What happened with Chaetomium?A comment and update.

What happened with Chaetomium?
A comment and update.

The genus Chaetomium is well-known for its elaborate ascomata, as beautifully illustrated in a series of publications by X.W. Wang and coworkers (Wang et al. 2016a,b, 2019a,b, 2022). The genus was described in 1817 by Gustav Kunze and typified with C. globosum. Members of the genus can easily be recognized by their ascomata covered with long, often branched or curled hairs. With this characteristic, around 430 names were introduced, and 270 of these were accepted in modern literature. Species were identified by the structure of the ascoma wall, ascomatal hairs, asci shape, ascospore germ pores, and the growth temperature. The genus Chaetomium has a worldwide distribution and resides in a wide range of substrates including cellulose-rich materials, dung, soil, and indoor environments. Numerous species gained attention because of their ability to produce an arsenal of enzymes and metabolites that have antimicrobial, anticancer, antioxidant, or anti-inflammatory activities. On the other hand, some species were reported to cause human disorders including allergic, superficial, subcutaneous, and even deep-seated infections. Out of the 270, only eight chaetomium-like species are included in the Atlas. Others can safely be used in industry and agriculture.

The relationship of Chaetomium is wider, with a family Chaetomiaceae that was introduced in 1885. It first accommodated genera that were phenotypically similar to ascosporulating Chaetomium, but now many fungi are added on a molecular basis, irrespective of morphology and presence of spores or conidia. As a result, the number of genera in this family has increased to 30 (see Table). DNA data showed that also the strictly hyphal genus Madurella, agent of human eumycetoma, is a close relative. From 2016 and onwards, Chaetomium and it is allies were dissected along phylogenetic lines by Wang et al. (2016a,b, 2019a,b, 2022). The authors first revised the C. globosum complex using multigene phylogeny of five markers and concluded that not only morphology, but also ribosomal markers of ITS and LSU were insufficient to delimit taxa in Chaetomiaceae. With this increased level of precision, a similar approach was followed with extensive studies of indoor and thermophilic species, and with large genera that were classically described without ascospores, such as Humicola. This led to the acceptance of 50 genera with 275 species by Wang et al. (2022). Thus, the number of genera in Chaetomiaceae had almost doubled, although the number of species remained the same. For many species, novel genera were described and a lot of names were changed. The number of species in the genus Chaetomium sensu stricto was reduced from 270 to 44. 

An implication of the above taxonomic changes is that almost all clinical Chaetomium species changed their names now, except for C. globosum (see Table below). Thielavia terrestris, reported from a cerebral infection, was changed to Thermothielavioides terrestris and Thielavia subthermophila moved to the genus Canariomyces. This was done because the type species of the genus Thielavia, T. basicola, appeared to be a member of the order Melanosporales.

The Chaetomiaceae is not the only group which appears nomenclaturally unstable. In the blog “Why fungal names are changing?” we elucidated the reasons for changing names by taxonomists. One of these reasons is that sequence data demonstrate that superficially similar fungi may be phylogenetically unrelated and thus cannot be maintained in a single genus. We gave examples in the blog “What are Coelomycetes?”, where we showed that in the genus Phoma only five or six species are currently maintained, instead of stunning number of 3292 names mentioned in Index Fungorum. Thus, fungi with classical phoma-like morphology can no longer be identified as Phoma, and routine identification requires sequencing.

Phylogenetic studies are likely to be affected by sampling: more species included, then rarely the structure of the resulting tree remains the same. For our Atlas policy we prefer to be conservative with changing names in groups where nomenclature is still unstable. Therefore, several of the new chaetomium-like genera are not applied in the Atlas, where the species are still listed under Chaetomium. In all cases, we mention both original and new name, as well as all synonyms used in the medical literature, as a convenience for users. We therefore advise to use “advance search” button in the online Atlas in case a particular name can’t be found with “chapter search”. Furthermore, care should be taken when ITS or LSU is used for identification of Chaetomium or relatives. The β-tubulin (TUB2) and RNA polymerase II (RPB2) genes are currently judged to be superior in delimiting species as well as for routine identification.

Generic names in 2015Generic names in 2021
Crassicarpon(introduced in 2015)
Pseudocanariomyces(introduced in 2021)
Names in AtlasNames according to Wang et al.
Chaetomium atrobrunneumAmesia atrobrunnea
Chaetomium brasilienseOvatospora brasiliensis
Chaetomium funicolaDichotomopilus funicola
Chaetomium globosumChaetomium globosum
Chaetomium murorumBotryotrichum murorum
Chaetomium perlucidumParachaetomium perlucidum
Chaetomium strumariumAchaetomium strumarium


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