Basidiomycetes and Ascomycetes

Last month, the Cascade Mycological Society asked me to do an educational piece of art for a fungi-themed show they were were organizing. I decided that I wanted to clearly illustrate some of the differences between Basidiomycota and Ascomycota, particularly with regards to reproductive structures. The show, and this painting, is now hanging up at Morning Glory Cafe!


 To help, I’ve written a limerick for each close up frame. Starting at the upper left, and proceeding clockwise:

Doctor Büller peered down at a spore
then gave a great shout and he swore:
“by God, that wee drop-lit
must be what propels it;
surface tension’s no force to ignore!”

The first close up pulls us into a small section on the edge of a gill; we see a line of basidia (singular = basidium), the reproductive structures of the Basidiomycota. Each basidium caries four basidiospores on the ends of little projections (called sterigmata for those of you that like to know the jargon). The limerick (adapted from Dr. Jeff Stone) makes reference to Büller’s Drop — the Surface Tension Catapult! Basidiomycetes have evolved a sophisticated, active dispersal mechanism for their spores that relies on the energy of a water droplet straining against its own surface tension: condensation builds up a drop on the end of each spore, and when it reaches a certain size, it joins with a drop on the other end, rapidly shifting the center of gravity of the drop-covered spore, catapulting it off the sterigma! 

Use the microscope, don’t be shy,
the fertile surface to magnify:
spores in little sacks,
Like tiny bivouacs.
The ascospores inside the asci!

Here, we’re looking at the reproductive structures of the Ascomycota up close and personally: asci (singular = ascus). Ascospores are contained within the asci; there are usually eight spores in each one, though this can be different in different species, but it’s always a multiple of four (the usual number of basidiospores on a basidium), because the starting point is the result of meiosis, the sexual division of a diploid cell into four haploid cells. Meiosis is the root of variation between organisms that sexually reproduce, because it mixes up the chromosomes and leads to novel recombination! After meiosis, there is usually one mitotic (just copying the genes, not mixing them) division, which is why ascomycetes tend to have eight spores per ascus; basidiomycetes don’t have this copying step, so they’re left with only four spores per basidium, usually. Some species of ascomycetes take this mitotic step pretty far, and have hundreds of spores per ascus! The most I’ve personally seen has been 128, but there are species out there that have even more.

Ascomycetes don’t have the active dispersal mechanism that basidiomycetes do, but some of them (in the subphylum Pezizomycotina, mostly) can coordinate releasing the spores out of their asci all together, which generates a cooperative wind, carrying the spores aloft! This is why some cup fungi give off a “puff” of spores when you blow on them, or pick them. Next time you see an orange peel fungus (Aleuria aurantia), blow gently across the surface of the cup, and see if you can see the coordinated release of spores that happens after.

Also, notice that the spore names reflect the reproductive structures: ascospores and basidiospores. What type of spores do you think zygomycetes have?

In the great state of Tennessee
And everywhere else, you see,
(look at the cross-walls,
like doors in long halls)
ascomycetes have simple septae!

In the ascomycetes, most of the thallus (the body of the fungus, which is usually hidden inside of the substrate) is haploid, which means it has only one copy of each chromosome in each nucleus. You, and most animals, are diploid, meaning that you carry two copies of every chromosome: one from each parent. Fungi deal with sex a little differently than animals, though, and do it a bit differently for each different group of fungi, too. Ascomycetes stay haploid most of their lives, and only come together to mate when it’s time to reproduce; two compatible fungi will grow close to each other, and then fuse together, but instead of immediately forming a diploid, the DNA stays packaged into separate nuclei, and each cell gets one of each. This is called being dikaryotic, from the same root as the word eukaryote, the greek word for seed. Being dikaryotic means that there are two nuclei — it’s not quite the same as being diploid, because the nuclei are separate still, but it’s close in terms of function. This doesn’t last long, though: the part of the fungus that’s dikaryotic almost immediately starts making reproductive structures, and as the asci develop the nuclei fuse into a diploid finally, only to immediately start the dance of meiosis, separating into four (brand new!) haploids, which serve as the start of spore development.

So, when you see ascomycete hyphae out in the world, it’s nearly always haploid. Because of that, they don’t need any elaborate connections between compartments in the hyphae, just simple septa (or septae, singular = septum). Septa are just walls between compartments; the protect the fungus from losing all of its cytoplasm if the hyphae is damaged, and allow for stuff to move back and forth through the hyphae.

If fusion don’t lead to rejection,
to achieve that hyphal perfection
the nuclei dance
and don’t miss the chance
to form a new clamp connection.

(Read this one as if it were part of a country song — it’s better that way!) Basidiomycetes live life a bit differently than ascomycetes: they spend most of their lives dikaryotic—but not diploid—with two distinct nuclei per hyphal compartment. This achieves much the same effect as diploidy does, giving each unit access to two potentially different copies of every gene in the genome. But, it adds some complexity: when hyphae with only one nucleus grows, the nucleus can just divide, and the septum can form between the two new nuclei. But, a hypha is a pretty narrow space, and when there’re two nuclei, there’s not space for them to divide side-by-side, so the fungus can’t just plop a wall between them, because they’re arranged with the identical copies right next to each other (A-A-B-B, for example); there needs to be some way to rearrange them (A-B-A-B) so that one copy of each ends up on either side of the septum. That’s where clamp connections come in: when the nuclei divide, a septum is laid down in between the rearmost pair, while the nucleus in front divides into a little backwards-pointing offshoot of the hyphae, which drops one copy back behind the new septum. This is what clamp connections are: a way of keeping one of each kind of nucleus in each compartment. If you’re having trouble visualizing that, there’s a pretty good illustration on Wikipedia, actually. Check it out!


Hopefully this helps clarify some of the important differences between ascomycetes and basidiomycetes. Because so many of us think of morels and other ascomycetes as “mushrooms”, I think it’s surprising for people to realize that the division between these two groups is so deep: different phyla! I mean, that’s as different as you are (phylum Chordata, those animals with a hollow dorsal nerve cord) from starfish (phylum Echinodermata) or or roundworms (phylum Nematoda).

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