Spongeworthy genes

Posted 21 December 2006 by PZ Myers

What are the key ingredients for making a multicellular animal, or metazoan? A couple of the fundamental elements are:

A mechanism to allow informative interactions between cells. You don't want all the cells to be the same, you want them to communicate with one another and set up different fates. This is a process called cell signaling and the underlying process of turning a signal into a different pattern of gene or metabolic activity is called signal transduction.
Patterns of differing cell adhesion. But of course! The cells of your multicellular animal better stick together, or the whole creature will fall apart. This can also be an important component of morphogenesis: switching on a particular adhesion molecule (by way of cell signaling, naturally) can cause one subset of cells to stick to one another more strongly than to their neighbors, and mechanical forces will then sort them out into different tissues.

These are extremely basic functions, sort of a minimal set of cellular activities that we need to have in place in order to even begin to consider evolving a metazoan. Fortunately for our evolutionary history, these are also useful functions for a single celled organism, and while the metazoa may have elaborated upon them to a high degree, there's nothing novel about the general processes in our make-up. The principles of signaling and transduction were first worked out in bacteria, and anyone who has a passing acquaintance with immunology will know about the adhesive properties of bacteria, and their propensity for modulating that adhesion to build complexes called biofilms.

So let's take a look at the distribution of signaling and adhesion molecules in single-celled organisms, multicellular animals, and most interestingly, a group that is close to the division between the two (although more on the side of multicellularity), the sponges.

Continue reading "Spongeworthy genes" (on Pharyngula)

20 Comments

Mike · 21 December 2006

So Panda's Thumb is now related to the show about nothing. Eh.

PvM · 21 December 2006

Excellent posting PZ. Love it how you slowly unravel the history of life, at the same time reducing the shadows of our ignorance in which Intelligent Design is forced to hide.

Pete Dunkelberg · 21 December 2006

Early evolution of animal cell signaling and adhesion genes
Scott A. Nichols, William Dirks, John S. Pearse, and Nicole King

Last sentence of the Abstract:
"From these data, we infer that key signaling and adhesion genes were in place early in animal evolution, before the divergence of sponge and eumetazoan lineages."

It's from the Nicole King lab, with choanoflagellate research coming soon.

Related musing

and don't miss the terrifying carnivorous sponge video, hat tip to Deep Sea News .

David B. Benson · 22 December 2006

As a non-biologist, I am puzzled. How are sponges different from kelp, which I currently believe is considered a society of unicelled algae?

PZ Myers · 22 December 2006

Sponges are in the animal kingdom. They aren't algae at all -- completely different.

Popper's ghost · 22 December 2006

I think you need a more specific question than just how two different things are different.

Popper's ghost · 22 December 2006

I currently believe is considered a society of unicelled algae?

Perhaps that's the problem. Why do you believe that?

David B. Benson · 22 December 2006

PZ suggests in this thread two criteria for multicellular organisms. I recall being taught, in the 1950s, that kelp were not considered to be multicellular.

However, the Wikipedia article on kelp and algae seems to sugggest that kelp is now considered to be multicellular. If so, then I have no question for PZ since it certainly appears that kelp meet his two criteria...

Popper's ghost · 22 December 2006

I recall being taught, in the 1950s, that kelp were not considered to be multicellular.

Either you recall incorrectly or you had a very bad teacher; the former is more likely. Take a look at http://www.geol.utas.edu.au/kelpwatch/facts_b.html and tell me if you think anyone could mistake that for a unicellular organism or a "society" of unicellular organisms that lack adhesion and cell signaling, even in 1950.

David B. Benson · 22 December 2006

Popper's ghost --- It was in either 1954 or 1955. One semester of biology, which I recall as being required of all students. The instructor was perfectly adequate and followed closely the text. The text which incidentally did not mention Darwin or the theory of evolution at all, despite the fact that this was Los Alamos High School...

At one point we learned that algae were single celled organisms. Later in the book we learned that kelp were colonies of algae. At no point was adhesion or cell signaling mentioned...

Thus my (poorly phrased) question. However, since it appears that some algae are multicellular, my question to PZ regarding classification is null and void. Ok?

Lenny's Deli Counter Gal · 22 December 2006

Guys! Yo, guys!

I just had to escort Lenny off the premises! If youse think fer a minnit I'm gonna allow anybody else to start behaving that way, youse better have another think, real quick-like!

So just put those things back in your pants or youse can just take yourselves right out the door! Got that?

I should hope so!

Multi-cellular! Uni-cellular! Kelp!

This is a deli already, not one of your college-town veggie loonie bins!

We don't even carry spirulina, fer cryin' out loud!

Popper's ghost · 22 December 2006

At one point we learned that algae were single celled organisms. Later in the book we learned that kelp were colonies of algae.

"kelp is a colony of algae" is not inconsistent with "kelp is multicellular"; it depends on what is meant by "colony", but that is your word. Previously you said you learned that they were a "society of unicelled algae"; which did the book say? Since you can't even keep track of what you are saying now, it is not unlikely that you can't accurately remember what you were told in 1954.

At no point was adhesion or cell signaling mentioned...

I didn't say they were; apparently your reading comprehension is as bad as your recollection and your ability to distinguish between two very different descriptions. The point is that kelp are obviously multicellular, and it was obvious in 1954, and in addition to having asked a bogus question, your recollection and report of what was taught in 1954 is most likely wrong.

Lenny's Deli Counter Gal · 22 December 2006

Are youse hard of hearing, or what?

Am I gonna have to get my broom out?

David B. Benson · 22 December 2006

Popper's ghost --- You are on a way to a ulcer and appearantly are having difficulty with reading comprehension.

I did not say earlier I was taught 'society'. Go read what I said.

Do you know where Los ALamos is? Perhaps I had never seen any kelp at the time? How is a young high school sophomore, not very interested in biology, going to know the difference or similarities between 'colony of algae' and something as different as the earthworm we disected?

Have an eggnog and go outside to cool off, ok?

Popper's ghost · 22 December 2006

Methinks thou doth protest too much, Mr. Benson -- in other words, it's you who is getting an ulcer, as well as prevaricating. You said you believed that kelp is considered a society of unicelled algae, I asked why you believed that, and you referred to what you recall being taught. It's really quite simple -- as is the fact that 50 year old memories aren't reliable.

David B. Benson · 22 December 2006

Popper's ghost --- Is a mushroom obviously multicellular? Demostrating adhesion and cell signalling? What about lichens?

Biology appears to be rather complicated, yes? Why did PZ write "... (although more on the side of multicellularity), the sponges"? Are sponges multicellular or not? Or perhaps the post is about this grey area?

Peace...

PZ Myers · 23 December 2006

I see the problem. Look at what I wrote again: those two factors are key elements in making a multicellular animal. I would most definitely not suggest that there is only one path to multicellularity: look at plants. Plants are multicellular, but they are not metazoans.

David B. Benson · 23 December 2006

PZ --- Thanks. I think I have it sorta straightened out now.

Popper's ghost · 23 December 2006

Popper's ghost --- Is a mushroom obviously multicellular?

Who cares? The point is that kelp are. Moron.

Popper's ghost · 24 December 2006

Look at what I wrote again: those two factors are key elements in making a multicellular animal.

But the explanations of why these are "key ingredients" are not specific to animals. "The cells of your multicellular organism better stick together, or it will fall apart" is just as true.

I would most definitely not suggest that there is only one path to multicellularity: look at plants. Plants are multicellular, but they are not metazoans.

But plants do display these key ingredients, which surely distinguish a "multicellular" organism from a mere aggregation of similar cells. I don't think the problem here has anything to do with metazoan vs. non-metazoan, but rather has to do with a) not recognizing that "algae" refers to a broad array of organisms from single cells to aggregations of similar cells to complex multicellular seaweeds and b) formulaic thinking, based on what one was taught (or thinks one was taught) about kelp while not paying any attention to the actual facts about kelp that make it bloody blatantly obvious that they are "multicellular" and considerably more complex than sponges.