Cell system evolution

[Improvision]

If you scrape your knee when you fall down, the blood cells and skin cells that are separated from the rest of the body and left behind on the ground will die while left in the open environment.

 

Current evolutionary theory would suggests the tissue and organs in a multi cellular organism evolved by cells in the past joining together to from those cell structures that you see in present day animals and plants. But since individual cells in such organisms can not survive on their own, how did they exist separately in the past to merge together in the first place?

[CharonY]

Essentially you are asking about the evolution of the multi-cellular state? There is quite some literature out there, if I find the time I can check if I can find a nice review. I think much research has probably be done on volvox. However even in a few bacterial species there are multi-cellular states. For instance during nutrient stress Myxococcus xanthus forms elaborate fruiting bodies in which spores are formed. It is likely that initially multicellular stages have evolved in otherwise free-living cells to overcome certain stress or to gains specific selective advantages.

[Improvision]

Essentially you are asking about the evolution of the multi-cellular state? There is quite some literature out there, if I find the time I can check if I can find a nice review. I think much research has probably be done on volvox. However even in a few bacterial species there are multi-cellular states. For instance during nutrient stress Myxococcus xanthus forms elaborate fruiting bodies in which spores are formed. It is likely that initially multicellular stages have evolved in otherwise free-living cells to overcome certain stress or to gains specific selective advantages.

Well when the stress is gone those cells separate again right?

[CharonY]

Not necessarily. Or rather, rarely. The multi-cellular state is usually an endpoint of sorts. In case of myxobacteria the spores then form independent cells again. The multicellular fruiting body, however exists merely to form the spores. Also in other similar forms of primitive multicellular stages the fused cells tend to differentiate to specific jobs and subsequently lose their individual free-living capabilities.

[granpa]

look up 'acellular' or 'multinucleate' or 'syncytial organism'. then rethink how cellular organisms evolved.

 

I dont doubt that in the past that simple cells fused and formed complex cells but I seriously doubt that it led directly to cellular organisms.

[MM6]

Here's a section of notes I wrote for a class I used to teach. I say that only because I've forgotten the details. But you can use it to explore further, perhaps?

 

3. The rise of multicellularity

 

Cooperation and competition have been emphasized throughout this unit. Symbiosis was important in the origin of eukaryotes, resulting in an expansion of life forms similar to what we now call protists, which in turn radiated to give rise to the entire eukaryotic domain—animals, plants, fungi and protists. Cooperation and competition were essential driving forces in the rise of multicellularity.

 

Single-celled organisms, such as bacteria (prokaryotes) and protozoa (unicellular protists [eukaryotes]) have been so successful in adapting to a variety of different environments that they comprise more than half of the total biomass on earth. Unlike animals, many of these unicellular organisms can synthesize all of the substances they need from a few simple nutrients, and some of them divide more than once every hour. What then was the selective advantage that led to the evolution of multicellular organisms?

 

A short answer is that by collaboration and by division of labour it becomes possible to exploit resources that no single cell could utilize so well. This principle, applying at first to simple associations of cells, has been taken to an extreme in the multicellular organisms we see today (Alberts et al 1994).

 

The Cambrian explosion—the rapid diversification of multicellular animal life around the beginning of the Cambrian Period (500-600 my ago), resulting in the appearance of almost all modern animal phyla—was driven by the dramatic geological changes occurring at that time. The atmosphere was becoming rich with oxygen, opening a host of new ecological niches on land. Multicellularity became cost effective and possible and evolved independently many times and in all three domains of life, Archaea, Bacteria and Eukarya (Carrroll, S. B. 2001).

 

In certain of these new niches, bigger was/is better. So how can you get bigger?

 

Complex body plans, multicellularity.

 

Possible gains:

 

· Division of labour (through cellular specialization)

· Increased dexterity and locomotion

· Increased environmental exploitation (fitter)

 

In bacterial and algal forms (plants) with cell walls, one of the simplest ways to become multicellular was (and still is) for the products of cell division to remain together to form colonies in the form of long filaments. Many early multicellular eukaryotes were millimeter scale, linear or branched, filamentous forms, such as cyanobacteria (see below: Figure 1 (adapted), History of major evolutionary events from the fossil record).

 

How may cellular co-operation have evolved?

 

Evidence of potential routes is not too difficult to find. A number of prokaryotes associate quite closely; fimbriae and pilli are used for the transfer of genetic material and Strepto/Actinomycetes often form filaments. Myxobacteria form fruiting bodies (spore-producing structure) and Anabaena forms heterocysts where nitrogen fixation occurs.

 

However, these are colonial rather than multicellular as they lack what biologists use to define multicellularity: specialized cells with different functions and cellular communication (septal pores in fungi, gap junctions in animals, plasmodesmata in plants and pores/pit plugs in red algae).

 

Clues to the origins of true multicellularity can be observed in many present-day “primitive” life forms, such as Volvox (reading assignment from Alberts 1994), among others (Trichoplax, sponges).

 

 

Here's a decent discussion about Trichoplax and evolution of multicellularity.

 

http://pharyngula.org/index/weblog/comments/mysterious_trichoplax/

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Consecutive posts merged

Also in other similar forms of primitive multicellular stages the fused cells tend to differentiate to specific jobs and subsequently lose their individual free-living capabilities.

 

Yes, exactly.

[granpa]

Division of labour (through cellular specialization)?

 

but complex cells have organelles. and acellular organisms also have organs. there is no sharp distinction here between cellular organisms and noncellular organisms.

 

I think the question breaks down into 2 questions.

why did single celled organisms become multinucleate?

why did multinucleate organisms become cellular (with cell walls between nuclei)?

 

cell walls in the interior of an organism would seem to just get in the way. but as a protection from viruses they would be very useful.

Edited May 26, 2009 by granpa

[CDarwin]

Division of labour (through cellular specialization)?

 

but complex cells have organelles. and acellular organisms also have organs. there is no sharp distinction here between cellular organisms and noncellular organisms.

 

I think the question breaks down into 2 questions.

why did single celled organisms become multinucleate?

why did multinucleate organisms become cellular (with cell walls between nuclei)?

 

cell walls in the interior of an organism would seem to just get in the way. but as a protection from viruses they would be very useful.

 

That might be a plausible way multicellularity could have evolved, but I don't believe that's the sequence most biologists imagine. I think the models more often involve certain free-living single celled organisms (like choanocytes) that come together in some circumstances and how those temporary associations could become a permanent part of how a whole, emergent organism lives. Part of that process involves cells specializing in ways that would be useless to them as free living organisms, like having some cells that just serve as conduits of communication between the outside and the inside of a body. That's the special thing about multicellular division of labor. (I think).

[CharonY]

What is a noncellular organism?

[Cay]

What about the environment that these cells develop in?

 

The saying "primordial ooze" is normally used?

[CharonY]

Nope. Those organism are very diverse, include pro- as well eukaryotes. As such they are found in a variety of environments.

[granpa]

What is a noncellular organism?

see post 5

[CharonY]

I have no idea what your argument is. Especially given the fact that nucleation only arose in eukaryotes (obviously), whereas multicellular structures are already found in prokaryotes.

[granpa]

I have no idea what your argument is. Especially given the fact that nucleation only arose in eukaryotes (obviously), whereas multicellular structures are already found in prokaryotes.

you believe that we were already multicellular before we were even eukaryotes? thats laughable. how do you explain syncytial eukaryotic organisms?

[CharonY]

Read again. I am saying that multicellularism is not exclusive to eukaryotes. Meaning that in the absence of nuclei (what appears to be one of you central points, if you cared to elaborate) there can also be multicelluarism. I have no idea why the existence of syncitial organsms should contradict that. Would mind to spell out your theory in a cohesive way or should I start guessing your thoughts?

I have the feeling that you see a distinct line between multi and single-celled life, which is not correct.

 

Edit: I think I may get your point. You mean that partial cell division is another means of formation of multicelluarism? Yes it is. Is it the only way? No it isn't.

Edited May 28, 2009 by CharonY

[granpa]

I think I explained myself pretty well in post 7. what part did you not understand?

 

heres something I found on the web in about 1 minute that refers to the possibility of multicellular life arising through sycytial organisms.

 

btw, post 5 was in response to the op. in case that wasnt clear.

[CharonY]

I was confused on your insistence of nucleation as an essential part of multicellularism as in the posts above already counter examples were given. You start off from some viewpoint that you apparently have but do not care to elaborate and string questions out from there. For example, what is the difference between an acellular and a unicellular organism (in your words).

 

In short: multicellular organisms can arise by incomplete separation, or the reverse action: fusion of different cells. For both there are examples. And it happens both in eu-as well as prokaryotes.

[granpa]

In my very first post I said to look up 'acellular' or 'multinucleate' or 'syncytial organism'

that shoud have made it clear what I meant by 'acellular'. then I explained myself fully in post 7. finally I gave a reference to a professional paper. what more do you want?

[jimmydasaint]

Do you refer to this?

Why would some biologists refer to single-celled organisms such as Amoeba and Paramecium as "acellular" rather than "unicellular"?

 

Stephen Tomkins comments:-

 

Acellular just means NOT essentially cellular, i.e. there are more important attributes to this levelof organisation than that it is a single (uni) cell. e.g. Amoebae have highly polyploid nuclear systems, with huge chromosome numbers, they are sometimes multinucleate, they sporulate, they behave as a whole animal capturing prey and defaecating undigested waste, etc, : similarly Paramecium is more than a single cell it has two well differentiated nuclei, it has a cell mouth and a cell anus, an anterior and posterior, it also often has sophisticated symbionts inside it like bacteria and algae. etc. i.e. neither are "simple cells", I guess.

 

http://www-saps.plantsci.cam.ac.uk/records/rec452.htm

 

Or to this for syncitial organisms?

 

Hexactinellid sponges are metazoans in which the major tissue component is a multinucleated syncytium. The preferred deepwater habitat of these sponges makes collection of hexactinellids in good condition difficult, and has hindered extensive examination of their body plan. Nonetheless, over the last three decades a number of studies have explored their ecology, histology and physiology. It has been shown that hexactinellids are extremely long-lived animals. Their cytoplasm consists of a giant, multinucleated tissue, the trabecular syncytium, which is connected via open and plugged cytoplasmic bridges to cells such as archaeocytes, choanoblasts, and cells with spherical inclusions. Because all of the sponge is cytoplasmically interconnected, electrical signals can propagate through the animal.

http://www.bioone.org/doi/abs/10.1093/icb/43.1.19?journalCode=icbi

 

And you cannot see how multicellular organisms arose from these forms. Would that be what you are trying to state granpa? Or am I just being obtuse ?

[granpa]

I was of the impression that sponges were the classic example of the colonial hypothesis. they've never impressed me much. I'm inclined to think that we evolved from some kind of worm which evolved from some sort of syncytial protozoa. I'm not sure exacly what kind of protozoa. it might have been something that no longer exists, having been outcompeted by its own descendants. the main clues are the flagellum on our sperm and the cilia in certain tissues of our bodies. I dont know much more than that.