Chapter 28: Phylum Acrasiomycota: Acrasid Cellular Slime Molds

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome to the Deep Dive, where we transform your source material into those essential nuggets of knowledge and insight you need to feel truly well -informed.

Today we're taking a fascinating deep dive into a microscopic world that's, well, often overlooked, the acrosid cellular slime molds.

Now, if slime molds doesn't immediately spark excitement,

just wait.

Prepare to be surprised.

By the end of this deep dive, you'll definitely have a new appreciation for these tiny squiggly wonders.

Our guide for this exploration is a really comprehensive chapter from introductory mycology, and it's packed with details on their unique biology, their life cycles, and even some evolutionary connections.

Indeed.

Our mission today is really to illuminate these organisms, help you grasp their distinct structure, how they reproduce, their physiological quirks, and where they ultimately fit within the vast tree of life.

We'll move from the broad strokes down to the intricate details, connecting their unique biology to their roles in the environment, and what it signifies for our understanding of life itself.

Okay, so to really appreciate these organisms, we have to start at their classification, right?

The phylum acrasiomycota, it's a group that's seen its share of taxonomic debate, hasn't it?

Oh, absolutely.

Early researchers, like the Olives back in 1902 and then later in 1975, they carefully distinguished them from the dictyostelid slime molds.

And while dictyostelids are generally seen as a clear single evolutionary group monophyletic, we call it, the acrosids are, well, almost certainly a polyphyletic assemblage.

Polyphyletic, right.

So less like a single family tree.

Exactly.

More like several distinct evolutionary branches that kind of independently arrive at similar traits, which makes their classification particularly complex, as Blanton's work in 1990 really highlighted.

What's fascinating, though, is that even though they're apparently quite common, you rarely encounter acrosids because, well, people just don't look for them.

Where do they usually hang out, then?

They thrive in moist environments, like moist chamber cultures of soil, decaying plants,

rotting mushrooms in their early stages, and especially dung.

Those are their prime hunting grounds.

Okay.

So what really defines an acrosid?

It's their amoebae, isn't it?

That unique limax type.

Absolutely.

That's key.

Imagine a cylindrical, almost slug -like amoeba.

Its distinctive feature is this single rounded pseudopodium up front for movement.

This gives it an explosive forward burst kind of motion as it literally engulfs its food, bacteria, yeasts, spores.

That's

Wow.

Explosive forward burst.

Sounds quite dramatic for an amoeba.

It is.

And this peculiar locomotion isn't just a quirky detail.

It's a highly efficient hunting strategy that really differentiates them.

And if you could look closer inside.

Right.

You'd see a clear internal separation.

Granular endoplasm in the middle, non -granular ectoplasm near the edge.

And at the posterior end, the uroid or tail, there's often a contractile super important for water balance.

And sometimes these thin thread -like pseudopodia is sticking out.

So this whole distinct structure and movement really sets them apart from other fungus -like protists.

It does.

And what's particularly noteworthy is how acrosids diverge from their dictyoscalid cousins, especially when they aggregate.

Ah, the aggregation.

How does that differ?

Well, both are cellular slime bolts, right?

Yeah.

But acrosid and amoeba come together individually or maybe in small clusters.

They lack those visible streaming patterns you see in dictyoscelids.

So if it's not that streaming pattern, what pulls them together?

Is it like the chemical signals in dictyoscelids, that cyclic AMP stuff?

Surprisingly, no.

It's not cyclic AMP.

Unlike some dictyoscelids that use that as kind of pheromone, acrosis just don't.

Really?

Yeah.

There's likely an unknown attractant at play.

We just haven't identified it yet.

And once they do aggregate, the structure they form, this pseudoplasmodium, it doesn't migrate around like the dictyoscelid ones do.

Okay, so no migration.

What happens then?

It immediately starts forming a sorocarp.

That's their spore producing structure.

Straight to business.

And the sorocarp itself is different too, you mentioned?

Very different.

Dictyoscelid stocks secrete cellulose, makes them rigid.

Acrosid stocks, no cellulose.

And what's even more unusual, maybe the biggest difference, is that all the cells in an acrosid sorocarp, even the stock cells, which seem pretty basic, they can all germinate to produce new amoebae.

All of them.

Not just the spores.

Exactly.

Which is totally unlike dictyoscelids, where only the specialized spores germinate.

Plus, some acrosids can produce flagellate cells, cells with tails for swimming, though we have no evidence of sexual reproduction in this group at all.

Okay, let's maybe illustrate these unique traits by zooming in on one example.

How about acrosis roseae?

You mentioned it earlier.

Beyond the basic spore germinating into that limax amoeba, what makes this species stand out?

Maybe its internal structure or how its sorocarp develops.

Good one.

Yeah, acrosis roseae's internal structure holds some fascinating clues.

Inside their amoebae, you'll find mitochondria, the powerhouses, but their internal folds, the cristae, are plate -like.

Plate -like.

Not the usual tubular shape.

Exactly.

Different from the cristae seen in many other organisms.

You also see rough endoplasmic reticulum involved in protein stuff associated with these mitochondria.

And most notably, there's this unique organelle called the pea body.

Pea body?

What does that do?

Well, that's the thing.

Its exact function is still

a persistent mystery.

It was once thought to be a pigment granule, maybe giving the cell its color.

But experiments showed much of it can be removed with proteases, enzymes, that break down protein, which makes the carotenoid pigment theory less likely.

So it's made of protein, not pigment.

Seems like it.

Mostly.

The cells themselves are actually tinted pink by other carotenoid pigments.

But the pea body might not be the source.

Huh.

So even in these seemingly simple things, there are still basic parts we don't understand.

Absolutely.

It really highlights how fundamental cellular structures can hold profound unanswered questions, challenges our assumptions.

And you mentioned a flagellate stage?

For a rosea?

Yes, that's right.

While it's not always shown in the simpler life cycle diagrams, it has been reported.

These have two anterior flagella, like tiny propellers for swimming.

Okay.

And it's important not to confuse these actual flagella with the long, thin pseudopodial extensions that sometimes pop out from the uroi, the tail end, especially when the amides are covered in water.

They look a bit similar, but are different things.

Right.

And while other genera also have flagellate cells, their detailed flagellar structure, or things like centrioles, which are related to flagella formation, they just haven't been described.

More unknowns.

And what about survival?

If conditions get tough?

Ah, yes.

To survive harsh conditions, especially drying out, individual amoebae can form microsists.

It's a neat survival strategy.

The amoeba just rounds up, secretes a tough, fibrous wall.

We don't actually know what the wall is made of.

Another mystery in weights.

A little survival pod.

Pretty much.

This pause lets them survive until conditions improve.

So,

okay.

Under suitable conditions, the amoebae of a roseae aggregate, they form that pseudoplasmodium, which quickly, you said, develops into a sorocarp.

What does that look like?

The mature sorocarp of Acrisis roseae is actually quite striking.

It's often described as resembling a tiny pink tree.

A pink tree?

Seriously?

Yeah, kinda.

The sorocarp has a stalk made of cells, and this supports multiple chains of spores branching out, giving it that tree -like look.

And except for the very tips of these spore chains, the whole structure is covered by this fragile, membrane -like sheath.

Its origin is also, you guessed it, a bit of a mystery.

Lots of mysteries with these guys.

There really are.

And as we noted, all the cells in this little tree's stalk cells included can germinate into new amoebae if things get moist again.

It's interesting that the stalk cells can germinate too.

Can you tell them apart from the spores if you look closely?

You can, actually, at the ultra -structural level.

So with powerful microscopes.

Spores are particularly distinctive because they have these structures called HILA.

H -I -L -A.

They're like these plate -like aggregations of globular material that develop on the spore wall, right where the spores touch each other as they mature.

So yeah, you can tell them apart.

Okay, so with such a unique group, how does classification work within the Acrasia mycota?

Are there clear subgroups?

When you look across the diversity, some patterns do emerge.

For instance, Acrasis itself and another genus, Gudelina, they seem pretty closely related.

They share features like those plate -like mitochondrial cristae we talked about.

And at least one species in each genus has those flagellate cells.

And you mentioned something really weird about Gudelina earlier, didn't you?

Something about multiple nuclei?

Ah, yes.

That's truly compelling.

One species of Gudelina, it's unusual because it's amoebae can develop into multinuclear protoplasts, basically a single -cell membrane containing multiple nuclei.

How does that happen?

It happens through mitosis, nuclear division, but without the cell itself dividing afterwards.

That's called cytokinesis.

So the nuclei multiply, but the cell doesn't split.

And this multinuclear structure, technically it fits the definition of a plasmodium, which you normally find in plasmodial slime molds, not cellular ones like these.

Whoa, so it's like blurring the lines between different types of slime molds.

Exactly.

It's a fantastic example of evolution seemingly playing with the boundaries of classification.

A cellular slime mold making something like a plasmodium.

Fascinating.

What other groups are there?

Right, so beyond those, Blanton's 1990 work pointed to three other distinct lineages that don't have flagella.

There's Gudelinopsis.

It also has the plate -like mitochondrial cristae, but its soracarp stalk is different.

It's mostly a non -cellular matrix like a gel with just a few cells embedded in it.

And then there are copromixa and copromicella.

These really stand out from all other acrosygenera because they possess tubular mitochondrial cristae, not plate -like ones.

The other type of cristae.

Yes, and as Dexter pointed out back in 77, mitochondrial morphology, the shape of those cristae, can be a really useful phylogenetic character, a clue to evolutionary relationships.

This difference strongly suggests that copromixa, at least, might actually be misplaced in this group altogether.

It might belong somewhere else entirely.

Wow, okay, so the mitochondria are key clues.

Anyone else?

Well, there's Fonticula.

It's a genus with uncertain affinity, meaning we're not quite sure where it fits within acrosyomycota.

Fonticula alba has only ever been isolated once, from dog dung, so it's poorly understood.

But according to DC and all its research, it does have plate -like mitochondrial cristae and an amebral aggregation phase, so it has some acrosyte features.

Only found once.

Makes it hard to study, I bet.

Extremely.

So, summarizing that, Blanton's 1990 categorization grouped these five genera and 15 species into three families.

Acracidae, copromixidae, and guttalinopsidae, with Fonticulaidae stuck in as a family of uncertain affinity.

It's worth noting he used the zoological code suffix, iidae, for family names, which isn't actually necessary under botanical nomenclature, but that's a minor detail.

Right.

The bigger question is whether these family distinctions, which are partly based on saurocarp structure, something that can be a bit variable or artificial,

sometimes will continue to hold up as we get more data, especially molecular data.

Okay, so if they're this distinct, even potentially misplaced within their own group, where do acrosids fit into the larger tapestry of life?

Are there any surprising evolutionary connections outside this group?

That's an excellent question, and yes, there are some intriguing possibilities.

Some studies propose relationships between certain acrosid species and another group of amoebae called volcampfit amoeba.

A well -known example is nigleria.

Nigleria, isn't that the one that can cause?

Yes, the brain -eating amoeba, unfortunately.

But most volcampfids are harmless soil dwellers.

They also show limax -type movement and have flagellate stages, like some acrosids.

But how do they differ?

Key differences.

Volcampfids have tubular mitochondrial cristae, like copromixa, not platelike.

And crucially, they don't aggregate to form saurocarps.

They stay as individual amoeba or flagellates.

Interesting similarities and differences.

And connecting this to the really big picture, Patterson and Sojen suggested back in 1992 that acrosid slime molds are among the early diverging eukaryotic lineages, lineages that split off early in the evolution of complex cells that possess mitochondria.

They tentatively grouped them with another group, the schizopyranoid amoebae, into a larger class called heterolibosa.

Heterolibosa, so that puts them quite ancient.

Potentially very ancient, yes.

It highlights their significant position way down near the base of the eukaryotic tree of life, offering potentially invaluable clues about how complex cells, cells with organelles like mitochondria, first evolved.

Wow.

Okay, so let's bring this all together.

What does all this mean for you?

From their unique limax amoebae bursting forward like tiny slugs, to those still mysterious bodies inside them, and their really quite pretty tiny pink tree -like saurocarps.

Acrosis' cellular slime molds are, well, they're a remarkable testament to the incredible diversity of life on Earth, even at the microscopic level.

Absolutely.

We've explored how these often overlooked microscopic organisms serve as decomposers in various moist environments, important ecological roles.

And we've seen how their distinct features, like the shape of their mitochondrial cristae, are crucial clues, helping us untangle complex ancient evolutionary relationships.

They really are a compelling example of life's endless variations on a theme, you know.

Taking a basic amoeba form and doing something quite unique with it.

And maybe the most provocative thought for you to mull over as we wrap up, is seemingly simple characters and life cycles like those of Acrosids can be so easily evolutionarily modified, tinkered with, really to produce fascinating, unexpected structures, like that plasmodium -like stage in Gutulina.

What does that tell us about the hidden potential for adaptation and change in even the most apparently basic forms of life?

What other evolutionary pathways, what other strange and wonderful forms might still be unfolding right beneath our feet, largely unnoticed?

That's it for this deep dive.

We really hope you enjoyed exploring the quirky world of Acrosid cellular slime molds with us.

Thank you for joining us on this exploration, and thank you for being part of the Last Minute Lecture family.

We'll be back soon with another deep dive into another fascinating topic.