Saturday, May 31, 2014

Which Came First, Sex or Death?

Bodies to do the dirty work: on the origin of germ cells, somatic cells, and bodies.

Imagine you are a termite colony. The vast majority of termites can not reproduce- they are disposable workers for the good of the whole, while the alates, or winged reproductive caste, are set aside to reproduce new colonies. Your body works the same way ... virtually all its cells are on a one-way trip to death, with only the germ cells, and a vanishingly small proportion of those, giving rise to descendents in a bid for immortality.

Once upon a time, all cells were immortal. That is to say, they had no intrinsic lifespan, and could live as well as reproduce endlessly. Why give that up? A recent paper discusses the ins and outs of germ-somatic cell and tissue separation which happened early in multicellular animal evolution.

There are several theories about why this division of labor would take place. One is simple practicality- that not all cells are in a position to reproduce. Would you like your bone cells to send out new embryos to form babies? Such a democratic system would be a big mess, though plants approach it with their capacity to generate reproductive structures from any active meristem. And anyhow, every cell shares the same genome, so it shouldn't make that much difference which one in a body makes the germ cells, right?

Other theories focus on what a germ tissue can be optimized for, such as going through relatively few rounds of replication, thus minimizing mutation, or being fed by other cells, so that they do not have to do their own metabolism, which is mutagenic, again reducing the rate of mutation. While female germ cells often adhere to the former theory, male germ cells tend not to, being produced in vast profusion, requiring high rates of replication. The authors of a recent paper focus on the latter hypothesis, called the "dirty work" hypothesis; that bodies are devised to do all the dirty work of life, while the germ cells are protected from dangers of many sorts, especially from metabolic damage.

Metabolism is a messy process, spewing free radicals and other damaging chemicals about the cell. Both the production of carbohydrates using solar energy to split water molecules, and the process of using carbohydrates by splitting oxygen and returning CO2 to the air are very high-energy processes, as one can see any time something burns. Metabolic enzymology controls them, but at some point, free radicals have to be made, high-energy electrons need to be shot from one place to another, and there is no perfectly safe way to do this. The mitochondrion is an attempt to keep the danger contained in a membrane-enclosed space, but that is not completely effective either.

The paper is computational one, modelling various ecological strategies connected with this hypothesis.
"We tested the dirty work hypothesis using digital evolution, an approach where digital cells are self-replicating computer programs that evolve in an open-ended fashion and are theoretically capable of performing any computational function. For this study, we consider a world of 400 multicellular organisms (“multicells”) that compete for space, where the rate of reproduction of each multicell depends on the number and rate of computational functions being executed by constituent cells."

Basically, if you impose a mutagenic cost of doing the basic processes of life- gathering food, digesting it, metabolizing it, etc., then it stands to reason that if germ cells are spared these risks in a multicellular colony or organism, they will be able to spread their genomes with less mutagenic risk to future generations. Of course, some mutations are needed. A minimal level is required for adaptation and evolution, and some organisms even ramp up their rates of mutation when under stress, in a somewhat desperate bid for success. But on the whole, evolution has put vast resources and endless optimization into the project of high-fidelity reproduction, because as any Darwinian knows, most mutations are harmful.

So it should not be surprising that multicellular organisms rapidly took advantage of this opportunity to increase fidelity by segregating germ cells away from everything else, and building bodies to do all the so-called dirty work, including sex and thinking- which is, incidentally, very metabolically expensive.

Segregation of germ and non-germ cells, in a virtual evolution program, plotted on the X-axis, vs bands of mutagenicity going downwards from  zero (top) to high levels (bottom). 

The experimenters set various levels of mutagenicity to their virtual life tasks, and found that at intermediate levels, their virtual life forms would segregate non-reproductive cells into a soma with pretty much complete probability. At very low mutagenisis levels, there was no advantage to such segregation, and it did not evolve. At very high levels of mutagenicity, specialization of some cells (i.e. the body) to deal with mutagenic activities was essentially lethal, so such environments were avoided entirely rather than adapted to. Their simulations are clever, if very schematic. But they get the simple point across, that the origin of divided somatic / germ bodies is easily understood as a natural consequence of multicellular evolution.

So what about sex? Many bacteria have some form of sex, exchanging bits of their DNA, despite being single-celled and having only one genome to play with. Therefore sex developed long before death did, in the sense of the programmed death of a somatic body after reproduction, as is common among all animals and plants. No cell lives forever, but those that reproduce by binary division have no clear point of death. In contrast, our bodies, specialized for the complex operations of getting a living in a hostile world up to the point when a new generation can be created and nurtured out of the germ cells, have a very definite, genetically programmed (if only by default) endpoint. The way life spans vary between species of animals, but much less within them, suggests strongly that life span is not an accident, but is part of the developmental program, making way for the new generation on an optimized schedule, for evolutionary, and even cultural, rejuvenation.

As one might imagine, these ideas are related to the selfish gene hypothesis, put forth by George Williams and popularized by Richard Dawkins. Which is that bodies are the robotic (and disposable) entities devised by genes for their propagation. That is a strongly skewed perspective, however, making of the gene an almost conscious, greedy, self-sufficient, anthropomorphised totem, which is more than a little fanciful. Rather, organisms are complex communities with delegated and divided functions, one of which is to keep safe the genetic code that the whole community relies on for high-fidelity propagation. Even if the body and its mind can be viewed as disposable cat's paws for that code, they are the only part of the community that can be cognitively selfish; the gene is just a large molecule.
"The same information-work tradeoff may also have motivated a switch from RNA to DNA as the molecule of heredity. According to the RNA world hypothesis, RNA initially served as both a carrier of genetic material and a catalyst for metabolic work. However, RNA instability may have motivated a shift to DNA genomes and catalytic proteins. This is a type of molecular division of labor, ensuring both high fidelity transmission of hereditary information and the execution of critical chemical work."

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