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What is the smallest oligocelluar organism?

How many cells does it have?

EDIT

The question is motivated by this comment@Philosophy.SE

EDIT as recommended in comments

I'm looking for an example of an organism made of very few (the fewer, the better) sister cells (obtained by mitosis or aggregation) that are morphologically and functionally different. I would accept examples of species that exist in different "conformation" (unicellular and multicellular).

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I thing the answer depends on what you will accept to consider as multicellular. If you use a very broad and open definition, than the answer will probably be "2 cells". –  Remi.b Jan 7 at 10:45
    
@Remi.b do you know an example with 2 cells? But in fact I thought about organisms where different cells serve different tasks... –  draks ... Jan 7 at 10:50
    
No I don't have an example right now. As someone already voted to close, I would suggest you to re-edit your question and make sure we understand what you ask for. For example, you might say that you're looking for an example of an organism made of very few (the fewer, the better) sister cells (obtained by mitosis) that are morphologically and functionally different. You can also say that you would accept examples of species that exist in different "conformation" (unicellular and multicellular). I think/hope this would be a valid question. –  Remi.b Jan 7 at 11:02
    
I can only support Remi's comment. The question is pretty broad at the moment. –  Chris Jan 7 at 11:22
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@Chris better now...? –  draks ... Jan 7 at 12:24
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2 Answers

up vote 3 down vote accepted

The classic example, though I am sure there are others that are smaller, is the slime mold Dictyostelium discoideum:

Dictyostelium life cycle

It can have up to 100,000 cells and exists as both single cells and as a multicellular organism (emphasis mine):

Dictyostelium amoebae grow as separate, independent cells but interact to form multicellular structures when challenged by adverse conditions such as starvation. Up to 100,000 cells signal each other by releasing the chemoattractant cAMP and aggregate together by chemotaxis to form a mound that is surrounded by an extracellular matrix. This mechanism for generating a multicellular organism differs radically from the early steps of metazoan embryogenesis. However, subsequent processes depend on cell-cell communication in both Dictyostelium and metazoans. Many of the underlying molecular and cellular processes appear to have arisen in primitive precursor cells and to have remained fundamentally unchanged throughout evolution. Basic processes of development such as differential cell sorting, pattern formation, stimulus-induced gene expression, and cell-type regulation are common to Dictyostelium and metazoans.

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100.000 is quite a lot, C. elegans has 956 cells which would surely classify into the smallest. Some of the algae mentioned above can have even lower cell numbers. –  Chris Jan 7 at 13:44
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@Chris well yes but dicty cell counts go from 1 to 100.000 while the worm is always at 956. The interesting part is that dicty exists as a single celled organism as well. –  terdon Jan 7 at 13:46
    
Slime molds are faszinating. And as we see, this question is not easy to answer :-) –  Chris Jan 7 at 14:04
    
great answer. Exactly what I was looking for... –  draks ... Jan 7 at 14:20
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Actually, a clever point. I would also mention syncitial organisms, which are formally multicellular, but have no cell boundaries between cellular "loci". –  har-wradim Jan 8 at 17:07
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Not sure, whether it fits in your requirement (since it was not clear for me with the terdon's answer), but I'd also mention plasmodia and other cases of species with syncytial organization: they have multiple nuclei, sometimes have even macroscopic sizes, but formally there is only one cell boundary within their body.

(If we broaden this definition, we may also include all kinds of protists with multiple nuclei: from diplomonads (which have two equal nuclei) and ciliates (most of which have two functionally differentiated nuclei) to Pelomyxa or giant foraminiferans with huge numbers of nuclei.)

Some examples:

  • Myxosporidia, once thought to be a group of protists and now believed to be highly aberrant metazoans - are parasites of fish (first generation) and some invertebrates (second generation), which form highly differentiated plasmodial bodies in their hosts.

  • Orthonectids, a group of "lower metazoans", have a biphasic life-cycle with sexual generation represented by oligocellular free-living males and females, which produce zygotes developing into parasitic plasmodia.

  • Myxogastrid slime molds are famous in having plasmodial fruit bodies: they are relatives of Dictyostelium and have a similar life-cycle with mono-nuclear amoebae, which conjugate and produce the plasmodium. So, these are still formally "unicellular" even as fruit bodies.

  • Some algae have plasmodial organization with the most famous case of Caulerpa, species of which "are unusual because they consist of only one cell with many nuclei, making them among the biggest single cells in the world.".

  • Cannot recall right now the name of the group (can someone help?), but there are amoebae, which are "normally" unicellular, but able to aggregate on rich food resources to form trophic plasmodium.

  • Lots of other cases.

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