Members of Ascomycota, or ascomycetes, commonly known as sac fungi, are named for their characteristic reproductive structure, the saclikeascus (pl., asci; Greek askos, sac).
Ascomycetes are ecologically important in freshwater, marine,and terrestrial habitats because they degrade many chemically stable organic compounds, including lignin, cellulose, and collagen.
Many species are quite familiar and economically important. For example, most of the red, brown, and blue-green molds that cause food spoilage are ascomycetes.
The powdery mildews that attack plant leaves and the fungi that cause chestnut blight and Dutchelm disease are ascomycetes. Many yeasts as well as edible morels and truffles are also asco mycetes.
The pink bread mold Neurospora crassa is an important research tool in genetics and biochemistry. A new ascomycete genus, Archaeorhizomyces, was described in 2011.
The group is globally distributed and is commonly found in association with the roots of pine trees but does not appear to be mycorrhizal, as it can be grown in pure culture. Like Neuros pora and Aspergillus, Archaeorhizomyces spp. are filamentous, producing slender hyphae.
Some ascomycetes are yeasts, while others have a life cycle that alternates between yeast and filamentous forms. The life cycle of the yeast Saccharomyces cerevisiae, commonly known ashas been invaluable for understanding the loss of cell cycle control that occurs in cancerous cells.
Filamentous ascomycetes form septate hyphae. Asexual re production is common and is associated with the production of conidia .
Sexual reproduction involves ascus formation, with each ascus usually bearing eight haploid asco spores, although some species can produce over 1,000. Such hyphae are said to be ascogenous.
Mating starts when two strains of opposite mating types form ascogenous hyphae into which pairs of nuclei migrate.
One nucleus of each pair originates from a "male" mycelium (antheridium) or cell and the other from a "female" organ or cell (ascogonium) that has fused with it. As the ascogenous hyphae grow, the paired nuclei divide so that there is one pair of nuclei in each cell.
After the ascogenous hyphae have matured, nuclear fusion occurs at the hyphal tips in the ascus mother cells. The diploid zygote nucleus then undergoes meiosis, and the resulting four haploid nuclei divide mitotically again to produce a row of eight brewer's or baker's yeast, is well understood . S. ceretfisiae alternates between haploid and diploid states. As long as nutrients remain plentiful, haploid and diploid cells undergo mitosis to produce haploid and diploid daughter cells, respec tively.
Each daughter cell leaves a scar on the mother cell as it separates, and daughter cells bud only from unscarred regions of the cell wall. When a mother cell has no more unscarred cell wall re maining, it can no longer repro duce and will senesce (die).
When nutrients are limited, diploid S. ceretfisiae cells undergo meio- sis to produce four haploid cells that remain bound within a com mon cell wall, the ascus. Upon theaddition of nutrients, two haploid cells of opposite mating types come into contact and fuse to cre ate a diploid.
Typically only cells of opposite mating types can fuse; this process is tightly regulated by the action of pheromones.
S. ceretfisiae is a valuable model organism. Research on this organism has revealed the importance of many cellular pro cesses. For instance, it is a favorite model system for studying cell cycling and the events during mi tosis.
This research is critical not only for our understanding of normal cell division, but it alsonuclei in each developing ascus. These nuclei are walled off from one another. Thousands of asci may be packed together in a cup- or flask-shaped fruiting body called an ascocarp.
When the ascospores mature, they often are released from the asci with great force. If the mature ascocarp is jarred, it may appear to belch puffs of "smoke" consisting of thousands of ascospores. Upon reaching a suitable environment, the ascospores germi nate and start the cycle anew.
Several Aspergillus species are noteworthy. A. fumigatus is ubiquitous in the environment, commonly found in homes and the workplace.
It is known to trigger allergic responses and is im plicated in the increased incidence in severe asthma and sinusitis.
It is also pathogenic, infecting immunocompromised individuals with a mortality rate of nearly 50%. A. nidulans is a model organ ism used to study questions of eukaryotic cell and developmental biology.
A. oryzae is used in the production of traditional fer mented foods and beverages in Japan, including saki and soy sauce. Because A. oryzae secretes many industrially useful pro teins and can be genetically manipulated, it has become an important organism in biotechnology.
Many ascomycetes are parasites of higher plants. Claviceps purpurea parasitizes rye and other grasses, causing the plant dis ease ergot.
Ergotism, the toxic condition in humans and animals that eat grain infected with the fungus, is often accompanied by gangrene, psychotic delusions, nervous spasms, abortion, and convulsions.
During the Middle Ages, ergotism, then known as St. Anthony's fire, killed thousands of people. For example, over 40,000 deaths from ergot poisoning were recorded in France in the year 943.
It has been suggested that the widespread accusations of witchcraft in Salem Village and other New England communities in the 1690s may have resulted from outbreaks of ergotism. The pharmacological activities are due to an active ingredient, lysergic acid diethyl amide (LSD).
In controlled dosages, other active compounds can be used to induce labor, lower blood pressure, and ease migraine headaches.
Although conidia are the major form of dissemination, some filamentous fungi also produce sclerotia. Sclerotia are compact masses of hyphae that can survive the winter.
In the spring they germinate to produce more hyphae or conidia. These structures confer a competitive advantage to the fungi that produce them.
For instance, some species of Aspergillus that infect plants form sclerotia to remain viable in the soil, where they can take advan tage of nutrient resources when the temperature rises.
Most fungal pathogens that infect animals, including hu mans, are ascomycetes . Many are opportunistic pathogens such as those in the genera Candida, Blastomyces,
and Histoplasma. In addition, the cause of"sick building syndrome;'
Stachybotrys chartarum is also an ascomycete .
Finally, the Aspergillus toxins known as aflatoxins are an important cause of food contamination. Exposure to aflatoxins can result in liver cancer.
