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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. During sexual reproduction, thousands of asci fill a fruiting body called the ascocarp    .

Art connection

 Ascomycetes have both sexual and asexual life cycles. In the asexual life cycle, the haploid (1n) mycelium branches into a chain of cells called the conidiophore. Spores bud from the end of the conidiophore and germinate to form more mycelia. In the sexual life cycle, a round structure called an antheridium buds from the male strain, and a similar structure called the ascogonium buds from the female strain. In a process called plasmogamy, the ascogonium and antheridium fuse to form a cell with multiple haploid nuclei. Mitosis and cell division result in the growth of many hyphae, which form a fruiting body called the ascocarp. The hyphae are dikaryotic, meaning they have two haploid nuclei. Asci form at the tips of these hyphae. In a process called karyogamy, the nuclei in the asci fuse to form a diploid (2n) zygote. The zygote undergoes meiosis without cell division, resulting in an ascus with four 1n nuclei arranged in a row. Each nucleus undergoes mitosis, resulting in eight ascospores, which are also arranged in a row at the tip of the hyphae. Dispersal and germination results in the growth of new mycelia.
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.
Micrograph shows asci, which appear as multiple, sphere-like shapes fused together into a structure about 7 microns across, and ascospores, which are small, light blue ovals about two microns wide by three microns long released from the asci.
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)

Basidiomycota: the club fungi

The fungi in the Phylum Basidiomycota    are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium    ), which are the swollen terminal cell of a hypha. The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn ( [link] ). These mushroom-producing basidiomyces are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The “gills” are actually compacted hyphae on which the basidia are borne. This group also includes shelf fungus, which cling to the bark of trees like small shelves. In addition, the basidiomycota includes smuts and rusts, which are important plant pathogens; toadstools, and shelf fungi stacked on tree trunks. Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycetes produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness.

 Photo shows toadstools growing in a ring on a lawn.
The fruiting bodies of a basidiomycete form a ring in a meadow, commonly called “fairy ring.” The best-known fairy ring fungus has the scientific name Marasmius oreades . The body of this fungus, its mycelium, is underground and grows outward in a circle. As it grows, the mycelium depletes the soil of nitrogen, causing the mycelia to grow away from the center and leading to the “fairy ring” of fruiting bodies where there is adequate soil nitrogen. (Credit: "Cropcircles"/Wikipedia Commons)]

The lifecycle of basidiomycetes includes alternation of generations ( [link] ). The club-shaped basidium carries spores called basidiospores. Eventually, the mycelium generates a basidiocarp    , which is a fruiting body that protrudes from the ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap.

Art connection

 The life cycle of basidiomycetes, better known as mushrooms, is shown. Basidiomycetes have a sexual life cycle that begins with the germination of 1n basidiospores into mycelia with plus and minus mating types. In a process called plasmogamy, the plus and minus mycelia form a dikaryotic mycelium. Under the right conditions, the dikaryotic mycelium grows into a basdiocarp, or mushroom. Gills on the underside of the mushroom cap contain cells called basidia. The basidia undergo karyogamy to form a 2n zygote. The zygote undergoes meiosis to form cells with four haploid (1n) nuclei. Cell division results in four basidiospores. Dispersal and germination of basidiospores ends the cycle.
The lifecycle of a basidiomycete alternates generations.

Which of the following statements is true?

  1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
  2. The result of the plasmogamy step is four basidiospores.
  3. Karyogamy results directly in the formation of mycelia.
  4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

Glomeromycota

The Glomeromycota    is a newly established phylum which comprises about 230 species that all live in close association with the roots of trees. Fossil records indicate that trees and their root symbionts share a long evolutionary history. It appears that all members of this family interact with the root cells forming a mutually beneficial association where the plants supply the carbon source and energy in the form of carbohydrates to the fungus, and the fungus supplies essential minerals from the soil to the plant.

The glomeromycetes do not reproduce sexually and do not survive without the presence of plant roots. DNA analysis shows that all glomeromycetes probably descended from a common ancestor, making them a monophyletic lineage.

Section summary

Chytridiomycota (chytrids) are considered the most primitive group of fungi. They are mostly aquatic, and their gametes are the only fungal cells known to have flagella. They reproduce both sexually and asexually; the asexual spores are called zoospores. Zygomycota (zygote fungi) produce non-septated hyphae with many nuclei. Their hyphae fuse during sexual reproduction to produce a zygospore in a zygosporangium. Ascomycota (sac fungi) form spores in sacs called asci during sexual reproduction. Asexual reproduction is their most common form of reproduction. Basidiomycota (club fungi) produce showy fruiting bodies that contain basidia in the form of clubs. Spores are stored in the basidia. Most familiar mushrooms belong to this division. Deuteromycota (imperfect fungi) belong to a polyphyletic group that does not reproduce through sexual reproduction. Glomeromycota form tight associations (called mycorrhizae) with the roots of plants.

Art connections

[link] Which of the following statements is true?

  1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
  2. The result of the plasmogamy step is four basidiospores.
  3. Karyogamy results directly in the formation of mycelia.
  4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

[link] D

Questions & Answers

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Source:  OpenStax, Bi 101 for lbcc ilearn campus. OpenStax CNX. Nov 28, 2013 Download for free at http://legacy.cnx.org/content/col11593/1.1
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