ASCOSPORES: CHARACTERISTICS, FORMATION, FUNCTIONS, EXAMPLES - BIOLOGY - 2024

The ascospores are spores product of sexual reproduction, cariogamia and meiotic division fungi asci class of Ascomycota. These, when germinating, are capable of giving rise to a new independent fungus.

The Ascomycota or Ascomycetes fungi are a class of fungi that comprises about 30% of all known fungal species. They are found most abundantly in terrestrial and aquatic environments. Only a few species are typical of marine habitats.

Photo of the ascospores of the species Morchella elata taken through a light microscope (Source: Peter G. Werner Via Wikimedia Commons)

The distinguishing feature of ascomycetes is the formation of an endospore-producing structure. This structure represents a special type of sporangium and is called "disgust." Therefore, all the fungi that produce disgust are of the class of Ascomycetes.

Asci are generally sac-shaped and represent the place where ascospores form. The more specialized ascomycetes, such as those found in lichens, have macroscopic asci and a fruiting body called the ascocarpus.

The shape of asci and ascospores is used by taxonomists to differentiate the different species of the Ascomycota class. For example, within ascomycetes are yeasts, unicellular fungi that do not form fruiting bodies.

A part of the agri-food industry has dedicated itself to preserving items and food from contamination by ascospores since, when they germinate and originate mature individuals, they deteriorate and decompose food.

characteristics

Ascospores are like a kind of "seeds" of Ascomycota fungi, analogous to those of plants, since they can remain inactive (in dormancy) but live for long periods of time.

These structures are very resistant, can give rise to new complete individuals and can remain alive for a long time after germinating, since they feed on endogenous substrates.

Photo of Schizosaccharomyces octosporus showing ascospores with protective envelopes through light microscopy. A = Ascospores, B = Ascas, C = Ascospores divided by fission with four ascospores, D = Ascospores with protective shells. Scale bar = 0.01 mm (Source: Photo of Schizosaccharomyces octosporus showing ascospores with protective shells through light microscopy. A = Ascospores, B = Ascas, C = Ascospores divided by fission with four ascospores, D = Ascospores with protective shells. Bar scale = 0.01 mm. Via Wikimedia Commons)

However, ascospores have unique characteristics that differentiate them from plant seeds, for example, the main stimuli for the germination of ascospores are chemical substances produced by the decomposition of substrates.

In plants, on the contrary, the stimulus for germination is water and light, in some cases. Ascospores have half the chromosomal charge of a normal cell, that is, they are haploid; meanwhile the seeds of the plants are mostly polyploid.

Ascospores are generally microscopic structures that are rarely slightly visible with low-power magnifying glasses. On the other hand, the seeds of vegetables are macroscopic and few exceptions can be named with microscopic seeds.

When looking under the microscope and detailing a typical ascospore, we observe that they are elliptical in shape, that they have their protoplasts enclosed by a three-level or layered chitin cell wall and that they have a germinal pore at each end of the cell.

Formation of disgust and ascospore

Germination of the ascospore and development of the mycelium

Ascospores are the end product of the sexual reproduction process of ascomycetes. The formation of mycelia in these organisms begins with the germination of an ascospore, and just afterwards the conidiophores begin to form.

The fungus begins a growth phase where a large number of conidia are produced that contribute to the spread of the fungus in the substrate. In this mycelium the formation of disgust begins.

Before this, a gametogenesis occurs by which the antheridia (male) and ascogonia (female) are formed. The nuclei of the antheridium are transferred to the ascogonium and the protoplasts of both cells fuse in a process called plasmogamy.

Within the same cytosol, male nuclei mate with female nuclei, but without fusing. Then the “hyphal” filaments begin to grow outside the ascogonium and the ascogenic hyphae lengthen.

In ascogenic hyphae, nuclei develop and multiply by simultaneous mitotic divisions in all hyphae of the ascogonium. Disgust forms at the end of one of the aschogenic dikaryotic hyphae that originated during this step.

Life cycle of an Ascomycota fungus. A - haploid stage (yeast); B - stage of dicariotica (mycelium); C - diploid stage (proasci); D - development of asci and sporogenesis. 1 - emergence of ascospores and blastospores (conidia); 2-dicariotization; 3 - dikaryotic mycelium in plant cells, forming an ascogenous layer; 4 - karyogamy; 5 - mitosis of the diploid nucleus, protoasci and basal cell formation; 6 - development of disgust after meiosis; 7 - mitosis of haploid nuclei, formation of ascospores; 8 - layer formation of plant cells (Source: Afanasovich Via Wikimedia Commons)

Disgust formation

One of the cells of the dikaryotic hyphae grows to form a hook called an "uncinulo". In this hook-shaped cell, the two nuclei divide in such a way that their mitotic spindles are arranged in a parallel and vertical orientation.

Two of the daughter nuclei are in the upper area of ​​the hook, one is near the end, and the other near the basal septum of the hook. There, two septa are formed that divide the hook into three cells.

The cell in the middle of the three is the one that will form disgust. Inside this cell, the process of karyogamy occurs, where the two nuclei fuse to form a diploid nucleus known as the zygote.

This diploid nucleus is the only diploid in the life cycle of Ascomycota fungi. After karyogamy, disgust begins to mature and increase in length (elongate).

Formation of the ascospore

In young asco cells, the diploid nuclei within them undergo meiosis and later mitosis. 8 new haploid cells originate from the original cell. These eight cells, as they develop, will transform into ascospores.

Each nucleus that originated from meiotic and later mitotic reproduction will be stored, along with a part of the cytosol of the cell where division occurred, in a chitin cell wall that is synthesized inside the cell.

In almost all ascomycetes the disgust is a very well structured rigid structure. As the ascospores mature, the disgust implodes and releases the ascospores into the environment.

Generally, ascospores spread for short distances, around a few centimeters, however, in some species they spread up to a few meters, it all depends on the environment where they are expelled.

Examples

The most abundant species of Ascomycota in nature and in agribusiness are yeasts, which can be found on the surface of soils, water, fruits and a large amount of food.

These organisms have the ability to metabolize sugars, producing alcohol and carbon dioxide in the process.

Fruiting bodies do not occur in yeasts, as these are single-celled organisms that reproduce most frequently by binary fission or budding. However, when the conditions in the medium are adverse, two compatible cells fuse to form a zygote.

The zygote develops directly inside the cell, this cell differentiates into a disgust and, inside it, 4 or 8 nuclei are divided depending on the species of yeast. These nuclei develop and become coated with chitin, transforming into ascospores.

All the fungi that make up the symbiotic association that represent lichens are from the Ascomycota family, therefore, they develop ascospores through their sexual reproduction.

Generally, when observing in detail a lichen that has already reached its stage of maturity, small cup-shaped structures can be seen. These structures are the fruiting bodies of the fungus, known as "apothecia." Inside the apothecia is the place where the ascospores are generated.

References

1. Bellemère, A. (1994). Asci and ascospores in ascomycete systematics. In Ascomycete Systematics (pp. 111-126). Springer, Boston, MA.
2. Dijksterhuis, J. (2007). Heat-resistant ascospores. In Food Mycology (pp. 115-132). CRC press.
3. Guth, E., Hashimoto, T., & Conti, SF (1972). Morphogenesis of ascospores in Saccharomyces cerevisiae. Journal of bacteriology, 109 (2), 869-880
4. Lindorf, H., De Parisca, L., & Rodríguez, P. (1985). Botanica Classification, structure and reproduction.
5. Lowry, RJ, & Sussman, AS (1968). Ultrastructural changes during germination of ascospores of Neurospora tetrasperma. Microbiology, 51 (3), 403-409.
6. Raven, PH, Evert, RF, & Eichhorn, SE (2005). Biology of plants. Macmillan.