Eurotiomycetes: Small but Significant Fungi

Mention the word 'fungi' and most people's thoughts will probably go to images of mushrooms or toadstools. A few may conjure up pictures of lichens. Nevertheless, the great majority of fungal species are microscopic and likely to pass unremarked by most observers. That does not, however, mean that they are of no consequence. Today's post involves one major group that, for all their visual insignificance, include some of the most significant fungal species for modern human society: the Eurotiomycetes.

Developmental stages of Aspergillus glaucus, with cleistothecia as figs 21–23, from Raper & Fennel (1965).

The class Eurotiomycetes has been recognised in recent years as including a diverse assemblage of fungi, associated with a wide range of morphologies and habitats, that are united as a clade by molecular analyses. Réblová et al. (2017) recognised five subclasses within the Eurotiomycetes of which the two largest (or at least the most studied) are the Eurotiomycetidae and the Chaetothyriomycetidae. The Eurotiomycetidae are, for the greater part, saprobes. They were largely recognised as a distinctive group even before the advent of molecular phylogenetic analysis owing to the production by sexually reproducing forms of a distinctive type of fruiting body, the cleistothecium. In cleistothecia, the fruiting body is completely enclosed with no openings to faciliatate the release of spores, which only escape when the fruiting body itself breaks down. Cleistothecia are most commonly produced by fungi that grow in enclosed locations such as underground (the Eurotiomycetidae are not the only group of fungi to produce cleistothecia though they are one of the most diverse). Within the cleistothecium, spores develop within globular asci with a single wall that breaks down shortly after maturity (Geiser et al. 2015).

Penicillium expansum on rotting pear, copyright H. J. Larsen.

For many people, though, the most familiar members of the Eurotiomycetidae are likely to be asexually reproducing forms. This is the clade containing the moulds of the genera Aspergillus and Penicillium. Even before a species of the latter achieved fame as the shource of the first known antibiotic, penicillin, members of these genera had a great impact on human lives. Species of Penicillium are the moulds used in the production of cheeses such as Roquefort and camembert. Species of Aspergillus are used to ferment soy beans and rice in the production of comestibles such as soy sauce and sake. On the flip side, a number of species of Eurotiomycetidae act as pathogens of mammals including humans, causing conditions such as respiratory illnesses or tinea, with the former being of particular concern in immunocompromised individuals. Eurotiomycetid moulds may also cause problems for food storage and the like, particularly as many species are capable of growing under remarkably hot and/or dry conditions. Some Aspergillus moulds produce dangerous toxins, capable of causing acute poisioning or cancer development.

Verrucaria maura, copyright Richard Droker.

The Chaetothyriomycetidae are less clearly defined morphologically than the Eurotiomycetidae but fruiting bodies are mostly produced as perithecia: flask-shaped structures with an apical pore through which spores are released. The asci within the perithecium usually possess a double wall. Like many eurotiomycetids, chaetothyriomycetids have a tendency to be associated with habitats where water availability is a concern such as in very dry and/or saline environments. A number of chaetothyriomycetid species form lichens. One genus, Verrucaria, is often found as a thin black lichen growing on rocks along the seashore. Some species grow within the cavities of myrmecophytes, plants that form mutualistic associations with ants (the plant provides food and/or accomodation for the ants and the ants help keep the plant clear of grazers or sap-suckers). The fungi are cultivated by the ants that use them for food.

The other three subclasses of the Eurotiomycetes are less well known and recognised as containing a single order each. The Sclerococcales were first recognised as such by Réblová et al. (2017) via molecular analysis. Fruiting bodies, where known, are apothecia (open bowls) bearing single-walled asci. Representatives are known from marine and terrestrial habitats, growing on wood or lichens, and some have been isolated from within the digestive tracts of bark beetles. The Coryneliaceae, living as parasites on podocarps, have been considered as morphologically intermediate between chaetothyriomycetids and eurotiomycetids. Molecular analysis positions them as sister to the latter (Wood et al. 2016). Finally, the Mycocaliciales live as parasites or commensals of other fungi, particularly lichens.

There are other representatives of the Eurotiomycetes that I haven't even had the time to gloss over, such as endophytes and ectomycorrhizal truffles. You may not know they're there but that doesn't mean they don't mean anything to you.

REFERENCES

Geiser, D. M., K. F. LoBuglio & C. Gueidan. 2015. Pezizomycotina: Eurotiomycetes. In: D. J. McLaughlin, & J. W. Spatafora (eds) The Mycota 2^nd ed. vol. 7. Systematics and Evolution part B pp. 121–141. Springer-Verlag: Berlin.

Réblová, M., W. A. Untereiner, V. Štěpánek & W. Gams. 2017. Disentangling Phialophora section Catenulatae: disposition of taxa with pigmented conidiophores and recognition of a new subclass, Sclerococcomycetidae (Eurotiomycetes). Mycological Progress 16: 27–46.

Wood, A. R., U. Damm, E. J. van der Linde, J. Z. Groenewald, R. Cheewangkoon & P. W. Crous. 2016. Finding the missing link: resolving the Coryneliomycetidae within Eurotiomycetes. Persoonia 37: 37–56.