Yeasts are a group of eukaryotic fungi with a well‐defined cell wall whose growth is either entirely unicellular or a combination of hyphal and unicellular reproduction. The approximately 1500 known yeast species belong to two distinct fungal phyla, the Ascomycota and the Basidiomycota. Within each these phyla, yeasts can be found in several subphyla or classes, reflecting the enormous diversity of their evolutionary origins and biochemical properties. In nature, yeasts are found mainly in association with plants or animals but are also present in soil and aquatic environments. Yeasts grow rapidly and have simple requirements, for which reason they have been used as model systems in biochemistry, genetics and molecular biology. They were the first microorganisms to be domesticated for the production of beer, bread or wine, and they continue to be used for the benefit of humanity in the production of recombinant proteins and other materials.

Key Concepts:

  • Yeasts share in common a primarily unicellular mode of reproduction.

  • Yeasts are phylogenetically diverse, being classified in several classes of two fungal phyla.

  • The concept of yeast is rooted in history and does not always follow logical biological lines.

  • Yeasts have a rigid cell wall primarily made up of β‐(1–3)‐glucan.

  • Sexual reproduction in yeasts involves the formation of internally formed ascospores or externally formed basidiospores.

  • Current yeast classification is based on phylogenetic relationships inferred by gene sequencing.

  • Yeasts grow in nature primarily in association with the plant–insect interface or with warm‐blooded animals, but also occur in soil and aquatic habitats.

  • Yeasts serve as important industrial model systems in research.

  • Yeasts are of great importance in biotechnology. The main products are alcohol, yeast biomass and recombinant proteins.

  • A few yeast species cause infections in humans and other animals, but most yeasts act in an opportunistic fashion, in individuals that are immunocompromised.

Keywords: reproduction; fungi; systematics; ecology; application; biotechnology; genetics; spores; budding

Figure 1.

Blastoconidia. A well‐developed pseudomycelium; all cells are formed by budding, remaining attached to each other and forming a tree‐like structure; the lateral and terminal cells are called blastoconidia or blastospores.

Figure 2.

Apiculate cells. Ontogeny or development of characteristic cell shapes in an apiculate yeast (e.g. Hanseniaspora) from oval to apiculate during repeated bipolar budding.

Figure 3.

Arthroconidia. A hyphal (filamentous) cell undergoing fission or cross wall (septum) formation and breaking up into individual cells called arthroconidia or arthrospores (e.g. Trichosporon).

Figure 4.

Ballistospores. Formation of an asymmetric balistoconidium or ballistospore on a pointed stalk or sterigma on a mother cell of a Sporobolomyces species. The asexual ballistospore is discharged forcefully by means of a droplet mechanism (arrows).

Figure 5.

Life cycle of Rhodosporidium. Simplified life cycle of a heterothallic species of Rhodosporidium: K, karyogamy; P, plasmogamy; RD, reduction division (meiosis).

Figure 6.

Life cycle of Filobasidium. Schematic life cycle of a heterothallic species of Filobasidium: K, karyogamy; P, plasmogamy; RD, reduction division (meiosis).



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Further Reading

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Lachance, Marc‐André(Nov 2011) Yeasts. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000380.pub2]