Lecture #18

Archiascomycetes: Early Diverging Ascomycetes

Open with impact of molecular data and polyphyly of yeasts

Archiascomycetes: Early Diverging Ascomycetes

Ascogenous hyphae

Members of the group lack ascogenous hyphae and ascocarps, and the asci have sometimes been homologized with sporangia. In some members of the group the asci are said to form within thick-walled ascogenous cells that have been called cysts or chlamydospores. Whether this is, in fact, merely a thick walled ascus or not is unclear.

Free cell formation

Studies of ascosporogenesis indicate that the process may occur either by formation of an EMS similar to the cylindrical type found in filamentous ascomycetes (in Pneumocystis) or by development of the EMS in association with individual nuclei in Taphrina and Schizosaccharomyces

Ascospore Release

Ploidy level of nuclei

Septal Pores

There is some question about the occurrence of simple septal pore and Woronin bodies in the mycelial archiascomycetes.




Taphrina and members of the Protomycetaceae have mycelial stages that are parasitic on angiosperms, causing galls, thickenings, blisters, lesions and witches' brooms.

Protomycetaceae has a restricted host range and none of the species infects important crop plants; primarily weedy species in the Umbelliferae and Asteraceae

Taphrina occur on a wide variety of hosts, including some economically important species; best known of these is Taphrina deformans, the cause of peach and almond leaf curl diseases that produces obvious symptoms on the leaves and may lead to almost complete defoliation of the host tree;T. coerulescens on oak


Taphrinales includes two families, the monotypic family Taphrinaceae with almost 100 species and the Protomycetaceae with five genera and twenty species


Life Cycle


Schizosaccharomyces, the fission yeasts, have been isolated from slime fluxes, honey, fruit, and fruit products, habitats that also are common to a number of the Endomycetales.


Classification. Although the four species of the Schizosaccharomyces were traditionally included with the yeasts of the Saccharomycetales the genus long has been set apart from the group by its method of cell division and wall carbohydrates. Kurtzman (1993) used rDNA sequence analysis as the basis for placement in a separate order, and Sugiyama and Nishida (1994) included the genus in the archiascomycetes.


Saccharomycetales (=Endomycetales): The Ascomycetous Yeasts


Members of Saccharomycetales are important in the baking, brewing, distilling, and related industries including fuel ethanol production, as supplements to food, in the manufacture of riboflavin and citric acid, as plant pathogens and mycoparasites, in the contamination and spoilage of foods, as human pathogens, as important components of ecosystems, and as model organisms for scientific study.

Food Yeasts

Fermentations and Alcoholic Beverages. Yeasts almost certainly have been used by humans since the dawn of civilization. Their use is first documented about 6,000 B.C. Fermentations were welcome discoveries because they have the effect of preservation by lowering the pH and, in some cases, producing alcohol, conditions in which few microorganisms prosper; yeasts are noted particularly for their ability to ferment carbohydrates; hence, the name saccharomycetes (Gr. saccharon = sugar + myketes = fungi) is applied to them.

Human Pathogens. Many of us harbor a yeast as part of our normal intestinal and urogenital mycota that may turn on us one day causing severe infection and even death. Candida albicans is seldom isolated outside the bodies of animals, and is known from 58 species including wild and domesticated mammals and birds.

Somatic Structures

Asexual Reproduction


Sexual Reproduction

Saccharomyces cerevisiae

Meiosis and Ascosporogenesis



  1. Have a predominantly unicellular thallus that may produce pseudomycelium,
  2. Reproduce asexually primarily by multilateral budding, and
  3. Produce ascospores in a free ascus that originates either from a zygote or parthogenetically from a single somatic cell.

The best known member of the family is Saccharomyces cerevisiae.

Life cycle of S. cerevisiae, an example of a species that may have both haploid and diploid budding stages

Nadsoniaceae and Saccharomycodaceae.

Eremotheciaceae and Metschnikowiaceae.

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