# Wallemiomycetes
**Wallemiomycetes** is a class of basidiomycete fungi erected in 2005 for the xerophilic mold genus *Wallemia* and its relatives, recognized on the basis of a distinctive mode of conidiogenesis (how a fungus forms and releases its asexual spores) and extreme tolerance of low water activity[1](https://pubmed.ncbi.nlm.nih.gov/15928984/). The class contains the order Wallemiales and the family Wallemiaceae, and sits within the subphylum Wallemiomycotina of the phylum [Basidiomycota](https://www.edgepedia.org/basidiomycota)[2](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=148960). Its species are among the most salt- and sugar-tolerant eukaryotes known, and one member, *Wallemia ichthyophaga*, is described as the most halophilic fungus known to date[3](https://pubmed.ncbi.nlm.nih.gov/24034603/).
| Key fact | Detail |
|---|---|
| Rank and authorship | Class Wallemiomycetes Zalar, de Hoog & Schroers, 2005, with the order Wallemiales, in the phylum Basidiomycota[1](https://pubmed.ncbi.nlm.nih.gov/15928984/) |
| Higher placement | Subphylum Wallemiomycotina; lineage: Basidiomycota, Wallemiomycotina, Wallemiomycetes, Wallemiales, Wallemiaceae, *Wallemia*[2](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=148960) |
| Defining traits | Unique conidiogenesis with basauxic fertile hyphae and conidial units disarticulating into mostly four arthrospore-like conidia; extreme xerotolerance[1](https://pubmed.ncbi.nlm.nih.gov/15928984/) |
| Species at establishment | Three species segregated in 2005: *W. ichthyophaga*, *W. sebi*, and *W. muriae*[1](https://pubmed.ncbi.nlm.nih.gov/15928984/) |
| Divergence times | Subphylum Wallemiomycotina estimated to have split away approximately 487 million years ago; whole-proteome analysis dates Wallemiomycetes as a 250-million-year-old sister group of Agaricomycotina[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC6027281/) • [3](https://pubmed.ncbi.nlm.nih.gov/24034603/) |
| Record physiology | *W. ichthyophaga* requires at least 10% NaCl and thrives in saturated salt solution[3](https://pubmed.ncbi.nlm.nih.gov/24034603/) |
| Compact genome | *W. ichthyophaga* genome 9.6 Mb with 1.67% repetitive sequences and 4,884 predicted protein-coding genes, among the smallest observed for fungi[3](https://pubmed.ncbi.nlm.nih.gov/24034603/) |
## Taxonomic history
The genus *Wallemia* was introduced by Johan-Olsen for *W. ichthyophaga*, discovered in 1885 on dried salted fish sent for examination by a fishery inspector named Mr. Wallem; *W. ichthyophaga* remained the only recognized species until von Arx recognized *Sporendonema sebi* as congeneric in 1970[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4446336/). The taxonomic position of the genus remained uncertain for over a century. Terracina showed dolipore-like septal structures in *W. sebi*, and Moore interpreted these as a special kind of parenthesome, describing the family Wallemiaceae, first classified in the Filobasidiales within the Basidiomycota[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC4370657/).
**Resolution by sequencing.** Zalar, de Hoog, Schroers, Frank, and Gunde-Cimerman applied nuclear small subunit ribosomal DNA (SSU rDNA) analysis and found that *Wallemia* belonged to a highly supported clade with [Ustilaginomycetes](https://www.edgepedia.org/ustilaginomycetes) and Hymenomycetes, in an isolated position distantly related to the Filobasidiales and marked by numerous nucleotide substitutions not shared by any other fungus[1](https://pubmed.ncbi.nlm.nih.gov/15928984/). To acknowledge its unique morphology, evolution, and xerotolerance, they proposed the new class Wallemiomycetes covering the new order Wallemiales[1](https://pubmed.ncbi.nlm.nih.gov/15928984/). The same study segregated three species on differences in conidial size, xerotolerance, and ITS rDNA sequence, proposing the combination *Wallemia muriae* for *Torula epizoa* var. *muriae* and neotypifying all three[1](https://pubmed.ncbi.nlm.nih.gov/15928984/).
The class then remained monogeneric until Nguyen and colleagues added two genera, *Geminibasidium* and *Basidioascus*, in a new order Geminibasidiales tentatively classified in Wallemiomycetes, although relatively distant from *Wallemia*[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4446336/). Later treatments have variously ranked these three genera as distinct orders or classes, with *Wallemia* as sister to *Basidioascus* + *Geminibasidium*[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC6899921/).
## Phylogenetic position
Published studies have placed Wallemiomycetes at the base of the Basidiomycota, as a sister group to Agaricomycotina or Ustilaginomycotina or both, or simply as incertae sedis within the phylum[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC3849046/). A phylogenomic analysis aligning the whole proteomes of 14 fungi resolved the position of *Wallemia* as a sister group to, or the earliest diverging lineage of, Agaricomycotina, and supported Pucciniomycotina as the earliest diverging lineage among the Basidiomycota analyzed[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC3849046/). A 71-protein dataset reached the same placement, with Ustilaginomycotina as sister to the branch uniting Wallemiomycetes and the remaining Agaricomycotina[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC6027281/). Genome-wide datasets preferentially support the sister relationship with Agaricomycotina, while ribosomal markers have given other topologies[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC6899921/).
The distinctiveness of the lineage was emphasized by its allocation to the newly distinguished subphylum Wallemiomycotina, estimated to have split away approximately 487 million years ago[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC6027281/). The same whole-proteome chronogram dated the Wallemiomycetes stem as a 250-million-year-old sister group of Agaricomycotina[3](https://pubmed.ncbi.nlm.nih.gov/24034603/). A 2024 phylogenomic outline of Basidiomycota treated Wallemiomycetes as one of the classes of the phylum within Wallemiomycotina[9](https://gredos.usal.es/bitstream/handle/10366/161602/He%282024%29_Phylogenomics%2C%20divergence%20times%20and%20notes%20of%20orders%20in%20Basidiomycota.pdf?isAllowed=y&sequence=1).
## Ecology and physiology
On media with low water activity, *Wallemia* forms brown, powdery colonies, and the fungus is regarded as xerophilic or, at minimum, xerotolerant.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC4370657/) The genus is known from air, soil, dried food, and salt, and can be isolated from the hypersaline water of man-made salterns on different continents[1](https://pubmed.ncbi.nlm.nih.gov/15928984/). Tests on xerotolerance indicated that *Wallemia* presents one of the most xerophilic fungal taxa, and xerotolerance is otherwise rare in the Basidiomycota[1](https://pubmed.ncbi.nlm.nih.gov/15928984/).
The three species differ in their tolerance profiles. *W. sebi* can grow in a wide range of water activities, *W. muriae* grows only on media with slightly reduced water activities, and *W. ichthyophaga* is obligately halophilic, requiring water-activity-lowering solutes for in vitro growth[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933). *W. ichthyophaga* requires at least 10% NaCl and thrives in saturated salt solution, and counterbalances the osmotic pressure of the surrounding medium by intracellular accumulation of a mixture of polyols, with glycerol as the major osmotically regulated solute[3](https://pubmed.ncbi.nlm.nih.gov/24034603/) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC3849046/). Morphological responses to high sugar concentrations differ from adaptation to high salt: *W. ichthyophaga* grew slowly in all sugar-based media, while *W. muriae* and *W. sebi* grew better[11](https://pubmed.ncbi.nlm.nih.gov/23931114/).
## The genus Wallemia and its species
Beyond the three species recognized in 2005, a multi-locus phylogenetic approach resolved four additional species: *W. mellicola*, *W. canadensis*, and *W. tropicalis*, closely related to *W. sebi*, and *W. hederae*, phylogenetic sister of *W. ichthyophaga*[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC6027281/).
**The W. sebi species complex.** Previous ITS analyses had suggested that *W. sebi* was genetically heterogeneous, with at least two subclades[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933). Multi-locus analyses using ITS and the protein-encoding genes MCM7, TSR1, RPB1, RPB2, and the new marker HAL2 confirmed that *W. sebi* presents a complex of at least four species, resolved as four strongly supported clades[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933). The revision recognized *W. sebi* sensu stricto and described three new species, *W. mellicola*, *W. canadensis*, and *W. tropicalis*; the species differ in conidial size, xerotolerance, halotolerance, chaotolerance (tolerance to MgCl2), growth temperature regimes, extracellular enzyme activity profiles, and secondary metabolite patterns[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933). Strains of the clade containing *W. sebi* sensu stricto typically originate from sea salt, hypersaline water, and air and dust in indoor environments worldwide[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4446336/).
## Genomics and health relevance
The genome of the *W. ichthyophaga* type strain is unusually compact: 9.6 Mb with only 1.67% repetitive sequences, and 4,884 predicted protein-coding genes covering almost three quarters of the sequence, figures among the smallest observed for fungi[3](https://pubmed.ncbi.nlm.nih.gov/24034603/). Of 639 differentially expressed genes across salinity limits, two thirds were more expressed at lower salinity[3](https://pubmed.ncbi.nlm.nih.gov/24034603/). Contrary to the closely related *W. sebi*, *W. ichthyophaga* appears to have lost the ability for sexual reproduction[3](https://pubmed.ncbi.nlm.nih.gov/24034603/). The species also has an unusually thick cell wall, reported as a possible mechanism of salt adaptation[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC3849046/).
**Health relevance.** *W. sebi* is the most frequently isolated and best-studied species, commonly involved in the spoilage of food with low water activity; its air-disseminated spores have been associated with allergies, bronchial asthma, and farmer's lung disease[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933). [Human antibodies](https://www.edgepedia.org/human-antibodies) react to compounds produced by *W. sebi* spores, and metabolites present in spore and mycelial fragments caused inflammation and affected lung biology in an in vivo mouse model[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933). The species produces the metabolites walleminol, walleminone, the azasteroid UCA1064-B, and the highly toxic wallimidione; walleminol is found in food contaminated by *W. sebi* and has an LD50 of 40 μg/ml for brine shrimp and a minimum inhibitory dose of 50 μg/ml for rat liver cells[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933). Species of *Wallemia* are among the few osmophilic fungi that can contaminate salted and dried cod[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4446336/).
## References
1. [Zalar P, de Hoog GS, Schroers H-J, Frank JM, Gunde-Cimerman N (2005). Taxonomy and phylogeny of the xerophilic genus Wallemia (Wallemiomycetes and Wallemiales, cl. et ord. nov.). Antonie van Leeuwenhoek 87: 311–328.](https://pubmed.ncbi.nlm.nih.gov/15928984/)
2. [NCBI Taxonomy Browser: Wallemiomycetes](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=148960)
3. [Zajc J and colleagues (2013). Genome and transcriptome sequencing of the halophilic fungus Wallemia ichthyophaga.](https://pubmed.ncbi.nlm.nih.gov/24034603/)
4. [The Genus Wallemia—From Contamination of Food to Health Threat, Microorganisms (2018)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6027281/)
5. [Jančič S and colleagues (2015). A Taxonomic Revision of the Wallemia sebi Species Complex (PMC version)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4446336/)
6. [Application of the Phylogenetic Species Concept to Wallemia sebi from House Dust and Indoor Air (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4370657/)
7. [Fungal evolution: diversity, taxonomy and phylogeny of the Fungi, Biological Reviews (2019)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6899921/)
8. [Genome and transcriptome sequencing of the halophilic fungus Wallemia ichthyophaga (PMC full text)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3849046/)
9. [He and colleagues (2024). Phylogenomics, divergence times and notes of orders in Basidiomycota. Fungal Diversity.](https://gredos.usal.es/bitstream/handle/10366/161602/He%282024%29_Phylogenomics%2C%20divergence%20times%20and%20notes%20of%20orders%20in%20Basidiomycota.pdf?isAllowed=y&sequence=1)
10. [Jančič S and colleagues (2015). A Taxonomic Revision of the Wallemia sebi Species Complex. PLoS ONE 10(5): e0125933.](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125933)
11. [Morphological responses to high sugar concentrations differ from adaptation to high salt concentrations in the xerophilic fungi Wallemia spp. Fungal Biology (2013).](https://pubmed.ncbi.nlm.nih.gov/23931114/)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Fungi and mycology › Basidiomycete taxa › Other basidiomycete classes › Wallemiomycetes*
*Initially written Sep 17, 2026 · Reviewed: Sep 19, 2026; Oct 21, 2026 · Edited: Sep 19, 2026; Oct 11, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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