Chapter 10: Phylum Ascomycota: Order Saccharomycetales—The Ascomycetous Yeasts

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Have you ever, you know, walked past a bakery and just been hit by that incredible smell, that warm yeasty aroma?

Oh yeah.

Or maybe felt that like satisfying fizz from a really good kombucha or a craft beer?

Little everyday things.

Exactly.

And we experience these things all the time, but we probably don't often think about the tiny powerhouses behind them.

Yeasts.

We really do take them for granted, but there's just this whole universe of amazing biology packed into them.

Well, that's what we're doing today.

We're taking a deep dive into what are called Ascomycetus

yeasts.

Specifically, we're focusing on one key group,

the order Saccharomycetales.

That's the plan.

And it's a really fascinating world once you get into it.

So let's unpack that term right away.

Yeast.

It's not like a single branch on the fungal family tree, is it?

It sounds more like a description.

That's a perfect way to put it.

It's really a growth form, a description of how they live.

You know, much like how a tree isn't one specific plant family, lots of different fungi can actually adopt this yeast -like single celled form.

Okay.

So it's functional, not strictly genetic.

Exactly.

And our goal today really is to explore what makes these particular yeasts, the Saccharomycetales, unique.

We'll look at how they reproduce.

They're incredibly diverse roles.

I mean, everywhere from ecosystems to industry, even our own health, and importantly, how scientists figure out how they're all related, especially with newer tools.

And we'll try to make all this biology, which can sound complex, feel, you know, clear and engaging step by step.

Absolutely.

Guide you right through it.

All right.

Let's start with the basics then.

When we're talking about these Saccharomycetales yeasts, what really sets them apart?

Because it's more than just being single celled and budding off new cells, right?

It is, yeah.

That's a key distinction.

While lots of fungi can bud and be unicellular sometimes, Saccharomycetales are, well, a distinct lineage.

And interestingly, one of their defining traits is actually something they lack.

What they don't have.

Right.

They generally don't produce those complex reproductive structures called Ascocarps or the specialized filaments, Ascogynous hyphae, that you see in many, many other Ascomycetes.

They're kind of simpler in their overall structure.

Okay.

And their cell walls are a bit different too.

They're primarily made of sugars like manans and beta -glucans.

There's less chitin, that tough structural stuff, compared to other fungi.

What chitin they do have is mostly concentrated in little rings left behind after budding the bud scars.

Okay.

Subtle differences, but important for classification.

Now you hear about some fungi being dimorphic.

That sounds kind of like a superpower.

What does that mean?

It sort of is.

Dimorphic just means a fungus can switch between two different growth forms.

It can grow as a network of threads, which we call a mycelium, or it can switch to that single celled yeast -like form.

Why would they do that?

It's usually triggered by the environment, things like changes in temperature or maybe the nutrients available.

It's a really neat adaptation strategy, but it's also important to remember that many of the yeasts we're focusing on today in Saccharomycetales spend most of their lives primarily as yeasts.

Even if they can sometimes form threads.

Exactly.

They might form some filament -like structures, especially if things get tough, like if nutrients are really low, but their default mode is often that single cell.

Got it.

Okay.

Let's zoom in a bit more on their

intricate lives, their basic structures, the somatic cells.

How varied are they?

There's quite a bit of variability, even at the fundamental level.

Some species are mainly unicellular and haploid, meaning they just have one set of chromosomes, like our sperm or egg cells.

Others have more complicated life cycles where they switch between being haploid and having two sets of chromosomes, like most of our body cells, and some can even be unaploid.

Having an abnormal number of chromosomes, not a complete extra set, but maybe one extra or one missing.

That can actually give them some unique characteristics or advantages in certain situations.

Wow.

When we picture yeast growing, we often think of just single cells floating around, but you mentioned something called pseudomycelium.

Is that like fake mycelium?

Yeah.

That's a pretty good way to think about it.

A pseudomycelium is a false mycelium.

It happens when a yeast cell buds, but the new cells don't fully separate.

They stay a patched, forming a chain that looks kind of like a fungal thread, a hypha.

But it's not a true hypha.

Right.

It's structurally different from a true septate mycelium, those proper cellular threads with internal walls, which as we mentioned, some yeast can actually form, especially if they're stressed or nutrients are low.

Inside these structures, even the true hyphae, they have internal walls, right?

SEPTA.

What do those do?

Yeah.

SEPTA.

Think of them as internal partitions dividing the fungal threads into compartments.

In most yeast hyphae, these walls aren't solid.

They have pores.

Cores.

To let things through.

Exactly.

Allows for communication, movement of nutrients, sometimes even cell contents between compartments.

There are different types.

Some have just one simple central pore.

Others might have lots of tiny micropores.

There's even one type with many tiny pores that kind of resembles structures in plants, but they form differently.

So even the internal architecture is specialized.

It is.

You don't need to memorize all the types, but it's just neat to realize that these tiny details tell us a lot about how different yeasts function and are related.

It's amazing the detail in such a small organism.

Okay, let's shift to the cell cycle, how yeasts grow and divide.

If we focus on maybe the most famous one, Saccharomyces cerevisiae baker's yeast, how does its cycle compare to, say, our own cells?

Right.

Saccharomyces is the classic model.

All eukaryotic cells, including yeast and us, go through these phases.

G1 for growth, S for DNA synthesis or copying, G2 for more growth, and M for mitosis, the actual division.

In Saccharomyces, there's a really critical checkpoint early in that first growth phase, G1.

It's called start.

And basically, the cell pauses there and checks.

Am I big enough?

Are enough nutrients?

Did the last division finish correctly?

Only if everything checks out does it commit to a new round of division.

Like a little quality control step.

Makes sense.

Very much so.

But what's particularly interesting, maybe even a bit unusual about Saccharomyces compared to, say, many animal cells, is how it coordinates things.

How so?

Well, in many cells, there's a distinct sequence.

Grow, copy DNA, grow some more, then divide.

In Saccharomyces, the new cell actually starts to emerge.

And the spindle, the machinery for separating chromosomes, starts forming during the S phase while DNA is still being copied.

Oh, so it overlaps.

Exactly.

And it doesn't really have a distinct G2 phase, that second growth period.

And its chromosomes don't condense up tightly before division like ours do.

It's a much more streamlined process, maybe helps it grow so fast.

Very efficient.

Okay, let's talk reproduction.

Asexual first.

Budding is the one everyone knows, but are there other ways?

Budding is definitely the most common for many yeasts.

A new cell just kind of sprouts off the mother cell.

But yes, they can also produce other asexual spores like quinidia or sometimes arthrospores, which are like fragments of hyphae that become spores.

But focusing on budding in Saccharomyces,

how does that actually happen?

It's quite elegant, really.

The cell wall softens in one spot, enzymes help with that.

A ring of chitin forms, then the cell contents bulge out, the nucleus divides, one copy moves into the bud, and eventually a new wall, an abscission plate made of chitin, forms to seal off the new daughter cell from the mother.

And this leaves a mark, right?

The bud scar.

Exactly.

A permanent mark, the bud scar, is left on the mother cell.

It looks kind of like a tiny crater with a raised rim.

You can count them.

You can.

Scientists can actually count these scars under a microscope to tell how many times that particular cell has budded, how many offspring it's produced, it's like a biological age marker, and even where the buds form is important.

Some yeasts have bipolar bedding, they're often lemon shaped, and buds only form at the two pointed ends.

Others have multilateral budding, where buds can pop up pretty much anywhere on the cell surface.

These patterns are useful for telling different yeasts apart.

And for yeast that only do this, like Candida albicans,

how do we classify them if they don't have the sexual structures?

That's where DNA really comes in.

For yeasts like Candida that we mostly see reproducing asexually, analyzing their DNA sequences has been absolutely crucial.

It allows us to see who they're related to, even if we haven't observed their sexual cycle, and place them correctly in the fungal family tree.

Okay, so DNA bridges that gap.

Now, what about sexual reproduction?

You said it's rarer, maybe more complex.

It generally is, yes.

It involves two compatible haploid cells fusing together, that's called plasmogamy.

Then, importantly, their nuclei fuse almost immediately, that's karyogamy, to form a diploid zygote.

Almost immediately, so no waiting around.

Right.

Unlike many other fungi that have a stage where two distinct nuclei hang out in the same cell for a while, the dicaryotic stage, yeasts typically skip that.

Karyogamy follows plasmogamy pretty quickly.

Okay, so they form this diploid zygote.

What happens next?

That zygote can then transform into an ascus, the sac that holds the sexual spores.

Or, in some cases, it might grow a little stalk first, an askafore, which then produces the assi.

Inside the ascus, meiosis happens, reducing the chromosome number back to haploid, and typically forming four aska spores.

Typically four.

Usually four, but some species are different.

They might undergo extra mitotic divisions after meiosis, ending up with eight, or even hundreds of spores in a single ascus.

Wow, hundreds.

And I heard these spores are visually diverse, too, like tiny sculptures.

They really are.

Aska spores come in an amazing variety of shapes.

You find globos, or ovoid ones, sure, but also hat -shaped, Saturn -shaped, with a little rim hemispherical, needle -shaped, kidney -shaped, even club -shaped.

Hat -shaped and Saturn -shaped, that's pretty specific.

Isn't it?

And some are smooth, others might have little warts or ridges on their surface.

The way they get released varies, too.

Sometimes the ascus wall just dissolves, especially for hat or Saturn spores.

Other times, the spores start germinating inside and burst the ascus open, or there's a special weak spot at the tip that ruptures.

And these spores are tough.

Surprisingly tough.

Much more durable than the regular vegetative yeast cells.

They can survive drying, heat, chemicals, even remarkably, passing through the digestive tract of a snail.

Those enzymes don't break them down.

It's a great survival mechanism.

Incredible resilience.

Okay, let's zoom out from the cell level and talk about where these yeast live in the bigger world.

What kind of habitats do they like?

They're often found in places with high sugar concentrations and, interestingly, relatively low water availability.

Think of things like sugary plant secretions, slime fluxes on trees, flower nectar, the surfaces of fruits.

So they like sweets.

They definitely have a sweet tooth.

But also surprising places, like salty brines used for curing foods.

They're also common in soils, in water, and significantly in the digestive tracts of animals, including insects and us.

How do they handle those sugary or salty places?

That sounds stressful for a cell.

It is.

It's osmotic stress.

They cope by producing things like polyols, sugar alcohols, and proteins inside their cells to balance things out.

It helps prevent them from losing too much water.

Interestingly, compared to many molds, yeast generally produce fewer toxic secondary metabolites.

Why is that?

Well, the thinking is that maybe because they specialize in these high sugar niches, there's just less intense competition from other microbes, so they haven't needed to evolve as many chemical weapons.

It's a big exception to that low toxicity idea, isn't there?

These killer yeasts.

That sounds intense.

It really is a dramatic phenomenon.

Killer yeasts were first discovered in, again, our friend Saccharomyces cerevisiae.

These strains actually secrete proteins that are toxic, lethal to other susceptible strains of the same species or closely related ones.

So they kill their competition?

Directly.

And crucially, they're immune to their own toxin.

It's like microbial warfare.

Wow.

How does that work?

Is it genetic?

It's fascinatingly complex.

The killer trait, like the K1 phenotype in Saccharomyces, isn't usually encoded in the yeast's own chromosomes.

It actually comes from a double stranded RNA virus living inside the yeast cell.

A virus gives it this power.

Yes.

The virus carries the genes for both the toxin and the immunity factor.

And often, its persistence depends on the yeast's genetic makeup and sometimes even another larger helper virus particle.

The toxin itself is secreted in an inactive form, gets activated on the outside, and part of it binds back to the killer cell, protecting it.

That's incredible.

And does this actually matter out in the wild?

Oh, definitely.

Different killer strains often have different optimal conditions.

Maybe one works best at a lower pH, another at a higher temperature.

This means they can carve out and dominate specific ecological niches by eliminating rivals that aren't immune.

Literally assassinating the competition.

OK, that's a definite powerhouse trait,

which leads us nicely into their role as industrial powerhouses for humans.

We've used them for ages, right?

Absolutely, millennia.

There's evidence going back maybe 6000 BC, maybe even earlier, primarily for fermentation, which was a great way to preserve food and drinks by lowering the pH and producing alcohol.

And the name saccharomycetes even means sugar fungi.

Exactly.

Because they are brilliant at fermenting carbohydrates, sugars.

In baking, we harness the carbon dioxide gas they produce to make dough rise.

In brewing and wine making, the main prize is the ethanol, the alcohol.

And how we use them has changed over time, hasn't it?

From just using leftover bits to pure cultures.

A huge shift.

For centuries, it was all about starter cultures, using a bit of dough from yesterday's bread or the dregs from the last batch of beer.

But then came the scientific approach, pioneered by people like Emil Hansen at the Carlsberg Laboratory in Denmark in the late

1800s.

He figured out how to isolate and grow pure cultures of specific yeast strains.

And that changed everything for industry.

Completely.

It allowed for predictable, consistent results.

Today, brewers, bakers, winemakers, they carefully select yeast strains based on very specific traits, how much sugar they can handle, how much alcohol they tolerate, and crucially, the subtle flavor and aroma compounds they produce, or sometimes the ones they avoid producing.

Which explains all the different beer styles like ales versus lagers.

I heard the pilgrims on the Mayflower were drinking ale.

That's the story.

Ales traditionally use top -fermenting yeasts, like saccharomyces cerevisiae, which tend to rise up and form a foam at the top during fermentation.

Lagers, which became popular later, especially with German immigrants, use bottom -fermenting yeasts, like saccharomyces pastorianus, which tend to settle at the bottom and work at cooler temperatures.

But even in these controlled industrial settings, it's not just one single yeast doing everything.

Rarely is it that simple.

Even in a commercial brewery or winery using a pure starter culture, there's often a complex ecological succession happening.

Other use, maybe some bacteria, play roles at different stages, contributing to the final product.

And then you have things like spontaneous fermentation.

That sounds wild.

It is.

Think of traditional Belgian lambic or Goez beers.

They don't add any specific yeast culture.

They rely entirely on the natural yeasts and bacteria present in the brewery environment, floating in the air, living in the wooden barrels.

So it's like inviting the local microbes in for a party.

Pretty much.

It involves a whole sequence of different organisms taking turns, often including yeasts from the genus Decara, which give those beers their characteristic sour, funky notes.

It's a beautiful example of microbial ecology in action.

Okay.

Switching gears slightly.

Kombucha.

Super popular fermented tea.

What's the deal there?

Right.

Kombucha.

It typically involves fermenting sweetened black or green tea with what's called an Esco -Y, a symbiotic culture of bacteria and yeast.

It forms this thick, rubbery pancake -like mat.

It's Esco -E -Y.

Yep.

It contains various yeasts and acetic acid bacteria working together.

Now, kombucha has a long history in folk medicine, particularly in Asia, and you hear a lot of health claims today.

Cures for this, benefits for that.

But the science.

From a rigorous scientific standpoint, most of the major health claims haven't really been confirmed in strong clinical trials.

It's important for us, just reporting on the source material, to note that.

The FDA has also at times investigated potential issues with contamination by unwanted microbes if it's not made carefully.

So it's a popular drink, a fermented product, but the jury's still out on many specific health benefits.

Got it.

Beyond drinks, yeast are also used as food sources themselves.

Single -cell protein.

Yes.

SCP, or sometimes just food yeast.

This idea got a big push, especially after World War II, looking for ways to boost protein supply.

The concept is to grow yeast on relatively cheap, low -protein raw materials, often industrial byproducts like whey from cheese making, waste from potato processing, spent grains from brewing, wood pulp waste or molasses.

Turning waste into food, essentially.

That's the goal.

Convert those carbohydrates into high -protein yeast biomass.

One technical hurdle was that yeast cells have a lot of nucleic acids, RNA and DNA, which can cause problems like gout in humans if you eat too much.

Methods were developed to reduce the nucleic acid content.

So do we eat much of this SCP?

Not directly, as much as initially hoped.

It's quite common in animal feed.

Clavermyses marxianus grown on whey is a good example.

For direct human food, things like marmite and vegemite, which are yeast extracts, have been popular for a long time, especially in places like the UK and Australia.

But generally, the production costs and sometimes issues with waste disposal have made it less competitive against other protein sources like soy or fish meal for widespread human food use.

Okay,

so mostly animal feed in those iconic spreads.

Now, unfortunately, yeasts aren't always beneficial.

Some are medically important in a negative way.

That's the other side of the coin, yes.

The prime example here is Candida albicans.

For many people, maybe most people, it's just a harmless member of the normal collection of microbes living on our skin, in our mouth, gut, vagina, part of the microbiome.

But it can cause problems.

It can become an opportunistic pathogen, meaning if the conditions are right, if the person's defenses are weakened, it can switch from being harmless to causing disease, which we call candidiasis.

What weakens the defenses?

Things that compromise the immune system are key conditions like HIV AIDS, chemotherapy, or immunosuppressant drugs,

long -term antibiotic use that disrupts the normal gut flora.

Also, hormonal changes, like during pregnancy or conditions like diabetes, can make people more susceptible.

What kind of infections does it cause?

Is it just thrush?

Thrush, those white patches in the mouth or throat, is a common superficial form.

Similar infections can happen on the skin or cause genital yeast infections.

Those are usually treatable.

But Candida can also cause much more serious invasive infections if it gets into the bloodstream or deeper tissues, things like peritonitis, infection of the abdominal lining, meningitis, infection around the brain and spinal cord, systemic blood infections, candidamia, even infections in joints or organs.

These can be very dangerous, even life -threatening, especially in already sick or immunocompromised patients.

How does the body normally fight it off?

We have barriers like intact skin and mucous membranes, and then we have our immune cells, particularly phagocytic cells in the lymph and blood, that engulf and destroy invaders.

But as we said, if those defenses are down, Candida can take advantage.

Interestingly, some studies suggest certain drugs, like morphine, might interfere with some of these defense mechanisms, potentially increasing risk.

And how do doctors diagnose it?

Just look for it.

Diagnosis involves several approaches.

Sometimes, yes, direct observation of the characteristic yeast cells, or hyphae, under a microscope from a sample.

Culturing the yeast from blood or tissue is crucial for invasive disease.

There are also biochemical tests looking at which carbohydrates the yeast can use.

And increasingly,

modern molecular methods, like DNA fingerprinting, are used not just to identify Candida albicans, but to track specific strains, maybe during an outbreak in a hospital.

Okay, it's clearly a major player in human health, for better and for worse.

Let's switch back to classification.

Mapping the fungal family tree.

You mentioned DNA has been a game changer for yeast taxonomy.

A massive game changer.

Really, the classification of yeast has just leaped forward thanks to molecular tools, especially sequencing ribosomal DNA genes.

It's resolved so many long -standing questions and debates.

So older methods, like looking at budding patterns or cell wall chemistry,

they weren't always accurate.

They were the best tools available at the time, and they provided valuable clues.

But sometimes they led to disagreements or misinterpretations.

For example, how exactly did these simpler yeasts fit in with the more complex filamentous Ascoma seeds, or with other groups thought to be early diverging fungi?

DNA analysis has largely clarified these relationships, showing the Saccharomycetales as a well -defined group within the Ascoma coda, closely related to the filamentous forms.

And it helped place those asexual yeasts, too.

Exactly.

Fungi, that we only knew from their asexual stage, like many Candida species, could finally be placed confidently with their sexual relatives based on their DNA similarity.

It filled in huge gaps in the tree.

And this DNA data also changed how we interpret similarities between different yeasts, right?

This idea of convergent evolution.

That's been a really profound insight.

We used to see certain features like, say, having hat -shaped ascospores, or always being associated with insects, and assumed that meant those yeasts must be closely related, inheriting that trait from a common ancestor.

But that's not always true.

No.

The DNA evidence often shows that yeasts with those similar traits are actually in very different branches of the family tree.

It means those traits evolved independently multiple times, probably because they offered a similar advantage in similar lifestyles or environments.

That's convergent evolution.

It really forces us to rethink relationships based solely on appearance.

Fascinating.

So, given this new molecular understanding, could you spotlight a few key families or groups within the Saccharomyces tails?

Maybe starting with the big one, Saccharomycetaceae.

Sure.

The Saccharomycetaceae family is characterized by that multilateral budding buds forming all over.

And it includes, of course, Saccharomyces cerevisiae, our bakers' and brewers' yeast.

And as we hinted, molecular work is showing cerevisiae isn't just one simple species, but likely a complex of very closely related species and natural hybrids.

Explains why different strains work differently.

Precisely.

This family also includes Claviromyces, found in dairy environments and used industrially, and Pichia, often linked with insects and known for those hat or Saturn -shaped spores.

And importantly, this is where DNA places the medically significant Candida albicans, showing its close relationship to genera like Pichia, even though,

historically, Candida was a bit of a catch -all bin for asexual yeasts.

So Candida isn't one neat group.

Not a monophyletic group, no.

The name is still used, but genetically, its species are spread among relatives of Pichia, the Baryomyces, and others within this family and related ones.

DNA untangled that.

Okay.

What about some of the others?

You mentioned those needle -like spores earlier.

Right.

There are families like Ermothicicia.

Some members are plant pathogens, like on cotton.

But they're also used industrially to produce riboflavin, vitamin B2.

They often have needle -like ascospores.

Then there's Metchnicoea, which also has needle -like spores, but lacks mycelium and is often parasitic on tiny aquatic animals.

DNA showed that despite both having needle spores, they aren't close relatives.

So Metchnicoea gets its own family.

Metchnicoea ac, another convergence example.

DNA clarifying things.

Any other interesting families?

Well, there's the Dipodoskiche.

This includes some fascinating fungi like Dipodoskiche, which forms spores by fragmentation, arthrospores, and has a sexual reproduction involving the fusion of whole branches, not just single cells.

And then Lipomycetaceae is considered an early diverging lineage within the Saccharomycetales, often found in soil or associated with insects.

They have some unique cell wall structures.

It just shows the incredible diversity bundled under this yeast umbrella.

It really does.

Wow.

From, I mean, making bread rise and beer ferment to causing really serious diseases, these intricate cell cycles, killer toxins, shaping entire industries and ecosystems.

It's just staggering how much these tiny Eskimosetaceae do.

They're absolutely not simple organisms.

Not at all.

And we've really just skimmed the surface today.

Their structures, how they reproduce, their impact on us and the planet, it's vast.

And what's really exciting is that even now, especially with molecular tools, we're constantly discovering new complexities, new connections.

It shows how much more there probably is to learn in the microbial world.

Absolutely.

It really underscores that even the simple things around us hold incredible secrets, and studying them could lead to who knows what future innovations may be in biotech, medicine, or even just understanding life better.

It really makes you wonder, doesn't it?

It leaves you with a thought, perhaps.

How might knowing a bit more about these tiny, powerful microbes change how you, the listener, see the everyday world?

That bread, that beer, maybe even just the soil in your garden?

That's a great point to ponder.

Well, thank you so much for joining us on this deep dive into the

really captivating world of Eskimosetaceae yeasts.

We hope you feel a little more connected to these vital, often invisible organisms.

Thanks for listening.

Until next time, keep exploring.