Talmudic Question #31

Many unicellular protists have a very complex body plan. One can find “legs” (see our posting on Euplotidium), “mouths,” food vacuoles, etc. Some of these structures reflect the feeding habits of the organisms. The question arises, why haven’t they have become multicellular?

Comments

[Ben Katz]

agreed, but that is why I added the word philosophically – one would assume the same advantageous selection pressure to evolve speech or multicellularity would be at work on all organisms, unless there were an advantage in some environments to remain mute or unicellular.

[Elio Schaechter]

Ben,

I appreciate your comment, but wonder if we weren't dealing with different questions. We were wondering why SOME complex eukaryotic cells hadn't evolved into multicellular ones, not why others remained unicellular. Certain ones could have become multicellular, others stayed as they are, no?

Elio

[Ben Z. Katz, MD]

It seems to me this question is related at least philosophically to the question of why there are still apes (and all other "lower" animals) - why didn't they all just evolve further (in the ape's case into humans)? Why did some of our hominid ancestors remain apes?

[Manuel Sánchez]

Hi

I think that the answer is related with the oxygen. Pluricellulars become abundant when the oxygen in the atmosphere reach values over 10%. Maybe pluricellularity arise as a defense mechanism against toxic effects. In a similar way as the endosimbiosis event that produces the mitochondria, maybe as oxygen level increased one microbe could have developed two different kinds of cells. One in the exterior layer protecting the cells of the interior from the oxygen.

Regards

PD: Elio, we meet each other in Miguel Vicente's lab. I was making my PhD during 1990-94

Manuel,

I think you have a point. It may be one of several factors, none mutually exclusive.

Lamento tener que admitir que no me acuerdo de nuestro encuentro. Se debe posiblemente a la carencia de sinapses.

Elio

[Mike Gray]

I hadn't thought of the growth rate problem, but that's huge. Surface area is a big deal for photosynthesizers, too. The amazing thing is that anything ever becomes multicellular at all. All of the advantages seem to be "downstream" - for the distant complex descendants of the bug which makes the jump. Hmmm.

What advantages does multicellularity confer? Is it really just size? Protists are already bigger than most cells (and some multicellular organisms), and the first multicellular organism wouldn't have had to deal with animal predators, obviously. Maybe the thing to think about is fungi or myxococci which have both single- and multi-cellular stages. Under what conditions do they make the transition, and what do they gain? Can that tell us anything interesting?

Thought-provoking question...

[Jeffrey Felton]

Becoming multicellular would have a negative impact on the surface-to-volume ratio of a microbe, and in turn, on the per-cell rate at which the organism could acquire nutrients across its surface, and finally on the rate at which it could grow. Growing faster is a common way by which a microbe defeats its competition. So it would not be a good idea for it to adopt a change that would slow it down.

[elio schaechter]

Mike,

From one multicellualrnik to another, your comment is on the mark but one can think that being MC allows one to grow bigger, thus escape predators, etc. No?

Elio

[Mike Gray]

Why should they?

They're extremely successful as single cells. Multicellularity is a neat trick, for sure, but there's no particular selective pressure for it that I can think of, in most environments. You gain some size, and eventually (but not right away) the ability to become much more structurally complex, but you also have to cope with coordinating development and signaling and the possibility of cancerous "selfish" cells.

I'm glad one branch of eukaryotes ended up going the multicellular route (some of my best friends are multicellular), but it doesn't surprise me that most of them haven't.