Chapter 15: Phylum Ascomycota: Other Filamentous Ascomycetes

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Imagine something so incredibly tiny.

You might walk past it every day without a second glance.

Yet, these microscopic architects are shaping the world around us in really profound ways.

Absolutely.

They influence everything from the health of your garden plants to the very insects buzzing through the air.

Exactly.

And today, we're embarking on a deep dive into the fascinating world of fungi, specifically a diverse and, well, often puzzling group within the phylum Ascomycota, what many call the sac fungi.

That's right.

Our mission is to take a close look at a chapter from introductory mycology and really pull out the most important insights for you.

We want to unravel their unique structures, how they reproduce,

their hidden genetics, and why they matter so much.

Precisely.

Why they are such vital players in our ecosystems and even, you know, in our daily lives.

Think of this as you're express lane to truly understanding a topic that's far more surprising and relevant than you might first imagine.

We're talking genuine aha moments packed with just enough intrigue to keep you hooked.

Now, let's unpack this.

Even for seasoned mycologist,

these fungal groups can be a real taxonomic puzzle.

Our source notes their poorly resolved phylogenetic status, which basically means historically many were lumped together more for convenience than because they were actually closely related.

Right.

But modern molecular studies have really shaken things up, revealing just how distinct and, well, surprising these organisms truly are.

It's a perfect illustration of how scientific understanding is evolving, refining our view of life on earth.

And today we'll explore some of these intriguing ascomycetes, starting with a group you've likely seen, maybe without even realizing it.

Okay, let's jump right into the first major group.

The powdery mildews, scientifically known as the order erycifoles.

If you've ever seen a plant leaf covered in what looks like a fine white dusting, you've probably met them.

These fungi are incredibly widespread plant pathogens.

And their name, powdery mildew, it really describes that telltale white coating they leave on infected plants pretty well.

So what's the key thing to know about them?

Well, what's crucial to understand about erycifoles is that they are obligate biotrophs.

Obligate biotrophs.

Okay, what does that mean exactly?

It means they are absolute parasites.

They simply cannot survive or reproduce without a living host plant.

They're like microscopic vampires, completely dependent on their host for nutrients.

Wow.

And that powdery look.

That's largely due to the sheer immense number of their asexual spores called canidia.

Okay, here's where it gets really interesting and a bit counterintuitive.

While those canidia look dry and powdery, making them seem perfect for wind dispersal.

Right, you'd think so.

The source points out that in large numbers they're actually quite wet and sticky.

Yeah, isn't that something?

One researcher even suggested white mildews might be a more accurate name because of that stickiness.

It makes you wonder how that plays into their strategy.

Maybe helping them cling to a new host once they land.

Could be.

It's a subtle but important detail about their dispersal and their impact is enormous.

Powdery mildews attack over 40 ,000 plant species.

40 ,000.

Yeah, with about 90 % being dicots, that's a massive group of flowering plants.

You know, everything from roses and sunflowers to apples and beans.

We're talking significant agricultural concerns.

Like what?

Any big examples?

Oh, definitely.

Consider Unsinulinaicator on grapevines.

This one fungus alone can devastate an entire grape crop, impacting vineyards worldwide.

Big economic impact.

But it's not always a disaster for the plant growth.

No, exactly.

Some powdery mildews like Microsfera alni on lilac might show up year after year giving the leaves that dusty look but causing little to no actual harm.

So it's a spectrum.

It really highlights the incredible spectrum of impacts within just one fungal group.

From economically devastating to surprisingly benign.

What about which plants they attack?

Are they picky eaters?

That's also fascinating.

Some species like Arasufra polygoni are almost omnivorous, reported on over 350 different plant species.

Just goes for almost anything.

Wow.

But then conversely, you have highly host -specific ones like Blumeria graminis, the major cereal crop pathogen.

This one has developed specialized forms, almost like different strains.

Ah,

right.

Formae specialis.

That only attacks specific cereals like only barley or only oats.

Very precise.

That level of precision is truly remarkable.

And our source shares a cool real world research example about studying these guys.

Oh yeah, the spore trap car.

Yes.

For barley mildew,

Blumeria graminis fsp hordei, in Europe, researchers used an automobile mounted spore trap, literally driving around sucking up spores.

And they used advanced DNA markers,

repd markers, to track their populations.

It's brilliant because these fungi are so parasitic, you can't just grow them easily in a lab dish.

You have to study them in the wild, essentially.

And what did they find?

This method allowed them to see that massive diverse populations exist across Europe, giving us a clearer picture of how they spread and how genetically varied they are.

Okay, so how do they actually interact with the plant up close, on a microscopic level?

Right.

So their fungal body, the mycelium, mostly grows on the host's surface, like a thin white web over leaves or stems.

Just sitting on top.

Mostly, yeah.

To stay put, they use specialized anchoring structures called apresoria.

Think of them like little holdfasts.

They can be simple or quite complex, with multiple lobes acting like tiny suction cups or anchors.

Okay, anchored.

But how do they feed?

And then, the real trick, they send in specialized hyphal branches called hostoria.

These penetrate the host cells.

Hostoria, those sound important.

They are.

That's how they absorb nutrients directly from the host cells.

Imagine a tiny slender neck region that penetrates the cell wall, and then it widens into a body inside the cell.

That body can be shaped like a sphere, or a pear, or even have multiple lobes.

Can you describe them a bit more?

What do they look like?

Well, the Blumeria graminis hostoria are particularly unique.

They have numerous long, finger -like lobes extending away from the main body inside the host cell.

Very complex structure.

And that shape must be important.

Absolutely.

This complex shape isn't just a biological detail.

It's likely the result of millions of years of coevolution.

It probably helps them maximize nutrient uptake and maybe even evade some host defenses.

These hostorias are separated from the host's living cytoplasm by a specialized boundary.

The extra -hostorial matrix and membrane, which is often thicker and convoluted, showing its specialized function.

And you mentioned they're hard to grow in the lab.

Extremely.

Just to reiterate that key point,

no powdery mildew species has ever been successfully grown in a pure lab culture on defined media.

Never.

Wow.

They are truly obligate parasites, only thriving on living host tissues, or sometimes tumor tissue cultures derived from hosts.

It really underscores how perfectly adapted they are to this parasitic lifestyle.

Okay, so they need a living host.

How do they spread so fast during the growing season, then?

That's mainly asexual reproduction.

They produce these clear, upright stalks called canidiofors, which generate a tremendous number of those asexual spores we mentioned earlier, the canidia.

Right, the canidia.

The powdery part.

Exactly.

Imagine these as tiny, one -celled, clear structures, often oval or cylindrical, that are super lightweight and easily blown around by the wind.

They can develop singly or with the oldest mature ones at the tip.

These canidia are the primary way they spread during the season, starting new infections.

You mentioned something about humidity earlier.

Yeah, interestingly, there's some debate about their optimal humidity for germination.

While many fungi need high humidity, some reports suggest powdery mildew canidia can germinate even at 0 % relative humidity, which is unusual.

Very strange for a fungus.

It is.

It also notes that these canidia are extremely fragile, susceptible to the slightest injury, making them delicate to handle and research.

And most powdery mildews also have this asexual stage, or anamorph, that's often referred to by the genus name oedium.

Okay, so let's walk through an infection.

The canidium lands on a leaf.

Then what?

Okay, so using Blumeria graminis on barley as our example, within just 20 minutes of contact, the canidia exude a sticky matrix to adhere firmly, like instant glue.

20 minutes?

That's fast.

Very fast.

They then quickly secrete enzymes, estraces, that start degrading the plant's protective outer layer, the cuticle.

They're essentially preparing a tiny infection court.

Okay, they've stuck on, they've weakened the surface.

Then a tiny primary germ tube emerges.

It penetrates the cuticle, but, intriguingly, it doesn't develop further, it just stalks there.

Why?

What's it doing?

Scientists believe this specific structure, which seems unique to B.

graminis, is crucial for absorbing water and maybe some elements from the plant's surface, basically fueling the pathogen's initial growth before the main invasion.

Right, getting established.

Exactly.

A few hours later, a second germ tube develops.

This one forms an appressorium, that anchor structure.

They're pulled fast, yeah.

And from that appressorium, a narrow penetration peg emerges.

This peg pierces the host cell wall, pushes into the cell membrane, and then develops into that nutrient -absorbing hostorium inside the cell.

And once that's in, the fungus is basically set up.

Pretty much, and the speed is incredible.

A single successful infection can lead to a visible colony that produces an astonishing 300 ,000 wind -disseminated knidia in just 5 to 25 days, ready to start the cycle all over again.

Wow, that's a masterclass in rapid widespread dispersal.

It really is.

But beyond this fast asexual spread, they also engage in sexual reproduction, typically later in the growing season.

Okay, shifting gears, how does that work?

This is when Asco carps, their sexual fruiting bodies, appear.

You might see them as tiny dots on a leaf.

They actually change color as they mature, starting white, then moving through orange and reddish -brown, finally turning black when mature.

And when do they mature?

These can mature very late in the fall, or sometimes not even until the following spring.

This timing plays a vital role in their survival over winter.

So how do they survive the winter?

Dormant spores.

Several ways.

They can survive as these tough Asco carps on fallen leaves on the soil, or lodged in bark crevices like Unsinula necator does on grapevines.

In some perennial hosts, they simply stay put as dormant mycelia hidden inside buds.

And in warmer climates, some species might just skip forming Asco carps entirely, relying solely on those asexual knidia for year -round survival.

What do the Asco carps themselves look like?

They're typically like tiny closed spheres, technically called clastopetia.

They don't have a preformed opening.

They eventually split open, usually irregularly, to release the Asco carps containing the sexual spores.

And I remember reading something about appendages.

Ah, yes, the appendages.

What's truly fascinating and really important for identification are the characteristic appendages on these Asco carps.

They vary wildly in shape, depending on the genus.

Like how?

Oh, they can look like simple floppy fungal threads mycelioid.

Or they can be rigid and spear -like.

Some have distinctively curled tips, almost like little hooks.

Others have tips that branch dichotomously, like tiny antlers.

So they're key for telling species apart.

Genera, mostly.

These appendages are crucial for generic identification, especially since the internal structure is quite similar across many groups.

And they aren't just for looks.

They can play a role in dispersal, too.

Like the shuttlecock thing.

Exactly.

The shuttlecock analogy for Felictinia dispersal.

This species has a crown of short, apical appendages that secrete mucilage.

They're sticky.

So when an Asco carp detaches from the leaf, these appendages help orient it as it falls, kind of like a shuttlecock.

So it lands sticky side down, presumably onto a surface where it can effectively release spores later.

That is ingenious.

It gets better.

When it overwinters, this Asco carp actually opens equatorially, splitting around the middle.

This ensures the spores, when released, are shot effectively upwards towards new leaves in the spring.

It's a brilliant example of form, perfectly matching function for dispersal.

Amazing adaptation.

But you mentioned classification is tricky.

Yeah, despite these helpful features, classifying Arecifales has historically been difficult.

DNA analysis has been absolutely vital in clarifying their relationships, showing they are not as closely related to some groups they were previously linked with.

So the family tree is still being figured out.

It's an active area of taxonomic work.

The number of accepted genera has fluctuated quite a bit, from maybe six or seven historically to around 28, based on more recent molecular and morphological studies.

It really demonstrates the dynamic nature of fungal classification.

Identification is much easier if you have those mature Asco carps with appendages.

Relying just on the asexual oem stage is less satisfactory, though progress is being made there, too.

Okay, we've seen how powdery mildews master plant parasitism.

But if you thought that was specialized, prepare for our next group, the Lobulbenials.

These fungi have taken an even more, well, bizarre and direct approach to living on other organisms.

Oh, the Lobals.

Yeah, what's truly mind -bending about the Lobulbenials is that this is a large group of specialized Ascomycetes, and many of them lack a mycelium entirely.

No mycelium.

How does a fungus work without a mycelium?

Right, they don't grow a typical fungal body.

Their structure is very reduced and unique.

And get this, their history is wild.

Yeah.

These fungi were once so puzzling morphologically.

Totally bizarre looking.

That one species was initially described as a parasitic worm of batflies, not even as a fungus.

And the whole group was even suggested at one point to be related to red algae because of some superficial similarities in morphology and biochemistry.

Red algae.

That's way off.

It's a fantastic example of how modern molecular studies have revolutionized our understanding.

DNA evidence firmly places them as fungi as Ascomycetes, despite their incredibly weird appearance.

Their defining characteristic is that they are obligate parasites of insects, mites, and a few millipedes, typically growing right on the exoskeleton of a single host.

So little insect hangers on, are they all the same?

No, if we connect this to the bigger picture, the Lobulbenials actually fall into two distinct lineages, which is pretty neat.

First, there are the traditional Lobulbenials groups like Lobulbenias.

These are the non -mycelial ones we were talking about.

Their fungal bodies, which we call thalli rather than mycelia, develop directly from an ascospore sticking onto the external surface of an arthropod.

They send these little peg -like or branched hostoria just into the host's cuticle, the hard outer shell, to discreetly draw nutrients.

Do they harm the insect much?

Generally, no.

While they are parasitic, they don't seem to cause serious injury, though there are some reports suggesting they might slightly decrease host survival or fitness in some cases.

Kind of like fleas, maybe?

Annoying, but not usually deadly.

Okay, that's one lineage, what's the other?

But then you have the Pixidiophoresi.

These are quite different because they do have a mycelial stage.

Ah, okay.

More conventional in a way.

Well, yes and no.

This lineage exhibits a really rare trait in fungi called heteroacy.

Heteroacy means different hosts.

Exactly.

It means different parts of their life story unfold on completely different types of hosts, a complex strategy shared only with a few other fungi like some cutrids and the plant rust fungi.

Wow.

So what are the hosts for Pixidiophoresi?

They have a mycelial state that lives in decomposing organic material like dung or rotting wood.

But here's the twist.

That mycelium acts as a parasite on other fungi living in that material.

A fungus parasitizing another fungus.

Yep, a mycoparasite.

This is where they produce their sexual fruiting bodies, the parithesia.

Then their ascospores are cleverly dispersed, usually by hitchhiking on insects and especially mites.

Hitchhiking mites.

Yeah, it's a phenomenon called phoracy, where mites literally catch a ride on larger insects like beetles and flies.

The mites act as fungal taxis, carrying the Pixidiophorespores to new patches of suitable organic matter with potential fungal hosts.

That is incredibly complex.

Can you trace that life cycle?

Sure, let's quickly trace the incredible life cycle of a typical Pixidiophora.

The ascospores ooze out passively, often accumulating in a sticky droplet at the tip of a long neck on their fruiting body, the parithesium.

Waiting for a ride.

Exactly.

They have a sticky attachment region that adheres to a passing mite.

Okay, spore meets mite.

Then what?

Once on the mite, that ascospore develops into a small thallus.

And this thallus produces asexual spores called canidia.

This stage is known as the thexteriola anamorph.

Still on the mite.

Still on the mite.

These canidia then divide into yeast -like cells.

When the mite reaches a suitable substrate, like dung, these yeast cells are deposited and they germinate to initiate the mycelial phase.

And remember, this mycelial phase parasitizes other fungi already growing in that dung or wood.

Fungus eats fungus, makes spores, spores catch a mite, make more spores on the mite, mite drop spores off.

Oh, wow.

It's a complex multi -host strategy.

And the speed is astonishing.

The entire life cycle can potentially be completed in just five days in dung.

Sexual reproduction forming the parithesia usually requires that fungal host to be present.

Any other weird things about them?

They also have some highly unusual features like a peritical wall made of just a single layer of cells, which is quite uncommon.

And they consistently produce only three

perascus, another oddity.

Okay, back to the other libules, the traditional ones without mycelia.

Right.

The non -mycelial forms that attach directly to arthropod hosts via those little hostoria just penetrating the cuticle, their host specificity is truly remarkable.

How specific?

Many are highly host specific, like the genus Herpomyces, where different species only infect specific species of cockroaches.

But then surprisingly, some can infect a range of very different arthropods.

There's one Labalbenia acetonus, which infects army ants and the mites and beetles that live with the army ants in their colonies.

So sometimes specific, sometimes less so.

Exactly.

And it gets even more precise.

Many demonstrate position specificity.

Position specificity.

You mean where on the insect they grow?

Precisely.

Different fungal species may only infect very specific sites on a single host's body, like only on the left antenna or only on the underside of the third leg segment.

No way.

Why?

It's often linked to the host's behavior, especially mating behavior.

The fungi get transferred during host copulation, so they evolve to grow near the points of contact.

Direct infection.

That is incredibly specialized.

It gets even wilder.

The actual growth form of the fungus can vary dramatically depending on the exact part of the insect it grows on.

For example, Herpomyces stylopige looks different depending on whether it's growing on long sensory bristles or short peg -like bristles on a roach's antenna.

The substrate dictates the morphology.

It truly showcases an extreme level of coevolution.

Can you grow these in the lab?

Almost impossible.

This extreme specialization is further underscored by the fact that only one species of traditional lablibiniales out of thousands described has ever been even partially grown on artificial medium in a lab.

Just one.

Partially.

Wow.

Underscores how tied they are to their host.

Absolutely.

They are incredibly dependent.

So what's their life cycle like, briefly?

The ones on the insects?

Well, even without a mycelium, it's still intricate.

It begins with a two -celled ascospore landing and adhering to the arthropod's integument, its outer layer.

Then a hostorium penetrates the cuticle just enough to draw on nutrients.

The main fungal body, the thallus, develops externally through very specific, genetically programmed patterns of cell vision.

It's not a random blob, it's a highly structured little organism.

This thallus eventually produces the parathasium, which holds the assi and ascospores, and often various sterile appendages as well.

Is it sexual reproduction?

Yes.

There's typically a female receptive structure called a carpogonium, often with a hair -like extension called a trichogine to receive male cells, or spermatia.

Fertilization, or plasmargamy, occurs, followed eventually by meiosis to produce the ascospores.

So even though they look weird?

Exactly.

Even with their bizarre appearance, advanced microscopy, like transmission electron microscopy, has confirmed they have internal structures typical of other ascoma seeds like warrenan bodies near septal pores and typical ascospore development within the ascus.

It solidifies their true fungal identity.

Okay, from creepy crawlies on insects, let's now switch habitats entirely.

Let's dive into the oceans and talk about the spatholos spiralis.

Right, a much smaller and lesser known order.

The spatholos spiralis are found exclusively growing on red algae in various marine environments around the world.

Red algae fungi.

Okay.

Interestingly, just like the loboles initially caused confusion, these were also initially thought may be related to lobobinials.

That idea came from early observations of dried herbarium specimens, which seem to suggest an external -only development similar to how loboles look on insects.

There's always a butt with these fungi, isn't there?

It seems so.

This raises an important point about studying fungi, what happens when new evidence emerges from looking at living material.

Studies on fresh living specimens of spatholospora ficophila completely overturned that assumption.

What did they find?

They discovered it actually forms a broad and coiled fungal body,

an intracellular thallus developing within a single cell of the red alga host.

Only later do the external parathasia, the fruiting bodies, develop and emerge.

So it's hiding inside.

Exactly.

It's a perfect example of how direct, fresh observation can radically change our understanding.

Revealing complexity hidden in dried specimens shows the importance of looking at things live when you can.

What do they look like?

Morphologically, spatholospora itself typically has dark, somewhat leathery -walled parathasia and often dark, incurved appendages around the top.

In contrast, another genus in this group, Hispidicarpomyces galexoria, has a dark intracellular mycelium that spreads widely between the cells within the algal body.

So different strategies, even within this small group, for interacting with their algal hosts.

Okay.

Fascinating.

Now, to wrap up the groups, let's briefly touch on some genera that are still a bit of a mystery.

The ones labeled genera of uncertain affinity.

Right.

These are the real head scratchers from mycologists currently.

We're talking about cathestas and subaromyces.

Uncertain affinity means we don't really know where they fit in the fungal family tree.

Pretty much.

These two genera have uncertain relationships based on current data, and they seem to lie outside the major established ascomycete groups.

Yet they share some intriguing traits that hint at possible connections or similar lifestyles.

Like what?

Well,

they both tend to have evanescent assae, meaning the spore -containing sacs dissolve or disappear very quickly after the spore is mature.

They also often have long, parathesial necks.

And why are those features interesting?

Those features, especially the long necks, are often associated with arthropod dispersal.

It suggests that insects or mites might be playing a role in spreading their spores, even if we haven't directly observed it in all cases.

Any other shared traits?

Yes.

They also tend to have single layered parathesial walls, similar to the pixidia forsy we talked about.

And they are often associated with other fungi, sometimes as outright mycoparasites, meaning they parasitize other fungi.

Or they might require the presence of other fungi just to stimulate the formation, even in their own fruiting bodies.

Makes them hard to study alone, I bet.

Very difficult to study in isolation, yes.

So tell us about cathistes.

Cathistes has, so far, only been found in dung.

Critically, its ascospores have been observed attached to mites found in the same habitat, strengthening that arthropod dispersal link.

They have these unusual incurved extensions inside the long neck, where the ascospores seem to collect before release.

And they also produce these other really tiny spore -producing bodies called spiridiomata, whose function is still completely unknown.

Truly a niche organism.

A dung fungus spread by mites.

Okay.

And subaromyces.

Subaromyces splendens is maybe even stranger because of where it's found.

It seems unique to human -created habitats, like trickling filter systems used for wastewater treatment and even reported from antibiotic manufacturing plants.

Weird places for a fungus.

Very weird.

It also requires contaminating fungi, other species growing alongside it, to be present for deformant's pyrethesia.

And while its morphology, like the long neck, suggests arthropod dispersal might be involved, it has never actually been directly observed.

It really makes you wonder what other highly specialized fungal interactions are thriving just out of sight, perhaps even in environments we've built ourselves.

Absolutely.

We've just taken quite a journey, a real deep dive into just a slice of the Ascoma coda phylum.

And it showcases fungal diversity and adaptation that's almost beyond imagination.

It really does.

From the widespread plant diseases caused by powdery mildews with their unique hostoria and incredibly efficient infection processes.

To the incredibly specialized lobobinials living directly on insects in ways that are just, well, bizarre and fascinating.

And we explored the surprising aquatic world of spatholus spiralis living on red algae and touched on those enigmatic genera like cathetes and subaromyces that still hold so many mycological mysteries.

We've seen how their unique structures, their often complex life cycles, and their highly specific adaptations make them absolutely vital players in ecosystems.

Yeah, whether as plant pathogens impacting agriculture, or as intimate associates of insects influencing their populations, or even as hidden components decomposing waste in our own systems.

This knowledge helps us understand not just basic biology, but also things like plant health,

insect ecology, and maybe even industrial processes.

It really feels like the world of fungi is still so full of undiscovered connections and unexpected survival strategies.

Oh, definitely.

You have to wonder what other hidden, highly specialized fungal interactions are out there waiting to be revealed, perhaps in places we least expect them.

Like what's going on in the deep sea, or high in the canopy, or deep in the soil.

And thinking bigger picture, how might understanding these intricate biological relationships, these parasites, these symbionts, these hitchhikers, how might that open new doors for us?

Maybe in biotechnology, or medicine, or even in finding new ways for ecological management.

It truly makes you wonder what else is thriving just out of sight.

A lot left to discover.

Thank you for joining us on this deep dive into the amazing and sometimes strange world of fungi.

Keep learning, keep exploring, and stay curious.