| Saccharomycotina | |
|---|---|
 | |
| Candida albicans | |
Scientific classification | |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Clade: | Saccharomyceta |
| Subdivision: | Saccharomycotina O.E. Erikss. & Winka 1997[1] |
| Classes | |
| Synonyms | |
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Saccharomycotina is a subdivision (subphylum) of the division (phylum)Ascomycota in the kingdomFungi.[2][3] It comprises most of the ascomyceteyeasts. The members of Saccharomycotina reproduce bybudding and they do not produceascocarps (fruiting bodies).[2][4]
The subdivision includes a single class:Saccharomycetes, which again contains a single order:Saccharomycetales.[2][3]
Notable members of Saccharomycotina are the baker's yeastSaccharomyces cerevisiae and the genusCandida that includes several human pathogens.
The name comes from the Greek word σάκχαρον (sákkharon), meaning "sugar" and μύκης (mukēs) meaning "fungus".
Historical records from ancient Egypt and China describe the processes of brewing and baking from 10,000 to 8,000 years ago, and the production offermented beverages and foods seems to have paralleled the beginning ofagriculture.[5] In the 1850s,Louis Pasteur demonstrated that yeasts are responsible for thefermentation of grape juice to wine.[6][7]
Saccharomycotina include some of the economically most importantfungi known. Members include species of industrial and agricultural importance (e.g.brewing,baking,fermentation of food products, production ofcitric acid, production ofrecombinant proteins,biofuel production,biological pest control of crops). Other species cause economic losses worldwide (plant pathogens, contaminants of foods and beverages). Yet others are animal and humanpathogens.[8][9]
Saccharomycete yeasts usually grow as single cells. Their cellular morphology is fairly simple, although their growth form is highly adapted.Asci are naked andascospores can have several forms. No species produceascocarps (fruiting bodies). Saccharomycete genomes are often smaller than those offilamentous fungi.[2][10][11][4]
Some species (e.g.Metschnikowia species) tend to form chains ofbudding cells that are termedpseudohyphae.[2] Yet other species are able to produce true septatehyphae.[4] Such species (e.g.Candida albicans) are termeddimorphic, which means they can propagate both as budding yeasts and as filamentous hyphae.
Asexual reproduction occurs mainlyvegetatively bymitosis andbudding. Saccharomycotina is characterized by holoblastic budding,[12] which means all layers of the parent cell wall are involved in the budding event. This leaves a scar through which no further budding occurs.
Asexual cells may vary in shape.[13] The shape of the cell may be informative in terms of detecting mode of reproduction ortaxonomic placement to genera or species.
Although not commonly known, some species form endospores (e.g.Candida species).[2] These are asexual spores that are formed within their mother cell (hyphal or single cell). Strains ofCandida andMetschnikowia may also form asexual resting spores calledchlamydospores.[2]
Sexual reproduction is not known for all species of Saccharomycotina, but may happen in certain species if environmental conditions favour it (e.g. deficiency innitrogen andcarbohydrate).[2] Sexual reproduction is well known inSaccharomyces cerevisiae. Here, the life cycle involves alternation between ahaploid and adiploid phase. The life cycle proceeds as follows: Two cells of differentmating type fuse and the nuclei undergokaryogamy. This results in a daughter cell with adiploid nucleus, functioning as anascus, wheremeiosis occurs to producehaploidascospores. When ascospores germinate, thehaploid phase is established, and is maintained by furthermitosis andbudding. In most natural populations this phase is fairly short sinceascospores fuse almost immediately aftermeiosis has occurred. This results in mostyeast populations beingdiploid for most part of their life cycle.[4]
In Saccharomycotina there are twomating types present. The mating types specify peptide hormones calledpheromones and corresponding receptors for each type. These pheromones organize the mating. The pheromones do not affect the same mating type ordiploids, but bind to receptors of different mating type. Interaction between pheromone and receptor results in alteredmetabolism to allow for fusion between cells of different mating type.[4][2]
Saccharomyceteyeasts are found in nearly all regions of the world, including hot deserts, polar areas, in freshwater, in salt water, and in the atmosphere.[2] Their growth is mainlysaprotrophic, but some members are importantpathogens of plants and animals, including humans. They are often found in specialized habitats, e.g. small volumes of organiccarbon rich liquid (e.g. flower nectar).[4]
Examples of ecological modes in Saccharomycotina:
Although yeasts are commonly isolated from soil, few are believed to have soil as a primaryhabitat.[2]
Accurate identification of species is important for understanding yeastecology, something that is now possible with the increased use ofDNA-based methods. Before molecular methods were available, identification was mainly based onmorphology, something that resulted in misclassifications and further prevented reliable results of ecological research.
Saccharomycotina is a subdivision (subphylum) of the division (phylum)Ascomycota. It is asister group toPezizomycotina.[2][3]
Yeasts were traditionally classified as a separate group of thefungal kingdom, but in recent years[when?]DNA-based methods have changed the understanding ofphylogenetic relationships among fungi. Yeasts are considered to be apolyphyletic group,[2][11] consisting of members ofBasidiomycota,Taphrinomycotina, as well as Saccharomycotina. This realization has led to major changes in thephylogeny andtaxonomy of Saccharomycotina.[2]
In addition, the recent[when?] changes in theInternational Code of Nomenclature for algae, fungi and plants[17][18] have had a major impact on theclassification of fungi, including Saccharomycotina. The changes imply that a fungus can only bear one correct name, i.e. separate names foranamorphs and teleomorphs are no longer allowed. This involves major changes in Saccharomycotinataxonomy, as many species are currently described from both anamorphic and teleomorphic stages.[18] The genusCandida is an example of a genus that is undergoing large-scale revisions.
Molecular identification methods are important tools for discovery of new species and subsequently give better understanding ofbiodiversity in this group. Much of the futureclassification of Saccharomycotina will rest on phylogenetic analysis ofDNA sequences rather than on themorphological and developmental characters.[citation needed]
Phylogeny by Groenewald et al. 2023[19]
| Saccharomycotina |
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