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A class of the phylum Ascomycota that includes the yeasts and yeastlike fungi. These are morphologically simple fungi; no ascoma is formed, and the asci are produced free on the host or substrate. Asexual reproduction occurs by the formation of blastospores (budding) or, less frequently, by fission arthrospores. Two main orders are recognized, the Saccharomycetales and the Taphrinales. See Yeast
The vegetative body (thallus) of the Saccharomycetales may be either unicellular (true yeasts) or mycelial. In unicellular species, asci form when two vegetative cells fuse, and then the fused cell undergoes meiosis to form ascospores. In mycelial species, the hyphae are not very extensive. Sexual reproduction occurs when adjacent cells extend short lateral branches that fuse to form the asci. Variations on these modes of ascus formation, however, are common among the yeasts.
The Saccharomycetales are common on substrates high in sugars, such as plant exudates, ripe fruits, and flower parts. Because they are microscopic, they are recognized mainly from cultures that have a homogeneous appearance and a characteristic odor. The most important genus is Saccharomyces; S. cerevisiae is the common bakery and brewery yeast, and S. ellipsoideus is used in winemaking. An important mycelial species is Nematospora coryli, which causes yeast spot disease of various crops.
The order Taphrinales includes the leaf curl disease fungi. The most widely recognized species are Taphrina deformans, cause of leaf curl of peach and almond trees, and T. caerulescens, cause of leaf blister of oaks. These fungi produce a well-developed mycelium in the host tissue but, when grown in culture, form only a yeastlike colony of single cells. Asci are produced when special binucleate hyphal cells beneath the host cuticle undergo nuclear fusion and the resulting diploid cell elongates to form an ascus on the leaf surface. The nucleus undergoes meiosis, and ascospores are formed. See Ascomycota, Eumycota, Fungi
On the basis of molecular data, some workers now propose separating the Taphrinales and the fission yeast, Schizosaccharomyces, into a new class, Archiascomycetes. These fungi are considered to be more primitive and phylogenetically basal to the rest of the ascomycetes.
a subclass of class Ascomycetes.
Hemiascomycetes characteristically develop asci directly on the mycelium or on sprout cells, without the formation of fruiting bodies and largely isolated from one another. Some researchers consider Hemiascomycetes to be primitive organisms, while others believe that they are regressive. Hemiascomycetes are divided into two orders—Endomycetales and Taphrinales—according to their method of ascus formation. In Endomycetales the asci develop from zygotes that had developed by parthenogenesis or by the copulation of two of the fungus cells. Many species of these fungi (the yeasts) cause fermentation and are widely used in the food industry. In Taphrinales the asci are formed from binuclear ascogenous cells which, like chlamydospores, develop from binuclear mycelia. The Taphrinales are parasitic to plants and cause a variety of diseases, including leaf curl of peaches, almonds, cherries, pears, birches, alders, and poplars, “plum pockets” in plums and cherry plums, witch’s brooms in plums, cherry plums, and birches, and swollen leaves in filberts.
V. A. MEL’NIK