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9.21: Ascomycota- The Sac Fungi - Biology

9.21: Ascomycota- The Sac Fungi - Biology



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The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Aspergillus oryzae is used in the fermentation of rice to produce sake. Conidia and asci, which are used respectively for asexual and sexual reproductions, are usually separated from the vegetative hyphae by blocked (non-perforated) septa.

Figure 1. Click for a larger image. The lifecycle of an ascomycete is characterized by the production of asci during the sexual phase. The haploid phase is the predominant phase of the life cycle.

Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores. Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains (Figure 1).

The “male” strain produces an antheridium and the “female” strain develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in plasmogamy without nuclear fusion. Special ascogenous hyphae arise, in which pairs of nuclei migrate: one from the “male” strain and one from the “female” strain. In each ascus, two or more haploid ascospores fuse their nuclei in karyogamy.

During sexual reproduction, thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus gives rise to haploid nuclei by meiosis. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia (Figure 2).

Practice Question

Which of the following statements is true?

  1. A dikaryotic ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  2. A diploid ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  3. A haploid zygote that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  4. A dikaryotic ascus that forms in the ascocarp undergoes plasmogamy, meiosis, and mitosis to form eight ascospores.

[reveal-answer q=”34326″]Show Answer[/reveal-answer]
[hidden-answer a=”34326″]Statement a is true.[/hidden-answer]


Ascomycota: The Sac Fungi

The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Many ascomycetes are of commercial importance. Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies. Aspergillus oryzae is used in the fermentation of rice to produce sake. Other ascomycetes parasitize plants and animals, including humans. For example, fungal pneumonia poses a significant threat to AIDS patients who have a compromised immune system.

Ascomycetes not only infest and destroy crops directly they also produce poisonous secondary metabolites that make crops unfit for consumption. Filamentous ascomycetes produce hyphae divided by perforated septa, allowing streaming of cytoplasm from one cell to the other. Conidia and asci, which are used respectively for asexual and sexual reproductions, are usually separated from the vegetative hyphae by blocked (non-perforated) septa.

Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores (see the figure below). Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains (see the figure below). The “male” strain produces an antheridium and the “female” strain develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in plasmogamy without nuclear fusion.

Special ascogenous hyphae arise, in which pairs of nuclei migrate: one from the “male” strain and one from the “female” strain. In each ascus, two or more haploid ascospores fuse their nuclei in karyogamy. During sexual reproduction, thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus gives rise to haploid nuclei by meiosis. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia (see the figure below).

Art Connection

The lifecycle of an ascomycete is characterized by the production of asci during the sexual phase. The haploid phase is the predominant phase of the life cycle.

Which of the following statements is true?

  1. A dikaryotic ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  2. A diploid ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  3. A haploid zygote that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  4. A dikaryotic ascus that forms in the ascocarp undergoes plasmogamy, meiosis, and mitosis to form eight ascospores.

Answer

A dikaryotic ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.

The bright field light micrograph shows ascospores being released from asci in the fungus Talaromyces flavus var. flavus. (credit: modification of work by Dr. Lucille Georg, CDC scale-bar data from Matt Russell)


Ascomycota: The Sac Fungi

The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Filamentous ascomycetes produce hyphae divided by perforated septa, allowing streaming of cytoplasm from one cell to another. Conidia and asci, which are used respectively for asexual and sexual reproduction, are usually separated from the vegetative hyphae by blocked (non-perforated) septa. Many ascomycetes are of commercial importance. Some play a beneficial role for humanity, such as the yeasts used in baking, brewing, and wine fermentation, and directly as food delicacies such as truffles and morels. Aspergillus oryzae is used in the fermentation of rice to produce sake. Other ascomycetes parasitize plants and animals, including humans. For example, fungal pneumonia poses a significant threat to AIDS patients who have a compromised immune system. Ascomycetes not only infest and destroy crops directly they also produce poisonous secondary metabolites that make crops unfit for consumption.

Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores. Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains. The “male” strain produces an antheridium and the “female” strain develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in plasmogamy, without nuclear fusion. Special dikaryotic ascogenous (ascus-producing) hyphae arise from this dikaryon, in which each cell has pairs of nuclei: one from the “male” strain and one from the “female” strain. In each ascus, two haploid nuclei fuse in karyogamy. Thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus in each ascus gives rise to haploid nuclei by meiosis, and spore walls form around each nucleus. The spores in each ascus contain the meiotic products of a single diploid nucleus. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia.


Ascomycota

Ascomycota is a phylum of the fungi state and, combined with Basidiomycota, forms the sub condition for diarrhea. Ascomycota, also called sac fungi, is a fungal film (Kingdom Fungi) characterized by a sacral national structure, ascus, which contains four to eight ascospores at the sexual stage. Its members are commonly known as sack fungi. They are the largest fungi film with over 64,000 species. The defining feature of this fungus group is “ascus” (in Greek (askos), meaning “sac” or “wineskin”), a microscopic sexual structure in which nonmotile spores known as ascospores are formed. However, some species of Ascomycota are miraculous, meaning that they do not have a sexual cycle and thus do not produce SC or ascospores. Previously placed on deuteromycota with other fungal taxa, the asexual ascomycetes are now characterized and classified by phylogenetic analyzes of DNA sequences on the basis of morphological or physiological similarities with taxa bearing Ascus. Psychodynamic markings of the majority of lichen (less commonly known as “ascolicin”), such as Cladonia, include Ascomycota.

The sac fungus is grown on the SC alone or carried in one of several different fruit structures, or ascospores, and the ascospores are subdivided based on the method of discharge. Many ascomycetes are plant microbes, some are pathogens of organisms, some are edible mushrooms, and many are living in dead organic matter (as saprobes). Asexual (or anamorphic) ascomycetes, formerly with other fungal taxa in Duteromycota, are now characterized and classified on the basis of morphological or anatomical similarities to Ascus-bearing taxa and by phylogenetic analysis of DNA sequences. Other ascomycetes include important plant pathogens, such as grape powdery mildew (Ansinula nectar), Dutch elm disease (Ophiostoma ulmi), chestnut blight (Cryphonectaria parasitic) and apple scab (Venturia inequalis). Some special uses of ascomycetes as raising the incredibly common conditions for bread, alcoholic beverages and cheese counseling for antibiotics. There are many plant-pathogenic ascomycetes, including apple scabs, rice blasts, ergot fungi, black knots, and powdered millipedes. Several species of ascomycetes are biological model organisms in laboratory studies. Examples of ascomycetes are antibiotic manufacturers for the treatment of cheese and bacterial infectious diseases of the Penicillium species.

Many ascomycetes are pathogens, both humans and plants, including animals. Examples of ascomycetes that can cause infections in humans include Candida albicans, Aspergillus niger, and decades of species that cause skin infections. The most famous Neurospora crassa, several species of yeast and Aspergillus species, are used in many genetics and cell biology studies. Examples of taxa included Ascomycota are antibiotic manufacturers for the treatment of Penicillium species and bacterial infectious diseases on cheeses.


Fungal Sex: The Ascomycota

This article provides an overview of sexual reproduction in the ascomycetes, a phylum of fungi that is named after the specialized sacs or "asci" that hold the sexual spores. They have therefore also been referred to as the Sac Fungi due to these characteristic structures that typically contain four to eight ascospores. Ascomycetes are morphologically diverse and include single-celled yeasts, filamentous fungi, and more complex cup fungi. The sexual cycles of many species, including those of the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe and the filamentous saprobes Neurospora crassa, Aspergillus nidulans, and Podospora anserina, have been examined in depth. In addition, sexual or parasexual cycles have been uncovered in important human pathogens such as Candida albicans and Aspergillus fumigatus, as well as in plant pathogens such as Fusarium graminearum and Cochliobolus heterostrophus. We summarize what is known about sexual fecundity in ascomycetes, examine how structural changes at the mating-type locus dictate sexual behavior, and discuss recent studies that reveal that pheromone signaling pathways can be repurposed to serve cellular roles unrelated to sex.


Watch the video: Ascomycota (August 2022).