The Sordariales: In the Soil and Under the Skin

Microfungi are a very important factor in our lives. They play a key role in assuring that we are not literally up to our armpits in shit. Their hungry little hyphae break down ordure, cleaning up the planet and unlocking nutrients that will then be made available to other organisms. And among the most significant lineages of these largely unseen decomposers are the members of the order Sordariales.

Lab culture of Sordaria fimicola, copyright BlueRidgeKitties.

Members of the Sordariales are, without exception, minute. Many species are coprophilous, growing on dung. Others may be found on rotting wood, or other decaying plant matter or soil. Fruiting bodies, when they appear, are flask-shaped perithecia protruding to a greater or lesser degree from the surface of their substrate. The walls of the perithecia are made up of large cells and have a membranous or coriaceous (leathery) texture. Within the fruiting body, the asci are single-walled and contain one- or two-celled ascospores that are often surrounded by a gelatinous sheath or bear various appendages. If the ascospores are two-celled, the cells are typically differentiated into an apical head and a basal tail (Kruys et al. 2015; Marin-Felix et al. 2020). Genera of Sordariales have historically been recognised on the basis of ascospore morphology but the advent of molecular data has indicated that such genera are highly polyphyletic. As a result, the Sordariales have seen (and are still seeing) a great deal of taxonomic reassessment. Miller & Huhndorf (2005) suggested that the structure of the fruiting body walls are more consistent with molecular phylogenies than ascospore morphology.

Cake of oncom-fermented beans, copyright Hariadhi.

Apart from their significant role as decomposers, most Sordariales have little direct impact on human economics. The mould Neurospora intermedia is used to make oncom, a fermented food similar to tempeh. A number of species of Sordariales such as Neurospora crassa and Sordaria fimicola have been widely used in genetic research, to the extent that they have been labelled the 'fruit flies of the fungal world'. Seriously, it's one of those expressions almost every publication seems obliged to crow-bar in somewhere. The analogy is made even more apropos by the fact that one of the most widely used species, Triangularia née Podospora anserina, has been made the subject of debate whether taxonomic considerations should be allowed to shake up the name of a popular model organism.

Molecular studies have also shown that the Sordariales encompass Madurella mycetomatis, a fungus causing subcutaneous inflammation in humans (van de Sande 2012). Seeing as sexual fruiting bodies are unknown in this species, and even asexual spore-producing structures are exceedingly rare, this organism would have previously been all but impossible to classify. Infection by M. mycetomatis is characterised by the production of granular swellings. It is most significant in central Africa but is also known from other tropical regions of the world. Madurella mycetomatis infects people via trauma such as animal bites and other wounds, and it has been isolated from soil and ant nests. In its normal state, M. mycetomatis is probably a quite innocent soil fungus. The trouble comes when it finds itself somewhere it shouldn't be.

REFERENCES

Kruys, Å., S. M. Huhndorf & A. N. Miller. 2015. Coprophilous contributions to the phylogeny of Lasiosphaeriaceae and allied taxa within Sordariales (Ascomycota, Fungi). Fungal Diversity 70: 101–113.

Marin-Felix, Y., A. N. Miller, J. F. Cano-Lira, J. Guarro, D. García, M. Stadler, S. M. Huhndorf & A. M. Stchigel. 2020. Re-evaluation of the order Sordariales: delimitation of Lasiosphaeriaceae s. str., and introduction of the new families Diplogelasinosporaceae, Naviculisporaceae, and Schizotheciaceae. Microorganisms 8: 1430.

Miller, A. N., & S. M. Huhndorf. 2005. Multi-gene phylogenies indicate ascomal wall morphology is a better predictor of phylogenetic relationships than ascospore morphology in the Sordariales (Ascomycota, Fungi). Molecular Phylogenetics and Evolution 35: 60–75.

van de Sande, W. W. J. 2012. Phylogenetic analysis of the complete mitochondrial genome of Madurella mycetomatis confirms its taxonomic position within the order Sordariales. PLoS One 7 (6): e38654.