Tom Volk's Fungus of the Month for February 2000

This month's fungus is Schizophyllum commune, the split gill fungus, perhaps the world's most widespread fungus-- and possessor of over 28,000 different sexes

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Schizophyllum tops and underside People often ask me how I choose the fungus of the month. Usually I try to pick a fungus that can be found during a particular month or has something to do with a holiday in that month. I chose this month's fungus based on a phone call I got from ABC News in New York just a couple days before I'm writing this. They called me because they were planning on doing a story for Valentine's Day on sex in fungi. (I'm not making this up!). They were only in the preliminary planning stages, and had tentatively planned on airing the story on February 11. If you want a fun time, try explaining sex in fungi to reporters someday-- over the phone. They actually got it pretty well, I thought. I told them I would put up a web page dealing with sex in fungi, so I chose Schizophyllum commune as the fungus of the month-- since it is known to have more than 28,000 distinct sexes.

older Schizophyllum fruiting bodiesBut first a little about Schizophyllum commune. It is probably the most widespread fungus in existence, being found on every continent except Antarctica, where there is no wood to be used as a substrate. There is a single common worldwide species, although there are a few less common species of Schizophyllum. The genus name means "split gill," and thus this is the split gill fungus. It does not appear to be very closely related to the other gilled mushrooms, and most researchers place it in its own order the Schizophyllales. The gills function to produce basidiospores on their surface. They appear to be split because they can dry out and rehydrate (and thus open and close) many times over the course of a growing season. The fruiting bodies to the right are probably a year old or more. This is a great adaptation for a climate with sporadic rains. Unlike other mushroom species, the mycelium only has to produce one set of fruiting bodies per year, which can then dry out and rehydrate and keep functioning. It's a great strategy for reproduction. You can probably even go out in the dead of winter and find sporulating fruiting bodies of this fungus. It's a very successful wood decay fungus that causes a white rot. Interestingly, this fungus has also been known to cause a human mycosis in just a few cases involving immunoincompetent people, especially children. In one case, the fungus had grown through the soft palate of a child's mouth and was actually forming fruiting bodies (mushrooms) in her sinuses!!!

We know that there is a single widespread species because of the work of John Raper and his colleagues at Harvard University in the 1950'2-1970's. They collected worldwide samples of this fungus. After collecting and germinating the spores into mycelium, they were able to get individuals from all over the world to mate with one another. During that time they were also able to divide the species in mating types (sexes) based on their mating reactions. As long as two strains are of different mating types they are able to mate and form fertile offspring.

general nuclear cycle of fungiIn order to understand something about sex in fungi, we need to look at the general nuclear cycle of fungi. Comparing this to the life cycle of an animal or plant you'll notice some big differences. For the most part fungi are either haploid, with one set of chromosomes in a nucleus, or dikaryotic, with two sets of chromosome, each set in a separate nucleus. Most animals are diploid, with two sets of chromosomes in a single nucleus. You'll notice the three major steps in the life cycle of a fungus to the left are plasmogamy (joining of cytoplasm from two parents), karyogamy (fusion of the two parental nuclei) and meiosis (reduction division, returning to the haploid state.) In most fungi the diploid consists of only a single cell in the life cycle. There are four phyla of fungi, each of which is distinguished by its sexual reproductive structures and the amount of time spent in each of the phases. Notice that in animals and in plants we usually talk about plasmogamy and karyogamy as a single event called fertilization, since the two events occur in rapid succession. In fungi plasmogamy may be separated from karyogamy for several minutes up to several centuries! This means that in some fungi the dikaryon may be the most long-lived part of the life cycle.

Schizophyllum on a seed in Puerto RicoIt doesn't sound very romantic in a Valentine's Day month, but sex in fungi primarily involves getting the two parental nuclei into the same cytoplasm. Many fungi, like Schizophyllum, don't even have differentiated sex organs! Wherever they touch they can exchange nuclei. There are genetic controls over which individual a fungus may "choose" for its mate. Many of the more primitive fungi have only two sexes. In some cases we can distinguish male from female gametes or gametangia, but in most cases they are morphologically identical. In these cases we must call them + and -, or A and a, or 1 and 2. Thus any spore from one mating will be sexually compatible with half of its siblings or half of the population. The mating type is determined in this case by a single genetic locus with two alleles.

Some species of fungi (and probably some slime molds) have gone one step farther, having multiple alleles at this single locus. For example in a population, a fungus may have three alleles at a locus, designated A1, A2, and A3. Any given spore would still be compatible with half its siblings, but compatible with 2/3 of the population in general. This encourages a spore not to mate with its siblings, or encourages outbreeding in a population. These numbers would change as the number of alleles at that locus increases in the population.

Another advancement has taken place in some fungi-- using two genetic loci to determine mating type. These are designated as the A and B loci (pretty clever, those geneticists...). Let's assume for the sake of discussion that there are two alleles at each locus and let's assume that two haploids are coming together for a mating. (dim the lights please...) We'll designate the first parental strain as A1B1 and the second as A2B2. As long as the mating types differ at both alleles, a mating can take place.

A1B1 X A2B2

Since the A and B loci are genetically unlinked (on different chromosomes), independent assortment takes place during meiosis, and the haploid offspring of this mating would have one of the following mating types.

A1B1, A2B2 and the new combinations A1B2 and A2B1.

Thus any of the offspring would be compatible with only 1/4 of its siblings, a significant improvement over the single locus system. In reality there are usually more than two alleles at any one locus. Each individual is compatible with any individual that is different at both loci. In Schizophyllum commune there are more than 300 alleles at the A locus and more than 90 known for the B locus. Thus there are more than 28,000 different combinations of A and B, or 28,000 different sexes! Each individual is compatible with 27,997 of the others in the worldwide population (99.98% outbreeding) compared with being compatible with only 1/4 of its siblings. Thus the enormous number of sexes in fungi is meant to encourage non-sibling mating and non-relative mating, which ensures genetic diversity in the population. This seems to have worked quite well in the widely distributed Schizophyllum.

Schizophyllum communeThere are probably other examples of fungi with more mating types, but Schizophyllum has been the best studied because of John Raper's work. Incidentally, John Raper is my academic great grandfather-- I got my Ph.D. from Tom Leonard at the University of Wisconsin-Madison (he's now at Clark University), who got his Ph.D. from Stanley Dick at Indiana University, who got his Ph.D. from John Raper at Harvard. If you'd like to see some other interesting Mycological genealogies see this page by Meredith Blackwell and Bob Gilbertson. I'm in the Farlow lineage.

I hope you enjoyed learning something about a very widespread fungus, and something about sex in fungi. Mating type studies in fungi are still ongoing and there's much more to be learned about the subject, especially on a DNA and enzyme level.

If you have anything to add, or if you have corrections or comments, please write to me at

This page and other pages are © Copyright 2000 by Thomas J. Volk, University of Wisconsin-La Crosse.

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