Hemiascomycetes

Hemiascomycetes [¦he·mē‚as·kō‚mī′sēd·ēz]

(mycology)

The equivalent name for Hemiascomycetidae.

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Hemiascomycetes

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.