Chapter 7: Phylum Ascomycota: Introduction to Ascomycetes

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Welcome to another Deep Dive.

Today, we're plunging headfirst into a world that, well, it's often unseen, but it profoundly shapes our planet.

We're talking about the Kingdom Fundai.

Specifically,

the phylum Ascomycota, a really fascinating group.

Absolutely.

Think about it.

You've got everything from the yeast that makes your bread rise, maybe the truffle you had last night.

If you're lucky.

Right?

To life -saving antibiotics, but also some pretty devastating plant diseases.

Ascomy seeds are just everywhere.

They really are incredibly diverse and they keep scientists on their toes sometimes.

Our mission today is to unpack this dense chapter on Ascomy seeds.

We want to explore their unique structures, how they reproduce, which is quite complex, their genetics, and really why they matter so much.

To the planet and to us.

Exactly.

We'll start big picture, then zoom in.

Hopefully you'll walk away with a really clear understanding and maybe a few aha moments.

Okay, let's do it.

So Ascomy seeds,

what's the absolute number one thing that sets them apart?

The hallmark.

Well, the defining characteristic, the thing you look for is the Ascus.

That's singular.

The plural is Ascus.

Ascus.

Got it.

Imagine a tiny, sort of sac -like cell, like a microscopic pouch, almost.

And inside that pouch, their unique sexual spores are produced.

The Ascus spores.

Precisely.

Ascus spores.

Typically you find eight inside one Ascus.

Eight.

Always eight.

Usually.

But it can vary.

Sometimes just one.

Sometimes, believe it or not, over a thousand.

But eight is kind of the classic number.

And they form after nuclear fusion and meiosis happen inside that Ascus.

Okay, so the Ascus is key.

What about their basic body plan?

How are they built?

Right.

So most Ascomy seeds are mycelial.

They grow as a network of these really fine thread -like structures.

We call them hyphae.

Hyphae.

And what's unique about their hyphae is that they're divided up into compartments by these internal walls called septa.

With little rooms.

Sort of, yeah.

But unlike some other fungi, these septa have simple pores in them.

Little holes.

Ah, so things can move between the rooms.

Exactly.

Cytoplasm.

Their internal goo.

Even nuclei, the genetic material, can flow through.

It keeps a whole network connected.

Let's it respond kind of like one unit.

Interesting.

Now I saw something mention waronan bodies.

Sounds kind of like science fiction.

Yeah, they do sound a bit dramatic, don't they?

But they're really cool.

Waronan bodies are these small, membrane -bound structures, often found right next to those septal pores we just talked about.

Yeah, what do they do?

Think of them as tiny, super -fast first responders, emergency plugs.

Plugs.

Yeah.

They're filled with protein.

If a hypha gets damaged, say it gets broken, these waronan bodies are thought to just instantly shoot over and plug the septal pore.

Wow.

To stop the leak?

Precisely.

Stops the cell from losing all its vital cytoplasm.

It's a really efficient little emergency system.

Protect the whole network.

That is clever.

Anything else unique inside them?

Well, some of them, especially the ones that form lichens, you know, the symbiotic partnership with algae.

They have another structure called concentric bodies.

We don't fully understand their purpose yet, honestly, but seeing them is often a clue you're looking at a lichen forming fungus.

Okay.

And talking about special structures,

some of them get pretty interactive, right?

Like those nematode trapping fungi.

Oh, absolutely.

That's a wild side of fungi.

Some ascomycetes have evolved these incredibly specialized bits of mycelium.

They can be like microscopic nooses or sticky traps or coils.

To catch worms.

Tiny roundworms.

Yeah.

Nematodes.

The traps are triggered when a nematode bumps into them.

Some even produce specific proteins, lectins, that bind to the nematode surface, basically glue them down.

Predatory fungi.

Who knew?

That's a tough world out there, even for microbes.

And can build bigger things, too.

You mentioned fungal tissues.

Yes, exactly.

The hyphae don't just stay as loose threads.

They can weave together to form more complex structures.

We call this fungal tissue plectenchyma.

Plectenchyma?

Yeah.

And there are two main types.

If the hyphaes are kind of loosely woven, you can still see the individual threads.

That's prosochema.

Think of it like felt, maybe.

Okay.

But if they pack together so tightly that they lose their thread -like look and start to resemble plant cells, sort of rounded or squarish, that's pseudoprenchyma.

Pseudo meaning false -prenchyma, because it looks like plant tissue, but isn't made the same way.

They use this to build what?

Mostly their reproductive structures, the things that make the ass, and also resting structures, ways to survive tough times.

Right, that makes sense.

So we've got the structure down, these intricate little machines.

How do they actually make more of themselves?

Reproduction.

I gather they have options, asexual and sexual.

They do, and it's quite the repertoire.

Asexual reproduction can be super simple.

In yeasts, it might just be fission splitting in two or budding like a little bleb growing off.

Like we see in baking yeast.

Exactly.

Or the mycelium itself can just break apart fragmentation.

Each piece grows into a new network.

But the most common and most varied asexual method is making knidia.

Knidia?

Those are spores, too.

Yep, asexual spores.

Think of a fungal factory just churning out billions of these tiny things.

Often they're made on these elaborate branched structures called knidia spores.

Like little sport trees.

Kind of, yeah.

Very diverse shapes.

And these knidia are all about rapid spread, dispersal, getting the fungus out there quickly when conditions are good.

Makes sense.

What's interesting, though, is we used to think most ascomycetes made knidia.

But recent work suggests maybe only about 10 % of the ones that reproduce sexually have well -documented knidial stages.

Oh, so still lots to learn there.

Definitely.

About their full life cycles.

And lichens.

They have their own trick, right?

They do.

Lichens produce ceredia.

These are amazing little packages.

Basically a few algal cells wrapped up in fungal hyphae.

Like a ready -to -go lichen starter kit?

Exactly.

Just break off, blow away, land somewhere new, and boom, start a new lichen.

Super efficient.

Okay, so that's the quick and easy asexual side.

What about the sexual reproduction?

How do they, you know, find a compatible partner and mix things up genetically?

Right, the sexual cycle.

It starts with plasmogamy.

That's just the fusion of the cytoplasm, bringing two compatible nuclei into the same space.

And they have four main ways to do this.

Four ways.

One is game -tangible contact,

two specialized reproductive cells, the game -tangia, just touch, and a nucleus passes from one to the other.

Simple contact.

In yeasts, the ordinary cells can even do this.

Just bump into each other?

Pretty much.

Second way, sometimes they have differentiated game of tangi, a distinct male one, the antheridium, and a female one, the ascogonium.

Male and female fungi?

Functionally, yes.

The ascogonium often has this little receptive hair, the trichogine, that the male nucleus enters through.

Okay, what's number three?

Spermatization.

This one's fascinating.

Tiny detached cells that might be specialized or even just a canidium acting like a sperm.

A canidium, an asexual spore acting sexually.

It lands on the female part or even just a regular hypha and delivers its nucleus.

They can be carried by wind, water, even insects, like fungal pollen almost.

And the last one?

Somatogamy.

That's where two regular unspecialized hyphae from compatible networks just fuse together.

Simple fusion of cells.

It's common in some other fungal groups, maybe less so in ascomycetes, but it happens.

Okay, so plasmagamy happens.

The nuclei are in the same cell.

Then what?

Do they fuse right away?

Not usually in the filamentous ones.

This is a key point.

The two compatible nuclei, they don't immediately fuse.

They stay separate but hang out together, dividing in sync as the hyphae grow.

So each cell has two different nuclei?

Yes, for a time.

This is the dicariotic stage.

Die for two, carry on for a nucleus.

It's a temporary but really important phase.

Eventually, usually inside a young developing ascus, those two nuclei do finally fuse.

That fusion is called karyogamy.

And that creates a single diploid nucleus.

It now has two sets of chromosomes.

Just like most animal cells.

Right, but only briefly.

Because immediately after karyogamy, that diploid nucleus undergoes meiosis.

Ah, the reduction division.

Exactly.

Meiosis produces four haploid nuclei back to one set of chromosomes each.

And then usually those four divide once more by mitosis.

Which gets us to the typical eight nuclei.

That's it.

The eight haploid nuclei that will become the eight ascus spores inside the ascus.

Now there's this other structure involved, right?

The cruzier sounds like a shepherd's crook.

It kind of looks like one.

Yes, the cruzier.

It's a really clever bit of biological plumbing found in many species.

Basically a cell in the hyphae that's carrying those two different nuclei at the karyotic cell.

It grows and bends over into a hook shape.

The two nuclei inside divide simultaneously but in a very specific orientation relative to the hook.

Then walls form, cutting off a tip cell, a basal cell, and crucially a central cell in the crook that gets one nucleus of each original type.

Ah, ensuring the new ascus starts with the right pair.

Precisely.

That central crook cell is the young ascus.

And the cruzier process can repeat, allowing one mating event to lead to a whole cluster of ascii.

Very efficient.

That is efficient.

Okay, so we have the eight nuclei in the young ascus.

How do they actually become spores walled off inside?

Right, this is ascus burogenesis.

Sometimes called free cell formation.

It's quite remarkable.

Inside the ascus, portions of the cytoplasm, each containing one of those haploid nuclei, get individually wrapped by a double membrane system.

Like internal sarin wrap?

Sort of, yeah.

The enveloping membrane system, EMS, it delineates each future spore.

Then the actual asco spore wall gets laid down between those two membranes.

And the leftover goo in the ascus.

That's called the epiplasm.

It might nourish the developing spores, and sometimes it helps create the ornamentation, the patterns you see on the outside of the spore wall.

Wow.

Okay, so not all asa are the same then.

Yeah.

Or the spores.

Oh, far from it.

Huge variety.

Asa can be spherical, club -shaped, cylindrical.

They can be scattered randomly or arranged neatly in a layer called the hyminium.

Hyminium, like a fertile layer.

Exactly.

And how they release the spores is a big deal for classification too.

There are three main types based on the ascus wall structure and release mechanism.

Okay, what are they?

First, you have prototunicate assi.

Proto, meaning early or primitive.

These have a very thin, delicate wall.

They don't shoot spores out, the wall just dissolves or breaks down to release them.

Right.

Then there are unitunicate assi.

Uni, meaning one.

These look like they have one functional wall layer, though it's actually two layers stuck tightly together.

They usually have a built -in weak spot at the tip, a pore, or a slit, or sometimes even a little hinged cap called an operculum.

Like a little lid.

Exactly.

The spores shoot out through that opening, often forcibly.

And the third type sounds like it might be dramatic.

It is.

The betunicate assi.

B, meaning two.

These have two distinct wall layers that separate.

The inner layer, the endotunica, is flexible.

It balloons out, stretches, bursting through the rigid outer layer, the exotunica.

Whoa.

Like a jack.

Like a jack in the box.

That's why they sometimes call that.

The stretching builds up pressure.

And then pop.

The spores are violently ejected.

You can actually see that.

Sometimes, yes.

With large fruiting bodies like morels, you can get a visible puff of spores if you disturb them.

Some people even say you can hear an audible hiss.

Powerful stuff.

Incredible.

So behind all this amazing structure and reproduction,

what's controlling it all?

The genetics must be pretty complex, right?

Determining who can mate with whom, things like that.

Absolutely.

Fungi have these sophisticated incompatibility systems.

They're genetic systems that control who can fuse with

and it affects both sexual reproduction and just the fusion of regular body hyphae.

Two kinds of control.

Essentially, yes.

For sexual reproduction, there's homogenic incompatibility.

This system basically enforces out -crossing.

Meaning they have to mate with someone genetically different?

Usually, yes.

It promotes diversity.

This is controlled by specific mating type genes.

The classic example is in Neurospora acrassa.

It's like the fruit fly of the fungus world, genetically speaking.

The model organism.

Right.

Neurospora has MATA and MATA mating types.

You need one of each to mate.

And what's really wild is that the DNA sequence at the metalochus is completely different from the sequence at the metalochus.

They're not just slightly different versions of the same gene.

Not alleles.

No, they're so different.

Scientists call them idiomorphs.

It's like comparing, I don't know, a gene for eye color with a gene for wing shape.

Totally different, but occupying the same spot on the chromosome.

That is weird.

And that drives out -crossing.

It does.

Though some ascomycetes are self -fertile, they don't need a partner.

That's called homothalism.

Okay.

What about the other incompatibility, the non -sexual kind?

That's heterogenic incompatibility, often called vegetative incompatibility or somatic incompatibility.

This is crucial for maintaining the genetic integrity of an individual fungal network, the mycelium.

How so?

It prevents the hyphae of genetically different individuals we call these individuals, genates, from fusing together if they meet, unless it's for sex.

Think of it like tissue rejection, but for fungi?

A fungal border patrol.

Exactly.

When two incompatible genets meet in a dish or in the soil, you can sometimes see a clear line between them where the cells die, a barrage reaction, or maybe one side ramps up making knidia right at the border.

It clearly marks their territory.

And that's genetic too.

Yes, and it's usually controlled by many genes, not just one mating type locus.

This creates a huge number of different incompatibility groups in a population, which has big implications, especially for plant pathogens, affects how they spread and interact.

So these genetic systems are like the social rules of the fungal world, who you can mate with, who you can even touch.

That's a great way to put it, yeah.

And studying these systems, particularly trying to force incompatible strains together in the lab, actually led to the discovery of something called the parasexual cycle.

Parasexual, meaning beside sex.

Kind of.

It's a way fungi can achieve genetic recombination mixing up genes without going through the normal sexual cycle of making aci and ascospores.

How does that work?

It requires getting two different types of nuclei into the same mycelium first.

That's called hetero -karyosis.

Then, rarely, those different nuclei might fuse to form a diploid nucleus.

Then, during regular mitotic cell division, sometimes chromosomes get swapped between homologous pairs mitotic crossing over.

And finally, over time, the fungus gradually loses chromosomes randomly until it gets back to the haploid state, but with new combinations of genes.

Wow.

Does that happen much in nature?

Probably quite rarely, because that vegetative incompatibility system usually prevents the initial hetero -karyosis between different strains.

But it was hugely important for early fungal genetics research, especially in things like Aspergillus nidulans, before we had modern molecular tools.

Okay, this is all fascinating biology.

Let's connect it back to the real world.

Roles.

Ascomyces seem like they're involved in, well, everything.

Good and bad.

They really are.

Their ecological roles are immense.

Huge numbers are sap robes.

Decomposers.

Exactly.

Breaking down dead leaves, wood, dung, anything organic.

They are critical nutrient recyclers.

You find them everywhere.

Soil, freshwater, even the ocean.

We're finding lots of marine ascomysteids now, actually.

Some producing potentially useful chemicals.

And they team up with others, too.

Oh, yeah.

Symbiosis is huge for them.

We mentioned lichens.

That's a massive group.

Almost half of all known ascomycytes are lichenized.

Wow, half.

Roughly.

Then there are mycorrhiza, the partnerships with plant roots, essential for most plants to get nutrients.

Many are endophytes living harmlessly inside plant tissues.

Some associate with insects helping them digest wood, for example.

And some even parasitize other fungi.

A complex web.

But what about the impact on us humans?

The good, the bad, the ugly.

Okay, let's start with the bad and ugly.

They are major plant pathogens.

Responsible for huge economic losses in agriculture and forestry.

Think apple scab, piteri mildews, brown rot on peaches and plums, the historic chestnut blight that wiped out American chestnuts.

Caused by Crephinectria parasitica.

That's the one.

And Dutch elm disease, caused by ophiostoma.

Still a big problem.

And they cause diseases in us, too.

They do.

Some cause superficial infections.

Familiar things like ringworm in athlete's foot.

Those are caused by ascomycetes.

Others can cause serious systemic infections, especially in people with weakened immune systems, like Pneumocystis pneumonia, or infections by Aspergillus.

And toxins.

Yes, mycotoxins.

Some ascomycetes produce really potent poisons.

The classic example is urcot, from Clavicev's perpurea, which grows on rye and other grains.

The alkaloids it produces can cause serious illness, even death, if contaminated grain is eaten.

Historically, it caused devastating epidemics.

Urgotism.

St.

Anthony's fire.

Right.

Though, interestingly, those same urgot alkaloids, when purified and used correctly, are also important medicines for migraines or controlling bleeding after childbirth.

So even the bad guys can sometimes be useful.

Which brings us to the good side.

Absolutely.

The benefits are enormous.

We rely on the fermenting power of yeast ascomycetes for baking bread and brewing beer and wine.

That's fundamental.

Can't argue with bread and beer.

Definitely not.

They're also a treasure trove of pharmaceuticals.

Penicillin originally came from an ascomycete.

Penicillium.

Many other antibiotics, anti -cholesterol drugs,

immunosuppressants used in organ transplants, they come from ascomycetes.

Amazing.

What else?

Some produce plant growth regulators, like gibberellin, which makes plants grow tall.

It was first discovered from a fungus that caused foolish seedling disease in rice, making the rice grow too tall and fall over.

So a disease symptom led to a useful discovery.

Exactly.

And then, of course, there's food.

The highly prized edible truffles and morels are ascomycetes.

Some of the volatile compounds they produce to attract animals to spread their spores are even being explored as potential commercial fragrances and flavors.

Truffle perfume.

Maybe someday.

And crucially, as we touched on with NeuroSpara, they are incredibly important model organisms in biological research.

Studying their genetics and cell biology has taught us fundamental things about how all eukaryotic cells work, including our own.

A lot of genetic engineering technology is built on foundations led by fungal research.

It's clear they're incredibly important, both ecologically and for us.

Let's talk about seeing them.

Often the way we notice fungi is their fruiting bodies, right?

What kinds do ascomycetes make?

Right.

The structure that actually bears the assi, the ascocarp.

Most felminisous ascomycetes produce one.

And there are, broadly speaking, five ways they present their assi.

Five main types of ascocarp, or lack thereof.

Kind of.

First, some have naked assi.

Just assi formed right on the surface, no protective structure around them, like some yeasts and leaf -curl fungi.

Okay.

Simplest case.

Then you have the kleistothesium.

Kleisto means closed.

This is a completely enclosed, usually spherical, ascocarp.

No opening.

Spores get out when it decays or gets eaten.

Think powdery mildew resting stages.

Sealed ball.

Got it.

Third, the parathesium.

This is also mostly closed, typically flask -shaped or pear -shaped, but it does have a small pore at the top called an osteole.

Osteole.

Yeah.

Spores are usually shot out through that pore.

Think neurospora or ergot fungus.

Like a little vase shooting spores.

Exactly.

Fourth, the apothecium.

A belt means away or separate, suggesting it's open.

These are typically cup -shaped or disc -shaped.

The fertile layer, the hymenium with all the ash out, is exposed on the upper surface.

Like morels or cup fungi?

Precisely.

Spores are often forcibly ejected from that whole surface, sometimes creating that puffing effect we mentioned.

And the last one.

The ascostroma, sometimes called a pseudothesium.

This is a bit different.

Here, the ashy develop inside cavities, called locules, which are embedded within a larger mass of fungal tissue called a stroma.

The stroma itself forms the wall of the fruiting body.

So the ashy are in little pockets inside a fungal cushion.

That's a good way to picture it.

Apple scab fungus does this, for example.

And these structures aren't just sacs holding ashy, there's other stuff inside.

Oh yes.

Often there are various types of sterile hyphae mixed in with the ashy.

Collectively, we call these the hamathesium.

They can be hairs or filaments growing down from the top or up from the base.

They're important for classification, and sometimes they help with spore release, maybe by swelling with water and pushing the ashy apart.

Wow, okay.

So much diversity and structure, reproduction, lifestyle.

How on earth do mycologists classify them?

It sounds like it must have been really tough, especially before DNA.

You are absolutely right.

It has been notoriously difficult.

For a long time, classification relied almost entirely on morphology, what they looked like.

Especially the type of ascocarp, the type of ascus, prototunicate, unitunicate, betunicate, spore shape, hamathesial threads.

What you could see under the microscope.

Exactly.

But now, molecular data, especially sequencing ribosomal DNA or DNA in other genes, has completely revolutionized fungal systematics.

How so?

Well, we're learning that some things that look similar aren't actually closely related.

They evolve similar features independently.

That's convergent evolution.

For example, the apathesium, that open cup shape, seems to have evolved multiple times in different lineages.

So looks can be deceiving.

Very much so.

Molecular data helps us trace the actual evolutionary branches.

For instance, it clearly shows that after some very early lineages branched off, the Ascomycota split into two main sister groups.

The true yeasts, order Saccharomycetales, and almost all the other filamentous Ascomycetes.

Yeasts versus the rest, basically.

Broadly speaking, yes.

And the filamentous ones kept some ancestral traits, like the mycelium and often forcible spore discharge.

But then they innovated like crazy with things like Crozier's complex ascocarps, the dekaryotic stage.

Okay, so that brings up a classic term, the fungi imperfecti.

Where do they fit in this modern view?

Are they still a thing?

Ah yes, the deuteromycota or fungi imperfecti.

That's a really important historical point.

It wasn't a natural evolutionary group.

It was basically a holding category, an artificial bin.

For what?

For any fungus where scientists only knew the asexual stage, the canadial stage, either because the sexual stage, the ascus or basidium stage had been found yet, or maybe because the fungus had actually lost the ability to reproduce sexually.

Over evolutionary time.

So if you only saw canadia, you threw it in the fungi imperfecti drawer.

Pretty much.

But now, with DNA analysis, we can figure out where these asexual fungi truly belong.

Most of them, it turns out, are related to known ascomycetes, or sometimes basidiumycetes.

So we can place them in the proper phylum based on their genetics, even if we've never seen their sexual structures.

The term deuteromycota isn't really used formally anymore.

Making the classification less imperfect.

Exactly.

It reflects actual relationships much better now.

So wrapping this all up,

what's the big takeaway for us?

I think the biggest takeaway is the sheer complexity and, frankly, the profound importance of this fungal group, the ascomycetes.

They are masters of biochemistry,

masters of adaptation.

From those tiny warren and bodies saving the cell.

To the intricate dance of sexual reproduction with croziers and different mating types, to their complex genetic controls like vegetative incompatibility.

Their absolutely enormous impact on ecosystems and human life.

Right.

As decomposers, as partners in symbiosis like lichens and mycorrhiza, as sources of food and medicine, but also as devastating pathogens.

They are constantly shaping our world in ways we often don't even notice.

Operating just beneath the surface or sometimes right inside us.

It really drives home how much incredible biology is happening in places we don't always look.

Makes you appreciate that slice of bread or that patch of lichen a little bit more.

Hopefully.

There's still so much to discover about them too.

Well this has been incredibly enlightening.

We hope this deep dive into the phylum ascomycota has given all of you listening a new appreciation for these vital, versatile, and sometimes very strange fungi.

Thank you so much for joining us and we look forward to our next deep dive with you.