Chapter 18: Basidiomycota

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Okay, get this.

One of the absolute largest organisms on earth.

It isn't a blue whale, it's not a giant sequoia, it's actually a fungus.

Right, this vast network underground.

Exactly.

We're talking about a single armillaria mycelium deep in a Canadian forest.

They think it's, what, at least 1 ,500 years old?

Yeah, something like that.

Spanning 15 hectares, maybe weighing over 10 tons.

It's just huge, hidden mostly.

Incredible.

And that hidden world is what we're diving into today.

Specifically, this huge group of fungi called the Basidiomycota.

We're using Introduction to Fungi by Webster and Weber, the third edition, as our guide.

It's a great resource.

Dense, but really comprehensive.

It is.

And our mission really is to kind of unpack all that dense science, make it engaging, make it clear for you so you can get a handle on their morphology, life cycles, ecological importance, all that stuff without needing to stare at diagrams.

And it's worth unpacking because honestly understanding these organisms is fundamental.

They're so vital to life on earth, often in ways we just don't see day to day.

It goes way beyond just the mushrooms you might pick up at the store.

Okay, so let's start unpacking.

Basidiomycota.

You said over 30 ,000 species.

That's a lot.

It really is.

And yeah, colloquially, we often just call them Basidiomycetes.

And when most people think fungi, they probably picture something from this group, right?

Like mushrooms.

Absolutely.

Your classic button mushrooms, toadstools.

But also things like bracket fungi you see on trees, those puff balls that explode with spores.

Yeah, kids love those.

Right?

Earth balls, earth stars too, even the kind of weird looking stink horns and those jiggly jelly fungi.

It's a really visually diverse bunch, even just the ones we notice.

But it's not just about the ones we see easily.

What about the less obvious members?

That's where you get into things like the rust and smut fungi.

And these are, well, they're hugely significant, not because we eat them, obviously, but because they're major pathogens.

They attack plants, including really important crops, causing devastating diseases.

Wow.

So you've got these beautiful mushrooms, and then these destructive pathogens.

Yeah.

All in the same group?

How does that work?

It's fascinating, isn't it?

They share fundamental biological traits that core evolutionary heritage, despite looking and acting so differently.

And where do they typically live?

Mostly on land?

Mostly terrestrial, yeah.

And their spores are usually spread by the wind, which helps them get everywhere.

But they're not exclusively land -based.

Some have adapted to freshwater or even marine environments.

Really adaptable then?

Incredibly so.

Okay.

So let's talk about what they do in the world.

Their ecological roles.

This is where it gets really interesting, you said, from vital recyclers to, well, serious threats.

Exactly.

Their role as saprotrophs is fundamental.

That means they feed on dead organic stuff, think dead leaves, fallen trees.

The decomposers.

Precisely.

They break down all that tough, woody material, lignin, cellulose.

Without them, nutrients would just stay locked up.

Ecosystems wouldn't function.

Of course, sometimes that decomposition isn't helpful to us.

Like dry, rot in -house timbers.

That's serpula lacrimens, a basidiomyces, doing its job a little too well from our perspective.

Right.

Not ideal in your floorboards.

So essential recyclers, but sometimes destructive.

Sometimes very destructive.

As pathogens, some are serious problems.

We mentioned the honey fungus or malaria.

It attacks loads of different trees.

Then there's hetero -basidian anosome, a nightmare for conifer plantations.

And rhizectonia hits a wide range of crops.

These can have huge economic impacts.

But it's not all destruction, is it?

They also form partnerships,

like mycorrhiza.

Yes, exactly.

The flip side is symbiosis, mutually beneficial relationships.

Think about many common woodland mushrooms.

Amanita species, boletus.

They form ectotrophic mycorrhiza with tree roots.

Ectotrophic.

Meaning they form like a sheath around the root.

Yeah.

A sheath and a network extending out into the soil.

The fungus gets sugars from the tree, and in return, it massively increases the tree's ability to absorb water and nutrients, especially phosphorus.

It's a partnership that underpins entire forest ecosystems.

Wow.

And interestingly, that rhizectonia we just mentioned as a pathogen, some species can also be mycorrhizal partners with orchids, shows you how complex these interactions can be.

And of course, for humans,

many of them are food.

Oh, definitely.

Lots of basidiocarps, that's the scientific term for the fruit body, the mushroom part, are edible and delicious.

Your standard white button mushroom, agarx, bispris, oyster mushrooms, shiitake, staples in many cuisines.

But, and this is a big but.

A very big but.

Extreme caution is needed with wild mushrooms.

While some are great eating, others are deadly poisonous.

The death cap, Amanita phalloides, is notorious for a reason.

You don't want to misidentify that one.

Absolutely not.

And then you have the hallucinogenic ones, like the fly agaric, Amanita muscaria, or psilocybe species, the magic mushrooms.

So the rule is simple.

Unless you are an absolute expert, don't eat wild mushrooms.

Period.

Good advice.

Okay, let's circle back to that mind -boggling scale we started with, the giant armillaria.

And you also see those fairy rings in fields sometimes.

Right, myrasmias aureates.

Those rings can be incredibly old, centuries even.

Just the visible edge of a huge ancient mycelial network growing outwards underground.

Just amazing persistence.

It really is.

But it's also worth remembering, not all basidiomycetes form these vast networks.

Some are actually yeast -like.

Or they can be dimorphic, switching between a yeast form and a mycelial form.

Dimorphic.

Two forms.

Yeah, depending on the conditions.

And sometimes that's relevant to human health, unfortunately.

Phthalobacidella neophormans, for example, is a dimorphic fungus that causes cryptococcosis, a really dangerous brain infection, especially in people with weakened immune systems.

So yeah,

huge diversity in form and function.

Okay, so given all this diversity, how do they actually reproduce?

Let's zoom in on that signature structure you mentioned, the basidium.

Right, the basidium.

It's the defining feature, really.

It's the cell where the spores are actually produced in sexual reproduction.

And crucially, unlike some other fungi, it produces its spores externally.

On little stalks.

Exactly, on these little pointed outgrowths called stergmata.

Usually four spores per basidium, but that can vary quite a bit.

Sometimes just one, sometimes two, like in the cultivated mushroom, Agaricus spores, sometimes many more, like up to nine in the stinkhorn.

And the basidium itself isn't always the same shape.

That helps classify them.

That's right.

Taxonomists look closely at basidium structure.

The simplest are hollow basidia, basically undivided, often club -shaped or cylindrical.

That's typical of most mushrooms and bracket fungi.

Then you have fragmobasidia, sometimes called heterobasidia.

These ones are divided by internal walls or septa.

They can be divided crosswise, like in the Jews ear fungus, auricularia, or lengthwise, like in fromella species, the jelly fungi.

So different internal structures.

Yeah.

And the rests and smuts have yet another variation where the basidium develops from a thick walled resting spore and forms a kind of septate tube with each section producing a spore.

It just shows how much variation there can be on the central theme.

Okay.

Let's get into the nitty gritty.

How are these spores actually made and how do they get launched?

It sounds like some kind of micro catapults involved.

It basically is.

So let's think about how a typical basidia spore develops.

Inside that basidium, you start with two distinct haploid nuclei.

Remember the dekaryotic state?

Right, the two nucleus state.

Okay.

So first those two nuclei fuse, that's karyogamy.

Then immediately meiosis happens, producing four haploid nuclei.

These nuclei then migrate out through the steragmata into the developing spores.

So each spore gets one nucleus, usually.

Usually.

Sometimes there's an extra division, so spores can end up with more than one.

But the development itself is fascinating.

The spore grows asymmetrically on the sterigma.

It's sort of perched off to one side.

This shape is really important for the launch.

Asymmetry is key.

Yes.

And many spores have surface patterns like little spines or ridges, but there's always a smooth patch near the attachment point called the superhiller plage, which is crucial for discharge.

Okay, the discharge,

the catapult mechanism.

How does that actually work?

It's elegant, really.

The main theory is the surface tension catapult, often called Buller's drop, after the scientist who described it.

Just before launch, a tiny, almost spherical droplet of water, Buller's drop, forms at the base of the spore, right near the attachment point.

Okay, one drop.

And simultaneously, a thinner film or flatter drop forms on the adjacent face of the spore itself.

These two bodies of water grow for a moment, and then they suddenly merge.

Snap.

They coalesce.

Exactly.

And that rapid coalescence causes a sudden shift in the spore center of mass.

It generates just enough momentum to break the spore away from the sterigma and launch it outwards.

It's pure physics happening on a microscopic scale.

Wow.

So it literally flicks itself off.

Where does the water come from?

That's the clever part.

It's not squirted out from inside.

The fungus exudes tiny amounts of like mannitol onto the spore surface, and these are hygroscopic surface.

They attract water vapor from the air, causing the droplets to condense and grow.

Ah, so it needs humid air to work.

Precisely.

That's why you often find the highest concentration of basidio spores in the air during the night or early morning when humidity is high.

The typical umbrella shape of a mushroom cap actually helps maintain that humid microclimate under the gills and prevents rain from washing the spores away or interfering with the drop formation.

Clever design.

Very.

And it also explains why this mechanism called ballista sporespore doesn't happen underwater.

No surface tension catapult possible there.

And once it's launched, it has a specific flight path.

It does called a sporabola.

It travels horizontally for a tiny distance, maybe 0 .1 to 0 .3 millimeters, just enough to clear the gill surface it came from, and then it sharply drops downwards due gravity.

So it shoots out sideways just enough to fall clear.

Exactly.

It ensures the spores can actually escape from between the tightly packed gills or out of the pores in a bracket fungus.

And the numbers, they're just astronomical.

A single field mushroom might release nearly 2 billion spores in a couple of days.

Billion.

Billion.

Yeah, like 40 million per hour.

Some puffballs release trillions.

It really underscores how incredibly low the odds are for any single spore to land in the right spot, find a mate, and establish a new successful mycelium.

It's survival by sheer overwhelming number.

Incredible odds.

So, okay, say one of those billions does land in a good spot.

What's the next step?

How does it go from a single spore to that vast network?

Well, first it has to germinate.

Spores can stay dormant for a long time, waiting for the right conditions, moisture, temperature, maybe specific chemical signals.

When it germinates, it usually puts out a germ tube which starts growing into a hypha.

And this first hypha is different.

It is.

Because the spore is haploid one set of chromosomes,

the hyphae that grow from it are also haploid.

We call this the monocariotic mycelium or primary mycelium.

Each compartment or cell in these hyphae contains just one nucleus.

Mono meaning one.

Got it.

Right.

And this monocariotic stage often doesn't last that long or it doesn't grow very vigorously on its own.

The real action starts when two compatible monocariotic hyphae meet.

Compatible meaning, like different mating types.

Exactly.

When they meet, they fuse their cytoplasm, that's called plasmagamy.

Now crucially, their nuclei don't fuse yet.

Instead, you get hyphae where each cell contains two distinct haploid nuclei, one from each parent.

This is the dichariotic stage or secondary mycelium.

Di meaning two.

So two nuclei per cell but not fused.

Correct.

And this dichariotic state is often the dominant long -lived phase of the life cycle, that massive armillaria.

That's a dichariotic mycelium.

The nuclear fusion, karyogamy, is delayed until right before spore formation in the basidia.

That delayed fusion seems really characteristic and these hyphae themselves have unique features, don't they?

You mentioned septa earlier.

Yes, the internal crosswalls, the septa.

In basidiomyces, they're typically dollopore septa.

Dollopore basically means barrel -shaped pore.

It's not a solid wall.

There's a central pore surrounded by a thickened, barrel -like rim.

Okay, so stuff can pass through the pore.

Cynoplasm can, yeah, and small organelles like mitochondria.

But the pore is usually capped on either side by a structure made of endoplasmic reticulum called the parenthesum or septal pore cap.

It looks a bit like parentheses around the pore.

Like little caps.

Kind of.

And the structure of this cap, whether it's perforated or solid, can actually be useful for classification.

These dollopore septa help maintain cell structure, allow communication, but they generally block the passage of nuclei between cells, which is important for maintaining that dichariotic state.

That makes sense.

It's like having compartments but with controlled doorways.

And what about clamp connections?

Those sound intriguing.

Ah, clamp connections.

They're really neat.

They're little backward growing loops or bridges you can often see on the sides of dichariotic hyphae, right next to a septum.

Little loop.

What's it for?

It's basically a biological mechanism to ensure that when a dichariotic cell divides, both daughter cells still get one of each type of nucleus.

Remember, you have two distinct nuclei, say nucleus A and nucleus B, in the cell.

Right.

When the cell gets ready to divide, both nuclei divide simultaneously.

That's called conjugate nuclear division.

The clamp connection forms as a little side branch.

One nucleus, say, moves into it.

The other A moves forward.

Both divide.

Then septa forms, separating the original cell, the clamp, and the new forward cell.

One daughter nucleus of B moves back from the clamp into the original cell segment, and the clamp fuses back.

The end result.

Both the original cell segment and the new cell segment have one A nucleus and one B nucleus.

Wow.

Okay, that's an elaborate little cellular choreography to make sure the pairs stay together.

It really is.

It guarantees the maintenance of the dichariotic state during growth.

Though it's worth noting you don't always see them, especially in the hyphae that make up the actual mushroom fruit body, but they're very characteristic of the vegetative dichariotic mycelium in many species.

So these individual hyphae are complex enough, but they also team up, right, into bigger structures.

They do.

Hyphae can aggregate together to form things like mycelial cords, basically bundles of parallel hyphae or even more complex, highly differentiated structures called rhizomorphs.

Rhizomorphs, like fake roots.

They look a bit like roots, yeah.

Often tough, dark colored on the outside due to melanin pigment with specialized tissues inside.

Think of the bootlaces of the honey fungus or malaria.

Those are rhizomorphs.

And what's the advantage of forming these cords or rhizomorphs?

Several advantages.

They can grow much faster and farther than single hyphae.

They can bridge gaps between food sources.

They're much better at transporting water and nutrients over long distances, often in both directions.

And the outer layers provide protection.

They're essentially fungal highways and pipelines.

Very efficient.

And what about sclerotia?

Are they similar?

Sclerotia are different.

They're more like survival structures.

Dense, compact masses of hyphae that are packed with food reserves and often have a hard protective outer rind.

They're designed to survive long periods of unfavorable conditions.

Drought, cold, starvation.

Like a fungal seed or tuber?

Kind of, yeah.

They can vary hugely in size from microscopic to, in some cases, really large, several kilograms.

And when conditions improve, they can germinate and produce either new mycelium or even a fruit body directly.

They're all about long -term survival and propagation.

Okay.

Survival strategies, growth strategies.

Let's talk about mating strategies.

How do they actually find a compatible partner to form that

mycelium?

Right.

The social life of fungi.

It boils down to two main systems.

About 10 % are homothallic.

Homo meaning same, basically.

A single spore has everything it needs to germinate, grow into a mycelium, and eventually produce a fruit body all by itself.

It's self -fertile, essentially.

It doesn't need a partner.

Correct.

But the other 90 % or so are hetero -thallic.

Hetero meaning different.

These require mating between two genetically distinct, compatible mycelia.

And that compatibility is genetically controlled?

Very precisely.

There are two main types of heterothalism.

The simpler one is bipolar, controlled by one gene, usually called the A -locus, which has two different versions, or alleles.

Let's say A1 and A2.

A mycelium with A1 can only mate successfully with one that has A2.

About 25 % of basidiomycetes are bipolar.

So a 50 -50 chance if you pick two random spores from the same mushroom?

Roughly, yeah.

But the majority, maybe 65 % or more, have a more complex system called tetrapolar heterothalism.

This involves two unlinked genes, usually called A and B loci.

And critically, each locus often has many different alleles in the overall population.

Two genes now, so it's more restricted.

Exactly.

For a successful mating, the two mycelia must have different alleles at both the A -locus and the B -locus.

So if you take spores from the same mushroom, only about one quarter of the pairings will be fully compatible.

Okay.

A1B1 can only mate with, say, A2B2, not A1B2 or A2B1.

You got it.

A and B control different parts of the process.

One often controls nuclear pairing and clamp connection formation.

The other controls nuclear migration after plasmodomy.

But you said there can be many alleles for A and B in the population.

Thousands, potentially.

That's why, even though siblings might have only a 25 % chance of mating, if you take mycelia from two different locations, they are almost certain to have different A and B alleles, leading to nearly 100 % mating compatibility.

It promotes outbreeding.

That's fascinating genetic complexity.

It really is.

Research on species like Schizophyllum commune and Caprinus cinereus has shown these A and B loci are actually complex regions with multiple linked genes encoding things like pheromones, pheromone receptors, and transcription factors that regulate the whole process.

Wow.

And then there's this other odd thing, the Buller phenomenon,

where a single nucleus mycelium can somehow become dichariotic just by touching an existing dicharion.

Yeah, that's a bit surprising.

A monokarion single nucleus type can actually steal a compatible nucleus from an established dicharion it comes into contact with.

The nucleus migrates from the dicharion into the monokarion, effectively converting it to a dichariotic state without the usual fusion of two monokarions.

So nuclei can move between established mycelia.

They can under certain circumstances.

It shows a certain fluidity.

But that leads us to another important concept, fungal individualism.

How does a vast mycelial network maintain its identity?

Right.

If nuclei can move around, what stops different individuals from just merging into one giant mix?

That's where vegetative incompatibility or somatic incompatibility comes in.

When hyphae from two genetically different mycelia of the same species meet and fuse,

the fused cells usually die.

Often there's a reaction where the cell walls thicken, pigments are deposited, and you end up with a clear demarcation zone, sometimes visible as dark lines like those black bands you can see in decaying wood where different fungal individuals meet.

So it's like a rejection mechanism, self versus non -self.

Exactly that.

It's a way for the fungus to recognize and maintain its own genetic integrity.

It prevents the uncontrolled transfer of not just nuclei, but also mitochondria, plasmids, potentially even harmful viruses between different individuals.

That makes a lot of sense.

It preserves the individual.

And scientists use this.

By testing which mycelial samples are compatible or incompatible in the lab, they can map out the extent of single genetic individuals in nature.

That's how they confirm that those armillaria networks and fairy rings really are single, massive, ancient individuals.

The incompatibility reactions define their boundaries.

It's just incredible.

Okay, let's zoom out one last time.

Where do these basidiomycetes fit in the grand scheme of fungal evolution?

Who are their relatives?

They're very closely related to the Ascomycota, the group that includes yeasts, morels, cup fungi, and many molds.

What's the evidence for that close relationship?

Lots of things.

Similarities in their cell walls, which are primarily chitin, aspects of their mating type genetics, many have similar asexual spore stages called canidia, and of course, lots of molecular data DNA sequence comparisons strongly support this grouping.

Plus, key structures seem homologous.

Homologous, meaning they share a common evolutionary origin.

Right.

The basidiospores produced externally are thought to be evolutionarily equivalent to the ascospores produced internally inside an ascus in the Ascomycota.

And perhaps most tellingly, those clamp connections in basidiomyces serve the exact same function as structures called croziers found in Ascomycota.

Both are involved in distributing the two different nuclei correctly during cell division in the reproductive phase.

Same solution evolved or inherited from a common ancestor.

Likely inherited.

Because of these strong links, basidiomycota and Ascomycota are grouped together in a major fungal clade called the Dicaria, reflecting that characteristic Dicariotic stage.

They're considered sister groups, and together they're sister to the Glomeromycota, the arbuscular mycorrhizal fungi.

They probably branched off from older fungal lineages, like the zygomycetes, maybe 400 to 600 million years ago.

A very long history indeed.

And we have fossils.

We do.

Fossilized hyphae, showing clamp connections, have been found from the Carboniferous period, about 300 million years ago.

And even more spectacularly, there are mushrooms preserved in amber from the Cretaceous period, around 90 -94 million years ago, looking remarkably like modern ones, complete with basidiospores.

Actual fossil mushrooms.

Amazing.

And just briefly, how are they classified within the group?

Traditionally, they were split, based largely on basidium morphology.

You have the homo -basidiomycetes with those simple, undivided hullabasidia that includes most mushrooms, puffballs, bracket fungi.

Then the hetero -basidiomycete with the septate fragmobasidia, mostly the jelly fungi and related groups.

And then the major parasitic groups, the uridinia mycetes, rusts, and usteleginomycetes, smuts, which also have distinctive basidial development.

Modern classification uses a lot more molecular data now, but those broad groupings based on basidia are still useful concepts.

Okay, wow.

We've covered a huge amount of ground.

From giant hidden networks and deadly poisons to essential partnerships and intricate micro mechanics.

It's clear these basidiomycota are way more than just the mushrooms we see.

They're incredibly diverse and just fundamentally vital to how our planet works.

Absolutely.

And I think if you connect all these dots,

the complex mating systems, the self -recognition, the varied lifestyles it really makes you think, what other intricate biological systems are operating out there may be just as complex, shaping ecosystems in ways we haven't even begun to grasp yet.

These fungi hint at a whole hidden layer of biological complexity.

That's a great thought to end on.

There's clearly so much more to discover in the fungal kingdom.

We hope this deep dive has sparked your curiosity to look a little closer at the fungi around you.

Thanks so much for joining us on the deep dive.