Sunday, March 18, 2012

Biological Revolutions: Multicellularity

(Picture from here.)

In our ongoing saga of revolutionary life, we've arrived at a world with both prokaryotes and eukaryotes. All are single celled.

But, when we look around, we see multicelled organisms literally all over the place. From the lowliest parasite to blue whales, multicellular organisms are everywhere.

How did that happen?

Well, that's a complicated question. After all, which one are we talking about?

That's right. Multicellular life forms didn't evolve once. They didn't evolve twice. They perhaps evolved as many as twenty-five times. Animals and plants evolved it separately. Slime molds and algae. Cyanobacteria and myxobacteria-- though multicellularity appears to be more the province of the eukaryotes than prokaryotes.

There are a lot of advantages of being multicellular. Cells can specialize and with specialization comes a higher efficiency at a particular function. Organisms consisting of cells can grow larger. They can exploit different niches, digest different materials. They can employ sacrifice as a tool-- one unit of the organism sacrificing itself for the good of the whole. It's so ubiquitous in multicelled organisms we don't even think about it. Who considers shed skin cells? Dead leukocytes? The discarded cells of the gut? Yet these cells have, in effect, given up their lives on the hope that their genome will be continued by other specialized cells.

These are advantages if the organism is already multicellular. But evolution never works with an eye to the future. The only advantage is current advantage. A What have you done for me lately? sort of world. Thus, the origin of multicellularity must occur in a framework where it is advantageous. Once we have it we can talk about how great it is. But we have to get there first.

Figuring out how this happened is a problem unto itself. By the time multicellular organisms left fossils they were already well established. Whatever happened occurred long before-- some authorities think as much as a billion years ago. Certainly more than 500m years, since that's when we first see whatever fossils there are.

Alright, then. What can we do with living organisms?

Ah. Let me tell you about Volvox.

Volvox is a very pretty algae (see the picture above) that congregates in a wonderful hollow sphere. The somatic cells are on the outside. Reproductive cells are on the inside and near the posterior, so they have some limited specialization. The somatic cells have flagella and move the ball around. Some species have actual specialized cells that act as eyespots and cause the ball to move towards the light. How they manage this without nerve cells is a bit of a mystery but they manage. During asexual reproduction the daughter colonies occur inside of the parent colony and then the parent dissolves and the daughter colonies are released. During sexual reproduction male and female gametes are released, join and become new colonies.

A close cousin of Volvox is Chlamydomonas, a single celled green algae. They also have flagella. When they reproduce asexually, they pull in their flagella and then divide within the same cell wall. Sometimes multiple times so that many daughters share the same space. Eventually, the daughters develop flagella and swim away. C. reinhardtii has a sort of sexual reproduction. Normally, C. r. cells are haploid. When stressed haploid gametes develop that can be one or the other of two mating types. These join to form a diploid "zygote" which is dormant in the soil. When favorable conditions occur, the zygote divides via meiosis into haploid daughter cells which then go on their merry way.

What's interesting here is that Chlamydomonas and Volvox share the vast majority of their genetic content. And there is an interesting structural similarity between them in that Chlamydomonas undergoes a division where the daughter cells are packed together and in Volvox the daughter cells don't separate except in reproduction. Could there be a link here?

This represents the "colonial theory" of multicellular evolution. Haeckel came up with this in 1874. The idea is that organisms of the same species could fail to fully separate during division. Most of the time this would be destructive but if other predispositions were to occur at the same time an advantage for the colony would occur.

Interestingly enough, Volvox practices what is called "multiple fission." The nucleus divides multiple times before the cytoplasm divides the daughter cells apart. It's not hard to imagine a cluster of stuck cells resulting from a failure to launch.

An interesting experiment was published this last January. (See here.) Scientists at the Whitehead Institute thought they might be able to force single celled organisms to evolve multicellular behavior. They took single celled yeast and grew it in a test tube. Then, every day they shook up the tubes and pulled out what fell to the bottom and grew that in a new test tube. This went on for a couple of weeks (100 generations) and, sure enough, yeast cells began to show clusters of cells with limited self-sacrifice and specialization.

In the article I read there was no discussion of the heritage of the yeast cells. Could this be a previously evolved dormant property brought to light under extraordinary selection? Or is this the expression of a predisposition of traits towards multicellularity now expressed? I don't know. But it is very interesting.

This doesn't get past the what's-in-it-for-me problem of multicellularity in general. Why should the somatic cells put up with this? Why shouldn't they reproduce on their own? A kind of cellular tragedy of the commons.

In Chlamydomonas the number of cells produced depends on the size of the parent-- a reflection on quality of the environment. Volvox limit the number of cells in a colony. Cheaters are not so favored since the total number of cells limits their advantage. There are mutants where the somatic cells start reproducing but then the colony collapses and sinks. Such mutants are detrimental and are selected against. Volvox manages this by separating cell types very early in the colony's development, limiting the opportunity of mutations to accumulate and have an effect.

However, these mutations are still seen and suggest the opposition has not been silenced. This is not terribly surprising since it's estimate that Volvox evolved multicellularity only 200m years ago. It's probably still having growing pains.

While simple multicellularity (as exemplified by Volvox) has evolved many times. It appears that complex multicellularity is much more rare. Animals. Green plants. Fungi. Algae. Such skills of organization require much more dedication on the part of the workers. The rewards regarding the individual cells don't seem to match what the individual cells have given up.

Whales, squids and redwoods all must face a similar problem: how do you handle revolution in the ranks? Most cells in a complex organism are bound to their role. Each cell is using only that portion of the DNA required by its functionality. Some cells are even barred from reproduction. If that fails, each cell is always on the trigger of suicide (apoptosis) if something should go wrong. Should that fail the secret police (the immune system or its equivalent) are watching. Always watching.

Evading all the safe guards must come as liberation albeit at the cost of the organism.

Is cancer the price we pay for the majesty of our organization as we suppress the relic desire of our cells for their old single celled freedom?

Additional reading:
From Simple to Complex, The Scientist

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