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Author Topic: Lack of Multicellular Archaea Life Forms  (Read 88 times)

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Offline LostInTheMistTopic starter

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Lack of Multicellular Archaea Life Forms
« on: July 19, 2021, 04:58:39 am »
Okay so... this question has bothered me for a while, and I've posted it everywhere except here without getting an answer. So I'm not really expecting an answer here, but if somebody knows, I'm really curious. There don't seem to be any multicellular life forms that follow the formula of Archaea, their cellular structure. Can anyone explain to me why this is? Random chance? Or is there something that makes life forms with the structure of Archaebacteria unsuited for Eukareote-type multicellular life?

For reference, I asked my Dad who has a PhD in microbiology, and he did not know the answer, so if anyone does, you get a kudos.

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Re: Lack of Multicellular Archaea Life Forms
« Reply #1 on: July 19, 2021, 05:29:26 am »
Might it be something as simple as Earth's environment?  I've only done a little bit of reading, but it seems that the Archaea primarily exist in more extreme environments - naturally, they have less competition there.  If Earth's environment had been more hostile to Eukaryotes, perhaps the Archaea might have become the dominant species and evolved to a multicellular state?

Offline LostInTheMistTopic starter

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Re: Lack of Multicellular Archaea Life Forms
« Reply #2 on: July 19, 2021, 12:38:02 pm »
Might it be something as simple as Earth's environment?  I've only done a little bit of reading, but it seems that the Archaea primarily exist in more extreme environments - naturally, they have less competition there.  If Earth's environment had been more hostile to Eukaryotes, perhaps the Archaea might have become the dominant species and evolved to a multicellular state?

I thought that might be the case, but after looking up Archaea, it turns out they're all over the place. It's just that we've been assuming they were bacteria for a long time because they look more or less the same until you get down to the very small levels. I mean, it's a good idea and there do seem to be more Archaea extremophiles (though they are by no means alone.)

Offline Hawkwood

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Re: Lack of Multicellular Archaea Life Forms
« Reply #3 on: July 21, 2021, 06:08:22 pm »
Ankaryotes / Prokaryotes: Bacteria + Archaea (literally prokaryote literally means "before the nucleus")
Eukaryotes: Everything else (Protoza, Chroimsta, Plants, Fungi, Animals)

Ankaryotes are very similar - pathways, structure, proteins, membrane (but archaea have one kind of membrane protein, and bacteria a different). What's fascinating is that the genomes of all three domains contain the genes necessary to make both kinds of membrane.

Ankaryotes differ markedly from Eukaryotes in that they lack organelles and different genetic structures. That means that there's an intermediate step between ankaryotes and eukaryotes.

How did archaea emerge from bacteria? Probably to evade getting eaten or viruses, and did so by becoming a wee bit more simple (thermoreduction). Archaea tend to look a lot like gram+ bacteria. So it's probably the case that archaea were an evolution from bacteria.

There are, broadly, two hypotheses as to how Eukaryotes came into existence:
1) Eukaroyte = Archea + Eubacteria. One engulfed the other, and you end up with a nucleus and a cytoplasm
2) The Eocyte / Crenarchaeota hypothesis. That eukaryotes emerged from a specific kind of archaea (these guys).

What makes ALL of the above really murky is what is called "horizontal gene transfer". So it's not clear if the links we see are actual relationships or just very promiscuous transfers across single layer membranes!

And then you have Asgard. What's really interesting about the Asgard superphylum is that they look sort of like how you might imagine an intermediate step between archaea and eukaryotes might look. Why aren't they more common? I suspect because being "almost a Eukaryote" doesn't present significant advantage over being an archaea in most situations. That the specific Asgard superphylum live only in thermal vent suggests that they have a special adaptive niche there. Eukaryotes and Archaea outcompete hybrids most of the time.

The major and dramatic change is the mitochondrion. Once you have them, the energy budget of your cell goes up massively.

So actually, the answer might well be that a Eukaryote is what a multicellular archaea actually looks like!

Offline Hawkwood

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Re: Lack of Multicellular Archaea Life Forms
« Reply #4 on: July 21, 2021, 06:26:12 pm »
And, actually, a second answer...

There are multicellular archaea - biofilms.

Offline Vekseid

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Re: Lack of Multicellular Archaea Life Forms
« Reply #5 on: July 21, 2021, 10:09:29 pm »
So I've done a crazy amount of research into eukaryogenesis recently. This is a bit roundabout but I'll get to your question eventually.

As I understand it, research samples of asgardarchaeota and their environments aren't going to be reported on in the general scientific community for at least another year, though we might see some soon from the original team. The samples were gathered in late 2020 and are being held and studied for a minimum of 6/18 months, before publication. That may give some more insight into this.

That said, a few things are known about why eukaryotic life is so dramatically unlike archaea. Archaea form biofilms, as mentioned, but they aren't remotely like eukaryotic life. To quote

Quote from: Phillip Bell
If we investigate the characteristic features of the eukaryotic domain, we find that meiosis is only one of a wide array of features that have such a wide distribution within the eukaryotes that they appear to have been present and almost fully developed in the ancestor of all living eukaryotes [3]. These features include the nucleus, endomembrane systems, linear chromosomes with telomeres, mitochondria, peroxisomes, the cell division apparatus, mitosis, nuclear pores, mRNA capping, introns, the spliceosomal apparatus and the nuclear pore proteins

Enter the viral eukaryogenesis hypothesis, by Phillp Bell and Masaharu Takemura. They both independently developed this hypothesis back in 2001, coming to the same conclusions with different evidence. Since then evidence has only built in their favor.

While we can't find fossil viruses the traditional way, we can find them through the genes they leave in DNA. Varidnaviria, of which these giant virii are members, are incredibly ancient, with individual families possibly predating LUCA.

Which is impressive considering LUCA seems to be around 4.4 billion years old.

So my personal hypothesis (of why eukaryotes are so strange, in every respect) goes like:

There was a species of asgardarcheota being parasitized by rickettsiales, or some close relative. For some pretty generous definitions of close, of course.

This host itself becomes infected again by a giant virus (presumably, by Takemura's latest paper, something closely related to a medusavirus).

For whatever reason, this secondary infection ends up turning the parasitism into a net benefit for the resulting chimera. Asgardarchaeota become an increasingly inhospitable environ for rickettsiales, while the new eukaryotes make for much better hosts, causing them to hop to the new life form.

The proto-eukaryote, meanwhile, has access to a hitherto unrivaled amount of genetic utility. The viral 'nucleus' can incorporate archaea genes from its host, eventually taking them all. It can incorporate bacterial genes from the parasite. It has its own 1-2 MB gene library to draw on.

Bell goes into this in great detail in this article about meiosis - the origin of sex, from where I take the above quote.

The article is fascinating and fairly readable, but the short of it is, eukaryotic impregnation evolved as a kind of co-infection at the cellular level. Viruses can recognize when they have already infected a host, but eventually this breaks down, and one or both of a eukaryotic pair, on meeting, will try to implant the other. The infected cell then needs to divide twice, making four daughter cells. Bell explains this in more detail.

Of course, sex is an enormous evolutionary advantage, producing and spreading new genetic material extremely quickly.



So to your question, this is a hypothesis of mine, but it revolves around the above situation.

Early on in eukaryotic evolution, organisms would need to be very closely related to avoid infecting each other. At the same time, getting infected can be an advantage. If it is too distantly related, however, it becomes too great of a risk.

Accordingly, eukaryotes would need to develop a lot of extracellular awareness, after a fashion, as well as abilities for response. These mechanisms simply aren't necessary in prokaryotes. They don't let themselves get infected by a genetic near-duplicate as a biological strategy, they don't need to protect against things being too different or things being too similar. As an example, just look at how enormously complex cell adhesion is in eukaryotes versus prokaryotes.

This in turn enables degrees of specialization that prokaryotes cannot easily achieve. So when eukaryotic biofilms became a thing, they could specialize further, and develop larger-scale 'communication'.

The multi-division of sexual replication is important here, as well. Sexual division can guarantee a number of cells available to the child organism - this isn't a feature available to prokaryotes. This means specialization is no longer purely an environmental thing - genes can influence specialization from the first fertilized cell.

Prokaryotes don't have these guarantees available and thus, a multicellular prokaryotic organism cannot be 'planned' from a single cell.




Offline BlueOrange

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Re: Lack of Multicellular Archaea Life Forms
« Reply #6 on: Today at 05:48:11 pm »
Wow, that’s fascinating.

My microbiology doesn’t equip me to do more than ask naive questions here, but I’m fascinated by morphogenesis, and cellular communication is critical to that.

So, individual eukaryotic cells reproduce sexually? I had this concept that sex only really occurred between macro-organisms, and that ‘sperm meets egg’ was a very unusual cellular event. But if I’ve understood correctly, then there’s a whole bunch of sex going on while a fetus is being formed out of undifferentiated cells?

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Re: Lack of Multicellular Archaea Life Forms
« Reply #7 on: Today at 07:17:05 pm »
So, individual eukaryotic cells reproduce sexually? I had this concept that sex only really occurred between macro-organisms, and that ‘sperm meets egg’ was a very unusual cellular event. But if I’ve understood correctly, then there’s a whole bunch of sex going on while a fetus is being formed out of undifferentiated cells?

'Sex' in the terms of exchanging and recombining genetic material also occurs in Paramecium.  The technical term is 'conjugation'.

Micro-porn