung disease known as San Joaquin Valley fever.

Many lichens are widely distributed in Antarctica, where more than 400 different symbionts have been found. The distribution of the parasitic groups is associated with that of their host.

Reproduction

Ascomycota have sexual and asexual reproduction. The asexual state (anamorphic) is constituted by the hyphae that are haploid, being the most common way in which we can find these fungi in nature.

In fact, for many species the sexual state (telomorph) is not known, which makes its correct classification difficult.

Asexual reproduction

This can occur by fission, budding, fragmentation, chlamydospores and conidia formation. Fission and budding occur in yeast and both consist of the division of a cell into two daughter cells.

The difference is that in fission two cells of equal size are formed and in budding the division is unequal, forming a cell smaller than the mother cell.

Fragmentation consists of the separation of a piece of mycelium that follows its growth independently. Chlamydospores are formed by thickening of the septa that originate a cell larger than the others that form the hypha, which is then released to form a new mycelium.

The formation of conidia (asexual spores) is the most common type of asexual reproduction in ascomycetes. They originate from a specialized hypha called conidiophore that can appear solitary or grouped taking various forms.

Conidia are very resistant to desiccation, thus facilitating the dispersal of fungi.

In the case of lichens (symbiosis between algae and ascomycetes), a group of cells of the algae are surrounded by the hyphae of the fungus, forming a structure known as soredium that is detached from the parental lichen and originates a new symbiont.

Sexual reproduction

When ascomycetes enter the sexual phase, a female structure called the ascogonium and a male structure, the antheridium, form. Both structures fuse (plasmogamy) and form the asco (sac where the ascospores will be produced).

Later, the nuclei of the two structures unite (karyogamy) and then this new diploid cell enters meiosis, originating four haploid cells.

The cells that originate divide by mitosis and form eight ascospores. In some species, a greater number of divisions may occur and numerous ascospores may occur.

Asci have various shapes and their characteristics are of great importance in the classification of ascomycetes. These can be open in the shape of a cup (apothecium), pyriform (perithecia) or be more or less rounded closed structures (cleistocecio).

The release of ascospores can occur through small pores or through a cap (operculum) in the ascus.

References

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