Chapter 16: Phylum Basidiomycota: Introduction to the Basidiomycetes

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Have you ever paused to truly look at a common mushroom?

You know, maybe just a simple button mushroom on your plate or like a vibrant red cap peeking out from the damp forest floor.

Yeah, most people probably just think, ah, mushroom.

Exactly.

But what if I told you that familiar cap is just the tiniest, tiniest tip of this, well, colossal hidden biological iceberg?

Oh, absolutely.

Because beneath that cap,

stretching through these vast underground networks and even sometimes residing unseen, you know, within our very bodies, there's this world of astonishing complexity and profound impact, often in really surprising ways.

Definitely.

Welcome to the deep dive.

Today, we're plunging headfirst into the truly fascinating phylum basidiomycota.

And it's not just about the mushrooms you find in the grocery store, right?

Not at all.

It's the sprawling kingdom that includes everything from this familiar fungi to microscopic plant pathogens, even serious human disease agents.

It's a huge group.

So our mission today is basically to take what could be a really dense, maybe overwhelming chapter from an introductory mycology text and sort of distill it for you.

Yeah, boil it down to the essentials.

We want to give you the key knowledge about their unique structures, how they reproduce,

their peculiar physiology, their kind of mind bending genetics.

How scientists classify them and crucially, why they matter so much in our world.

Exactly.

Think of this as your personal shortcut, your guided tour to being truly well informed about these incredible, often overlooked organisms.

And there's a central key to understanding them all, isn't there?

Right.

And here's our first crucial insight.

The core characteristic that defines this entire group,

all Basidiomycetes produce their sexual spores, they're called Basidiospores, on a specialized microscopic structure.

The Basidium.

The Basidium.

That tiny detail really is the key to unlocking their entire world.

It really is.

That tiny detail, the Basidium, it underpins this incredible diversity we see in the group.

When we talk Basidiomycetes, we're not just limited to the, you know, familiar mushrooms, bullies, puffballs, stars, stinkhorns, usual suspects.

We're also talking about the more obscure forms, things like bird's nest fungi,

jelly fungi, or those bracket and shell fungi you see growing on trees.

And critically, some really important plant attacking rust and smut fungi.

You might rarely see them clearly, but their impact is enormous.

So it's this Basidium and specifically how it releases spores that truly defines them.

What's so special about that structure and process compared to other fungi?

It's really about where the spores are formed.

Unlike some other fungi,

Basidiomycetes produce their Basidiospores

on the outside of that microscopic Basidium.

Outside, okay.

And typically each Basidium makes four of these spores.

These spores are the result of a very precise sequence of biological events.

First, you get two parent cells meeting and merging their internal fluid that's plasmagamy.

Right, sphatoplasmic fusion.

Exactly.

Then their nuclei, which hold the genetic blueprints, fuse together.

That's karyogamy.

Nuclear fusion.

And immediately after that, the newly combined genetic material undergoes a special kind of cell division, meiosis, to have its components.

This ensures the resulting spores have just a single set of genetic instructions ready to start a new life cycle.

And you mentioned many of these spores are ballistospores.

That sounds dynamic, like a fungal cannon.

It certainly does.

And it means they are forcibly discharged, literally shot from the Basidium through a really elaborate mechanism.

Wow.

And these spores themselves are incredibly varied, too.

They can differ widely in size, shape, color, even the thickness and characteristics of their cell walls.

And that diversity isn't just random, it's functional.

Oh, absolutely.

It's often key to how they survive, how they spread, how they colonize new environments.

It's all part of their strategy.

Okay, so we've got to handle on what Basidiomycetes are, this defining Basidium, their spores.

But beyond being scientifically fascinating,

why should we really care?

What's their impact on our world, both the good and the bad?

Let's maybe start with the more unsettling side first.

Well, their impact on agriculture alone is just immense.

Think about plant diseases caused by rust and smut fungi.

These aren't just minor annoyances.

They are major economic threats.

They destroy millions upon millions of dollars worth of crops every single year.

Like wheat rust, for example.

Exactly.

Notorious examples include things like stinking smut and black stem rust of wheat.

It's like this silent but utterly devastating war happening in our fields.

Yeah, it's not just crops, right?

Their destructive reach extends to forests and even the trees in our parks and yards.

They certainly do.

Many Basidiomycetes are powerful tree pathogens.

Take Armillaria astoiae, for instance.

Ah, the fungus humongous.

That's the one, famous for its massive underground networks that can spread across huge areas of forest, just relentlessly attacking trees.

Wow.

And beyond living trees, they're also primary agents in breaking down wood products.

Think lumber,

railroad ties, utility poles.

Things we rely on.

Exactly.

The costs associated with just protecting these materials from fungal decay are tremendous, impacting industries all over the world.

So they're reshaping ecosystems, hitting agriculture, affecting our timber supply.

Do they get even closer to home?

What about, like, direct impacts on human health?

Crucially, yes.

There's Philebesidiala neoformans.

It might know it better by its asexual stage name, Cryptococcus neoformans.

This is a serious human pathogen.

It's particularly dangerous for people with weakened immune systems like AIDS patients.

So yes, their impact stretches directly into human health concerns, not just crops and forests.

It really sounds like these fungi are, well, industrial scale destroyers when they turn their attention that way.

It's almost a bit unsettling thinking about them silently working away underfoot.

It can be, but it also highlights their incredible power and efficiency.

They are nature's ultimate recyclers.

It's just that sometimes we don't appreciate their recycling efforts when it's our crops or houses.

That's a good way to put it.

Which naturally brings us to their hugely beneficial side.

They're not all doom and gloom.

Far from it.

If we connect this to the bigger picture,

Basidiomycetes are absolutely essential components of, say, forest ecosystems.

They are the primary decomposers.

The recyclers you mentioned.

Exactly.

They break down the really tough stuff like cellulose and lignin in didwitty plants,

effectively recycling vital nutrients back into the ecosystem so new things can grow.

Without them, forests would choke on their own debris.

And they have broader industrial uses too, beyond just breaking things down naturally.

Indeed.

Some species are used in biotechnology.

For example, as biological pulping and bleaching agents in paper production.

Offering greener alternatives.

Precisely.

More environmentally friendly alternatives to harsh chemical processes.

They even have this remarkable ability to help remove toxic substances from the environment, showing real potential in bioremediation.

And they're also vital partners in the natural world, forming relationships with plants.

Absolutely fundamental partners.

Many are mycorrhizal species, forming these critical symbiotic relationships with plant roots.

Mycorrhizae, the fungus root connection.

Right.

These partnerships are fundamental for nutrient uptake.

They help plants access water and minerals they couldn't get on their own.

It allows them to thrive in both natural forests and managed ones too.

It's a true underground economy that sustains our forests.

And of course we can't talk about Basidiomycetes without talking about food.

The mushroom industry seems huge now.

It is.

The cultivation of mushrooms is a considerable and still growing global industry.

Yeah.

We see Agaricus brunessens.

That's the common button mushroom Portobello cremini.

Alongside others like Shiitake, Lentinula edodes, and oyster mushrooms.

Pyrrhota species.

Becoming staples in grocery stores and on restaurant menus worldwide.

And for the more adventurous, there are wild mushrooms, often prized by chefs and connoisseurs for those unique, intense flavors.

This brings up a really critical point though.

While many wild mushrooms are highly valued and delicious, caution is absolutely paramount.

You have to be careful.

Because some are dangerous.

Very dangerous.

A significant number of species are highly poisonous.

And sadly, reports of mushroom poisonings, some of which are fatal, are unfortunately common news items.

So the message is clear.

Don't guess.

Never.

Ever guess.

Never consume a wild mushroom unless you are 100 % absolutely certain of its identification by a qualified expert.

It's just not worth the risk.

A vital safety warning.

Okay, so beyond just eating them, Bacidium icetes are also known for producing this huge variety of secondary compounds.

We're talking interesting scents, unique tastes, even vibrant colors.

Yes, the chemical diversity is just stunning.

Some produce quite pleasant odor, balsamic, almond, fruity, floral, others.

Well, they can generate rather unpleasant scents like sweat or sulfur.

And these smells can actually help identify them.

Psychologists use them.

They've even named species based on these smells, like mabellina for its almond scent or maladora for a particularly noxious one.

Think of stinkhorns, too.

Their smell is key to their sport dispersal by attracting insects.

Right.

And then there are unique tastes.

Yes, like pungent sesquiter peens.

Some animals, like opossums, are apparently highly sensitive to these tastes, maybe as a defense mechanism for the fungus.

So these compounds aren't just random chemical byproducts.

They have real biological significance and maybe even potential uses, right?

Exactly.

They play roles in defense, communication, influencing other organisms.

For instance, there's this fascinating case where low levels of tolerance to amitoxins,

those deadly compounds in aminida mushrooms and certain drosophila fruit flies,

actually helps them escape nematode parasites and competition.

Wow.

It just hints at the complex web of interactions these chemicals mediate.

And of course, it points towards significant biotechnological potential, finding new medicines, industrial enzymes, things like that.

We've covered their incredible impact, the good and the bad, and that tiny basidium that defines them.

But let's zoom out again.

All this influence comes from their physical structure.

We picture a mushroom, but that's just the, well, the fruiting body, right?

What's the main event happening beneath the surface?

What's the actual anatomy?

Yeah, think of an almost invisible underground web.

It's this intricate, sprawling network called the mycelium.

Mycelium, okay.

It's woven from countless microscopic branching threads.

Those are the hyphae.

And these hyphae are typically divided by internal cross walls, the septa we mentioned.

So individual threads are too small to see.

Right, you need a microscope for a single hypha.

Yeah.

But their collective mass, the mycelium, is often plainly visible.

You might see it spreading in a fan shape through a rotten log or in dead leaves, often appearing white or bright yellow, sometimes even orange.

And some species take this network building to another level with rhizomorphs.

They sound robust.

They really are.

Imagine these dense, shoestring -like structures.

They're made of many hyphae growing tightly packed and parallel to each other, sometimes even wrapped in a protective outer sheath.

And what's their function?

Why build these shoestrings?

Their function is quite remarkable.

They are vital for the spread of certain species, almost like fungal highways.

They allow the fungus to explore new territory, search for resources, and efficiently transport and accumulate nutrients over distances.

Faster than just growing as single threads.

Much faster.

For instance, the rhizomorphs of Armillaria, that fungus humungous, extend way faster than its individual hyphae could.

They're typically about half a millimeter to two millimeters thick, but some can get up to four or five millimeters.

And where do you find them?

Just underground?

Usually in soil, especially the organic layers, or in leaf litter.

But surprisingly, some tropical Merasmia species can actually have them growing up into tree canopies, exploring the air as well as the ground.

Wow, fungal highways and the treetops.

Okay, you mentioned the internal class walls, the septa in the hyphae.

Are those just simple dividers, or is there more complexity there?

Oh, there's definitely more complexity.

What really stands out is that they aren't solid barriers.

Almost all Basidiomycetes have septa with a single tiny central pore.

But many have this very distinctive structure called the dollopore septum.

Dollopore?

Yeah, dollopore.

Think of it like a miniature, reinforced donut or barrel shape right around that central pore.

The wall thickens there.

Okay.

And then, often covering this dollopore is something called the septal pore cap, or parenthosome.

It looks like a little dome or screen, often made of modified endoplasmic reticulum membranes.

And what does this intricate setup do?

Well, its exact function is still a bit of a mystery, honestly.

But the leading idea is that this whole structure, especially the cap, acts like a selective filter or sieve.

It carefully controls what organelles or molecules can pass between adjacent hyphae compartments while maybe blocking larger things.

It helps maintain the fungus's integrity and coordinate growth.

It's really a tiny, elegant piece of cellular engineering.

So this tiny internal structure, the way this pore is built, is actually really important for figuring out who's related to whom.

For classification.

Absolutely critical.

The fine structure of these septal pores, whether it's a simple pore or a complex dollopore with a specific type of cap, is a key feature in Basidiomycetes systematics, their classification.

Why is it so useful?

Because these ultra -structural features are considered evolutionarily conserved.

That means they change very, very slowly over long periods of evolutionary time.

And what we're finding is that their morphology, how they look under an electron microscope, strongly aligns with the relationships suggested by modern molecular data, like DNA sequences.

It's proving to be a very powerful tool.

OK, and speaking of complex structures, the part we do see, the fruiting body, the mushroom, the bracket, the puffball, that's actually a highly organized tissue called a Basidiocarp, right?

Formed from what you call tertiary mycelium.

That's exactly right.

The tertiary mycelium isn't a different type of hypha.

It's the term for these highly organized, specialized tissues that arrange themselves to form the visible stridor, the Basidiocarp.

And these can vary wildly.

Incredibly so.

They range from microscopic specs to truly massive structures.

There was a Rigidipore's Ulmerius polypore found that measured nearly 150 centimeters across,

or single mushrooms weighing over five pounds.

That's enormous.

And their textures are just as varied.

You get thin, paint -like crusts, gelatinous blobs, fleshy mushrooms, quirky brackets, even tough woody conks.

And how they release their spores differs, too.

Some seem very open about it, others more closed off.

Exactly.

Basidiocarps can be open from the very beginning, exposing their spore -producing surfaces, the Basidia, early on.

Others might open up later in development.

And then you have forms like puff balls that remain completely closed.

So how do the spores get out of those?

In those closed forms, the spores are only liberated when the Basidiocarp itself disintegrates.

Or maybe it's fractured by external forces stepped on, chewed by insects or rodents, even the impact of raindrops sometimes.

Okay, so within these diverse Basidiocarps, where are the spores actually produced?

You mentioned the Basidia.

They're produced in a very specific organized layer called the hymenium.

Think of it as the fertile surface within the Basidiocarp.

Like an ascomycetes.

Similar concept, yes.

This hymenium is where the Basidia are formed, usually packed closely together.

But they're not alone in there.

There are also other sterile elements mixed in.

Sterile elements, like what?

Two main types are Basidials, which are basically immature or supporting cells that look a bit like Basidia, and Cystidia.

Cystidia.

Yeah, Cystidia.

These are often larger, quite distinctive cells that frequently protrude from the hymenium.

Their exact function isn't always fully understood.

Maybe they help with moisture evaporation, maybe defense.

But they're very important for taxonomy, for identifying different species or groups.

So the hymenium's features are key for classification too.

Definitely.

Traditionally, the way the hymenium is arranged and located within the Basidiocarp was used to classify Basidiomycetes into higher taxonomic categories.

And the microscopic details of the hymenium, the shape of the Basidia, the types of Cystidia, the spore characteristics are absolutely vital for identifying specific groups like mushrooms and boletes, the Agaricales.

Let's shift focus now to the life cycle itself, especially sexual reproduction.

You keep coming back to the Basidium as being central.

Can you walk us through how a typical Basidium actually develops and produces those spores, maybe step by step?

Okay, let's visualize this intricate little process.

It usually starts with a terminal cell, the very tip of a binucleate hysa.

Remember, that's a hypha with two compatible nuclei inside.

This tip cell is destined to become the Basidium.

Often with a clamp connection at its base.

Often, yes, we'll get to those.

So this cell enlarges, then crucially the two nuclei inside it fuse together, that's Karyogamy, forming a single temporary deployed nucleus.

Deployed meaning two sets of chromosomes.

Exactly, but it doesn't stay deployed for long.

That nucleus immediately undergoes meiosis, that special reductional division, which results in four haploid nuclei, each with just one set of chromosomes.

Okay, so fusion, then immediate division back to haploid.

Right.

Next, four small stalk -like outgrowths called sterigmata push out from the top surface of the Basidium.

And the tips of these sterigmata swell up.

These become the Basidiospore initials.

Like little balloons inflating?

Kind of, yeah.

And then those four haploid nuclei migrate, one into each developing Basidiospore initial through the sterigma.

One nucleus per spore.

Typically, yes.

Though sometimes a nucleus might divide again by mitosis after migrating, so you can end up with binucleus spores upon discharge in some species.

And it's worth remembering, while four spores is the classic number, some Basidia might only produce two, others maybe more, depending on the specific fungus.

And are all Basidia the same simple club shape we might picture?

Or are there different types?

Oh, there's variety there too.

We talk about holobasidia, which are the typical single -celled, often club -shaped ones.

But there are also phragmobasidia, which are divided by internal septa, typically into four cells.

And the rust and smut fungi have a really unique Basidium development too.

It actually starts from a thick -walled resting spore called a Tiliospore.

So lots of variation on that fundamental structure.

What about the Basidiospores themselves once they're made?

How do they get around, and what different forms do they take?

Well, Basidiospores are typically unicellular and, as we said, haploid.

When they land somewhere suitable, most undergo direct germination.

They just start growing a new primary mycelium, those initial fungal threads.

But not always direct.

No.

Some show indirect germination.

They might form secondary spores first, or they might bud off smaller asexual spores called knidia or microknidia, and those then produce the primary mycelium.

It's another layer of complexity.

And their shapes and appearances?

Incredibly diverse.

They can be globose, round, oval, elongated, even sausage -shaped, or angular.

Their colors range from completely colorless to various shades of brown, yellow, pink, even black.

Often, pale pigments are only really visible when you collect a large mass of spores together, what mycologists call a spore print.

Right.

Like taking a mushroom cap and letting the spores fall into paper.

Exactly.

All these features, shape, size, color, surface markings like warts or ridges, are vital for taxonomic identification, especially at the species level.

And you mentioned that ballista spore discharge earlier, the forcible ejection.

How does that amazing biological catapult actually work?

It sounds fascinating.

It really is a neat piece of biophysics.

The spore has this tiny little projection near where it attaches to the sterygma, called the Hiller appendix.

At this point, a small liquid bubble, famously called Buller's drop, starts to form.

A water droplet.

Essentially, yes, exuded by the fungus.

As this drop rapidly enlarges, it's thought to cause a sudden shift in the spore's center of gravity and maybe surface tension changes that create a burst of momentum.

This force rapidly propels the spore off the sterygma, launching it away from the hymenium, often just far enough to clear the gills or pores and get caught in air currents.

An elegant, tiny slingshot.

A perfect description.

Though it's worth noting, not all Basidio scores are ballista spores.

Some groups, like the Gastromycetes, the Puffballs and Earthstars, have lost this ability and rely on other methods for dispersal.

Okay, so that covers the intricate details of sexual reproduction.

But like many fungi, Basidio mycetes also have asexual ways to reproduce and spread.

Indeed they do.

It gives them flexibility.

Asexual reproduction happens through various methods.

Simple budding, like yeast do.

Fragmentation, where bits of the existing mycelium just break off and grow.

Or by producing different types of specialized asexual spores.

Things like knidia, which are very common in, say, the smut fungi.

And also in the repeating summer spore stage of rust fungi,

those are called uridinia spores.

Or you might get arthrospores, which form when hyphages break up into individual cellular sections.

Or odia, which are small spores produced from specialized hyphal branches called odiophores.

And sometimes this asexual stage is all we know.

Exactly.

What's fascinating is that some Basidio mycete species are primarily, or even exclusively, known from their asexual forms.

Many yeasts fit this bill, as does the important plant pathogen Rhizoctonia and the human pathogen Cryptococcus we mentioned earlier.

So how do we even know they are Basidio mycetes if they don't show the sexual stage?

Good question.

Their identity as a Basidio mycetes in these cases is often determined by looking for other telltale signs, like the presence of those complex doloporcepta in their hyphae.

Or sometimes by unique staining reactions of their hyphae walls.

Or, increasingly, through DNA analysis.

Let's delve into their genetic blueprint now, and how they manage compatibility finding the right partner.

You mentioned primary, secondary, and tertiary mycelium earlier.

Can we revisit those stages specifically thinking about their genetics?

Absolutely.

So the primary mycelium, also called a homocharion, is that initial stage that typically grows from a single germinating Basidiospore.

The key genetic point here is that all the nuclei within this mycelium are genetically identical clones of that original spore nucleus.

One genetic type.

But then you get the secondary mycelium.

This is usually a heterocharion, meaning it contains nuclei from different genetic backgrounds.

Most characteristically in Basidio mycetes, it's a decarion, meaning each cell or compartment contains exactly two compatible but genetically distinct haploid nuclei.

And this forms when two compatible primary mycelia meet.

Typically, yes.

It usually arises from an interaction, most commonly the fusion, plasmagamy, between cells of two compatible primary mycelia.

This brings two different nuclear types together in one cell.

Okay, so you get one cell that's dichariotic.

How does that state then spread throughout the entire fungal network?

It seems like it would need to coordinate.

It does need to coordinate, and there are two main ways this dichariotization spreads.

In one method, that initial binucleate cell can just branch and grow, and as it does, the two nuclei divide simultaneously, that's called conjugate division, and in new septa form, perfectly, dividing the growing branch into new dichariotic cells.

Each new cell gets one of each nuclear type.

Okay, direct growth?

What's the other way?

The other way, which is often more common, involves a fascinating migration.

After the initial fusion creates a dichariotic cell,

the nuclei in that cell undergo rapid division.

Then, daughter nuclei from one parent type actually migrate into the primary mycelium of the other compatible partner.

Migrate through the existing network.

Yes.

This often requires temporary breakdown or modification of the septal pores to allow the nuclei to pass freely between cells.

This nuclear migration continues until both of the original pair of mycelia are fully dichariotized, containing pairs of compatible nuclei throughout.

That sounds incredibly complex and coordinated, and this leads us to something you mentioned briefly, clamp connections.

These sound really specific and may be important for maintaining that dichariotic state.

They are absolutely ingenious and vital for many species.

Clamp connections are these special small hook -like structures that form at the growing tip of a hypha, specifically during that conjugate division of the two nuclei in the dicharion.

What's their purpose?

They function as this incredibly elaborate mechanism, almost like a biological buckle or bypass, to ensure that each new cell or compartment formed behind the growing tip receives exactly one of each of the two compatible nuclei.

It guarantees the maintenance of that essential dichariotic condition.

Can you describe how it works?

It sounds intricate.

It is.

During nuclear division, one nucleus divides along the main hyphal axis.

The other divides into this small backward projecting hook, the clamp initial.

A septum forms, cutting off the clamp cell with one nucleus.

The tip cell now has two nuclei.

Then the clamp cell fuses back with the sub -terminal cell, the one just behind the tip, delivering its nucleus, so that cell also becomes dichariotic.

Then a final septum forms behind the tip cell.

It's a little dance that ensures perfect distribution.

Wow.

Nature's little quality control check for nuclear pairing.

Exactly.

And while their presence is generally a very reliable indicator of a dichariotic mycelium and Bicidia mycetes, as always in biology, there are some exceptions some groups manage without them.

Okay, so they have this mechanism to maintain the dicharion, but how do they actually find a compatible partner in the first place to even start this genetic dance?

What are their sexual compatibility systems like?

Right.

It's not just any two primary mycelia that can successfully pair up.

While some Bicidia mycetes are homothallic, meaning they are self -fertile and can essentially mate with themselves.

No partner needed.

Correct.

But most species that have been studied are heterothallic.

This means they have genetically determined mating types and absolutely require a partner with a compatible, different mating type for successful sexual reproduction.

And how is compatibility determined genetically?

Their two main systems, about 20 -25 % of heterothallic species, have what's called unifactorial or bipolar heterothalism.

Here, compatibility is controlled by different versions or alleles at a single genetic location, a single gene or gene complex, often called the A locus or mat locus.

Think of it like having two mating types, A1 and A2.

Only A1 can mate with A2.

Simple enough.

What's the other system?

The other system, which is more complex and actually more common, especially in mushroom -forming fungi, is bifactorial or tetrapolar heterothalism.

This involves two unlinked genetic loci, usually called A and B, each typically with multiple components or genes.

Two separate genetic switches need to match up.

Or rather, they need to be different.

For a fully compatible mating to occur, leading to clamp connections and fruiting body development, the two partners must have different alleles at both the A locus, A and E, the B locus.

Can you give us a concrete example of how this works?

It sounds like it could get complicated fast.

Schizophyllum commune, the split -kill mushroom, is the classic well -studied example.

Its mating and sexual development are controlled by these A and B loci, which are actually complexes of genes on two different chromosomes.

Let's call them AYA, A, BAE, and BAE.

Okay.

Now, any two primary mycelia, homocharions, can physically fuse.

But for the resulting secondary mycelium, techarion, to be fully compatible, meaning it forms clamp connections correctly, allows nuclear migration, and can eventually produce basidiocarps, the two fusing partners must differ at both the A complex and the B complex.

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

What happens if they only differ at one locus, say A, but have the same B?

Ah.

That leads to what are called partially compatible or common locus interactions.

Studying these have been incredibly useful.

For example, if they differ at A, but share B, like A1B1XA2B1, you get nuclear pairing and clamp formation, but nuclear migration is blocked, and the clamp doesn't fuse properly.

This tells us the B locus controls nuclear migration and clamp fusion.

And if they share A, but differ at B?

If they share A, but differ at B, like A1B1XA1B2, you get nuclear migration, but nuclear pairing is unstable, conjugate division is messed up, and clamp formation doesn't complete properly.

This shows the A locus controls pairing, division timing, and clamp septation.

It's like different genetic modules controlling different steps of the process.

That's incredibly intricate control, and you mentioned multiple alleles.

Yes.

This key to promoting out -crossing.

Species like schizophyllum don't just have two versions of A and B, they have hundreds of different naturally occurring alleles for both the A and B loci in the wider population.

This means that any two random spores landing near each other have a very high probability, often over 90%, of being compatible, ensuring genetic mixing.

So it's a system designed to maximize diversity.

Are there variations?

Does everything follow this decarion model?

Not entirely.

Nature loves exceptions.

For instance, some armillaria species actually maintain a stable deployed mycelium after mating, rather than a decarion.

And as we said, some species are homothallic and might be multinucleid, but this heterothallic out -crossing condition is generally considered the ancestral state for basidiomycetes.

Okay, finally, let's talk about putting these amazing fungi in the proper place,

their taxonomy and evolution.

You hinted that classifying the higher groups of basidiomycetes has been, well, a bit messy historically.

That's putting it mildly.

It absolutely has been.

Historically, the taxonomic treatment of the major basidiomycete groups has been in a real state of flux, making it a particularly challenging area in mycology.

Different experts propose different systems, often conflicting.

What were some of the older approaches based on?

Early schemes relied heavily on characteristics like the overall morphology of the basidium and how the basidiospores germinated.

For example, Petuiar's system from the early 20th century divided them into hetero -basidiomycetes, with variable basidia, often cepate, and homo -basidiomycetes, with simpler club -shaped basidia.

Later approaches, like Talbot's work in the 70s, placed more emphasis specifically on whether the basidium itself had septa, cross walls.

This led to provisional classes like holobasidiomycetes, non -septate basidia,

fragmobasidiomycetes, septate basidia, like jelly fungi, and telomycetes, basidium developing from a teliospore, like rusts and smuts.

And there were also those broader descriptive terms, gastromyces and hymenomycetes.

Right.

Those are more general practical terms based on the fruiting body structure.

Gastromycetes traditionally grouped fungi, where the hymenium isn't exposed when the spores are mature.

Think puffballs, earth stars, stinkhorns, bird's nest fungi.

The spores are released passively from a closed structure.

Hymenomycetes, on the other hand, grouped those where the well -defined hymenium is exposed during spore maturation and release,

like typical mushrooms, boletes, shelf fungi.

But these traditional groupings weren't always reflecting true evolutionary relationships.

Increasingly, it seemed not.

There were always fungi that didn't fit neatly, and different characters, basidium -shaped septal pores, fruiting body type,

sometimes suggested contradictory relationships.

It was clear a new approach was needed.

And now, modern science, especially molecular data, is really revolutionizing our understanding, giving us a much clearer picture of their evolutionary family tree.

This is where the story gets truly exciting, yes.

Recent advancements in ultra -structure studies using electron microscopy, but critically,

the explosion of DNA sequencing data, especially analyzing ribosomal DNA, our DNA sequences, which evolves relatively slowly, are clarifying the true evolutionary relationships in ways we just couldn't before.

What are the key insights coming from this molecular revolution?

Well, the prevailing hypothesis now, strongly supported by molecular data, is that the basidium mycota as a whole is a sister group to the Ascomycota, meaning they share a relatively recent common ancestor, distinct from other fungal lineages.

Okay, so they're close relatives.

And within the basidium mycota?

Within the basidium mycetes, the molecular data strongly suggests the three main evolutionary lineages diverged early on.

There's one major group that includes the smuts, which are the leginales, and their relatives, like the exobesidiales.

There's another distinct lineage that encompasses the rust fungi, the uredinales, and various other orders that typically have simple septal pores, not dollopores.

And finally, there's the huge lineage that includes most of the fungi we think of as mushrooms and brackets, the traditional hymenomycetes.

Interestingly, this group also includes the jelly fungi, like tremella, tremelles, nestled within it, showing they aren't as separate as once thought.

So what's the really big takeaway message from all this for how we classify them now?

It sounds like some long -held ideas based on morphology were maybe misleading.

That's the crucial, perhaps provocative, revelation.

The big takeaway is that basidium morphology, whether it was club -shaped or septate, for example, which was traditionally a primary tool for classifying these major groups,

has actually turned out to be a poor predictor of relationships at these higher taxonomic levels.

So relying just on how the basidium looked led scientists down the wrong path sometimes.

In some significant cases, yes.

It caused fungi that weren't actually closely related evolutionarily to be grouped together and separated others that were.

The molecular data, however, is providing a much more robust, reliable,

and internally consistent picture of fungal evolution.

It's truly reshaping how we understand their family tree and revealing the deep, sometimes surprising, connections between these incredibly diverse organisms.

And that really wraps up our deep dive into the incredible phylum basidiomycota today.

We've journeyed from just appreciating their sheer diversity to understanding their really profound ecological and economic impacts, both good and bad.

We looked at their unique and complex cellular structures, like those amazing rhizomorphs and that enigmatic dollopore septum.

We explored their intricate sexual and asexual reproductive strategies, including those fascinating clamp connections.

And we touched on the sophisticated genetic systems governing their compatibility and how modern science, especially molecular data, is constantly refining our understanding of their evolutionary history.

It's been quite the journey through this fungal kingdom.

And maybe as you go about your day now, you might consider this.

These often overlooked organisms, hidden beneath our feet or fruiting quietly in the woods,

they aren't just silent decomposers, you know, patiently breaking down dead stuff.

They're more active than that.

Much more.

They are active, genetically complex engineers of our planet.

They're constantly interacting with their environment, communicating, competing, cooperating, evolving in ways we are honestly only just beginning to fully appreciate.

What more might these silent, hidden masterminds reveal about life itself?

That is a truly mind -expanding thought to leave us with.

Thank you so much for joining us on this Deep Dive into the fascinating world of Besidio My Seats.

My pleasure.

And thank you for being a part of the Deep Dive family.