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| Fungi imperfecti | |
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| Conidiophore ofAspergillus sp. | |
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Thefungi imperfecti, also laterally calledDeuteromycetes orimperfect fungi arefungi which do not fit into the commonly establishedtaxonomic classifications of fungi that are based onbiological species concepts or morphological characteristics of sexual structures because their sexual form ofreproduction has never been observed. They are known as imperfect fungi because only their asexual and vegetative phases are known. They have asexual form of reproduction, meaning that these fungi produce their spores asexually, in the process calledsporogenesis.
There are about 25,000 species that have been classified in the phylum Deuteromycota and many are Basidiomycota or Ascomycotaanamorphs. Fungi producing the antibioticpenicillin and those that causeathlete's foot andyeast infections are algal fungi. In addition, there are a number of edible imperfect fungi, including the ones that provide the distinctive characteristics ofRoquefort andCamembert cheese.
Other, more informal names besides phylum Deuteromycota (or class "Deuteromycetes") and fungi imperfecti areanamorphic fungi, ormitosporic fungi, but these are terms without taxonomic rank. Examples are Alternaria, Colletotrichum, Trichoderma etc. The classPhycomycetes ("algal fungi") has also been used.
Although Fungi imperfecti/Deuteromycota is no longer formally accepted as ataxon, many of the fungi it included have yet to find a place in modern fungal classification. This is because mostfungi are classified based on characteristics of the fruiting bodies and spores produced during sexual reproduction, and members of the Deuteromycota have been observed to reproduce only asexually or produce no spores.
Mycologists formerly used a unique dual system of nomenclature in classifying fungi, which was permitted by Article 59 of theInternational Code of Botanical Nomenclature (the rules governing the naming of plants and fungi). However, the system of dual nomenclature for fungi was abolished in the 2011 update of the Code.[1]
Under the former system, a name for an asexually reproducing fungus was considered aform taxon. For example, the ubiquitous and industrially important mold,Aspergillus niger, has no known sexual cycle. ThusAspergillus niger was considered a form taxon. In contrast, isolates of its close relative,Aspergillus nidulans(Eidam) G. Winter 1884, revealed it to be the anamorphic stage of ateleomorph (theascocarp or fruiting body of the sexual reproductive stage of a fungus), which was already namedEmericella nidulans. When such a teleomorphic stage became known, that name would take priority over the name of ananamorph (which lacks a sexual reproductive stage). Hence the formerly classifiedAspergillus nidulans would be properly calledEmericella nidulansVuill. 1927 – note there's no reference to the original author. The system since 2013 instead treats both as the same species typified by the anamorph, and hence the author citation would include the original author asEmericella nidulans(Eidam) Vuill. 1927[2]: Appendix.F.8.1.Ex.2
Phylogenetic classification of asexually reproducing fungi now commonly usesmolecular systematics.Phylogenetic trees constructed from comparative analyses ofDNA sequences (such asrRNA ormultigene phylogenies) may be used to infer relationships between asexually reproducing fungi and their sexually reproducing counterparts. With these methods, many asexually reproducing fungi have now been placed in the tree of life.[3]
However, because phylogenetic methods require sufficient quantities of biological materials (spores or fresh specimens) that are from pure (i.e., uncontaminated) fungal cultures, for many asexual species their exact relationship with other fungal species has yet to be determined. Under the current system of fungal nomenclature,teleomorph names cannot be applied to fungi that lack sexual structures. Classifying and naming asexually reproducing fungi is the subject of ongoing debate in the mycological community.[citation needed]
These groups are no longer formally accepted because they do not adhere to the principle ofmonophyly.[citation needed] The taxon names are sometimes used informally. In particular, the term 'hyphomycetes' is often used to refer to molds, and the term 'coelomycetes' is used to refer to many asexually reproducing plant pathogens that form discrete fruiting bodies.
Following, a classification of the Fungi imperfecti: Saccardo et al.(1882-1972)[4]
Other, according to Dörfelt (1989):[5]
Other systems of classification are reviewed by (Kendrick 1981).
Many fungi imperfecti are unable to reproduce in a sexual way because of identified mutations that disable their ability to sexually reproduce, others may be sterile due to epigenetic changes. These kinds of changes can arise very quickly in laboratory conditions that force 10–19 generations of exclusive asexual reproduction. Female-sterile strains ofMagnaporthe oryzae appear to have a selective advantage in the form of faster growth with more efficient conidia transfer.[13]
Nieuwenhuis and James (2016) also discusses the costs and benefits for sexual and reproduction in fungi as well as mechanisms that have evolved to reduce the costs of either. They also describe the so-called "parasexual cycle" in some fungi imperfecti, which allows recombination without sex.[14]
