Photo by Landon Parenteau on Unsplash

Why having 27,000 mating types makes dating easier for one funky fungus.

Schizophyllum commune, also known as the split-gill mushroom, may be the most widely distributed and common fungal species on the planet. Not only this, but the intricacies of its reproductive and degradative systems are incredible and unique characteristics which have helped cement it as an important fungal model organism. Here, I’ll present an array of current knowledge about S. commune, as well as diving deeper into some of its most intriguing features.

In the 1950s, the mycologist Dr John Raper of Harvard University conducted some of the most important early studies into the sexual reproduction of S. commune. He discovered that spores germinated from samples from all over the world could successfully mate with each other, demonstrating the species’ global distribution. The only barrier to successful mating was a feature called mating type, which at that point was undefined. Since the gametes are morphologically indistinguishable, mating pairs were given + and – types rather than being labelled by sex. These were arbitrarily described by the allele present on a single locus, which was believed to exist in one of two states. Further research into the sexual compatibilities of these individuals and other species revealed that in a number of species including S. commune, not only could there be two or more separate loci involved in determination of mating type, but these loci could also have hundreds of unique alleles, resulting in huge numbers of distinct identities. It was discovered that in S.commune, mating type is governed by two separate loci, matA and matB. The first of these has roughly 300 alleles, and the second has around 90. This means the number of possible mating types is a staggering 300 x 90 = 27000.

The next question to ask is what is the point of having this many sexual identities? As discussed in the episode of The Curios Cases of Rutherford and Fry listed in further reading, when individuals of species with only two sexes meet, there is a 50% chance they can reproduce to give viable offspring: they’re either the same sex or not. In the case that meeting or mating with another individual of your species is rare, any changes to DNA that increase the chance of successful reproduction may prove beneficial, and will thus spread through the gene pool by natural selection. In this way, the many sexual identities of S. commune increase the chance that it can successfully mate with another individual of its species: in some situations, it can play the + partner, and in other situations the – partner. S. commune isn’t unique in this attribute; fungi reproduce in a range of incredible ways, but currently S. commune is currently thought to have the greatest variation of genes in mating type loci of any fungal species. The four major fungal phyla (a taxonomic group) are defined by their distinct methods of producing sexual spores. S. commune is a member of the Basidiomycota, which are defined by their externally borne spores on a club-like structure (the basidium).

Having received extensive study into its reproductive methods, S. commune emerged as an especially interesting model fungus for further characteristics. One particular attribute which stands out as unique is its wood-degrading capabilities by the process of white-rot, which degrades all cell wall components, compared to brown-rot which only degrades cellulose and not lignin. This prompted the sequencing of its genome by Ohm and colleagues in their 2010 Nature Biotechnology article (see further reading). By studying the roughly 13000 genes of S. commune, 16 genes encoding a diverse assortment of Lignin-degrading enzymes (commonly named FOLymes) were identified, and a further 11 genes encoding further enzymes related to FOLymes. They also concluded that S. commune has the most powerful cellulose-degrading machinery of any basidomycetes studied to date, and after searching the Carbohydrate-Active Enzyme database (CAZy) more than 350 candidate cellulose-degrading enzymes were identified.

Understanding the white-rot formation of S. commune will be crucial to further understanding the mechanisms of fruit formation and natural degradation by mushroom-forming fungi. This has important implications for both the commercial production of edible mushrooms, as well as their industrial use as cell factories, which may become a focus of synthetic biology efforts in coming years.

Further Reading:

• I first heard of this topic on a recent episode of my favourite podcast, The Curious Cases of Rutherford and Fry: The Good and Bad in Fungi. Here Dr Adam Rutherford and Dr Hannah Fry discuss a range of fungi topics with humour and wit. I also recommend listening to other episodes since they’re great fun!

• This fungus fact-file by Tom Volk at the University of Wisconsin gives a good overview of Schizophyllum commune, with a number of key concepts in a digestible format. Consider that it was written in the year 2000, so some information may have since been updated – it is nevertheless worth a read.
• Finally, this important 2010 Nature Biotechnology paper by Ohm et al. describes the sequencing of the Schizophyllum commune genome, and a number of things we can learn from it. It really gets into fungal genetics in detail and I found it a great introduction to the field.

Written by Joshua Williams for the UCL Genetics Society:

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