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Knowing when not to cheat

This week's paper is Facultative cheater mutants reveal the genetic complexity of cooperation in social amoebae published in Nature by Lorenzo Santorelli and colleagues at Rice University and Baylor College of Medicine, both in Texas.

The evolution of cooperation is a central problem in the history of life. Darwin explained how sophisticated adaptations -- "the structure of the beetle which dives through the water... the plumed seed which is wafted by the gentlest breeze" -- can evolve in a series of small improvements over generations. But some of the major transitions in evolution are harder to explain, because It seems that they should have been opposed, rather than supported, by natural selection. The origin of multicellular life is a good example. It's not that hard to imagine independent cells working together in loose groups for mutual benefit - huddling together for defense, say - but why would a cell give up the ability to reproduce, as most of the cells in our bodies have done?

Supernatural intervention, perhaps? If Intelligent Design were a scientific field, rather than a religion, its researchers would be doing experiments to test this hypothesis, rather than making movies and whining. The problem, of course, is that "test" means "subject to possible disproof."

Cooperation among a group of genetically identical cells is easy to understand. Then, kin selection will favor some giving up individual reproduction, if that increases their collective reproductive success. But, when multicellular life first evolved, how often would cells in a cluster all be the same strain? "Hi, I'd like to join your cluster! How about if I reproduce while the rest of you fight off predators?" Cells that behave this way are known as "cheaters" because they don't cooperate themselves but benefit from cooperative activities of others. They don't really talk like that, of course.

Dictyostelium life cycle (David R. Caprette, Rice University)

Cooperation in the "social amoeba" Dictyostelium can be disrupted by such cheaters, just as we assume early multicellular cooperation could have been. These amoebae live much of their lives as independent cells. But, when starved, they form resistant spores, which eventually resume growth as the next generation of independent cells. The spores are held at the end of a stalk, which presumably helps dispersal by animals to environments that may be more favorable.

The problem is that the cells in the stalk are left behind and die without reproducing. (I previously discussed a paper showing that when "slugs" composed of multiple Dictyostelium cells crawl together through the soil, some cells sacrifice themselves to protect their clonemates.) Suppose you have a 50:50 mixture of two Dictyostelium strains. A "cheating" strain would be one that manages to get mostly into the spores, letting the other strain form the stalk. (This is how the researchers measured cheating, using a fluorescent version of the control strain so they could see what percent of the spores it made.) So would cheaters become more common in each generation? Maybe not, if the cheater wasn't able to form a good stalk on its own. Then, its evolutionary success would be entirely dependent on infiltrating other strains.

But what if cheating is facultative? That is, what if a strain forms stalks to hold up its own spores, but relies on the stalks of another strain when it can? I would have guessed that such a sophisticated form of cheating would be beyond the capabilities of an amoeba. (Similarly, in our own research, I have assumed that rhizobium bacteria that "cheat", by providing their host plant with less nitrogen, don't check first to see whether other rhizobia on the same plant are taking up the slack.)

It was certainly worth checking for facultative cheating, however, and that's what the researchers did. They let 10,000 different strains compete over ten "generations" (life-cycles; see diagram), all mixed together. This mixing would allow cheaters to become more common, via spores, at the expense of cooperative strains that contribute more cells to stalks. Then they tested a couple thousand strains - try doing that with meerkats! -- to make sure they could form spore-bearing stalks on their own. Only 1% of the strains couldn't. So if there were any cheaters in the other 99%, they were facultative, only cheating when they could get away with it, i.e., when another strain was forming a stalk they could use.

Such facultative cheaters were apparently fairly common. When the cooperative control was mixed 50:50 with the evolving population, only 40% of the resulting spores were of the control strain. Testing 40 different strains, they found that 31 were cheaters (making more than their share of spores), 5 were "losers" (making less than their share of spores), and the rest were neutral.

Is there a cheating gene? For these amoebae, cheating clearly had a genetic basis, but more than one gene was involved. In fact, mutations in any one of more than a hundred different genes could convert a cooperator into a cheater. Does this mean that, when cooperation among cells first evolved, it would have required hundreds of mutations to happen all at once? No, that would be another version of Behe's mousetrap fallacy. Just because a complex system has a lot of parts that are now essential, that doesn't mean that a simpler version couldn't have worked with fewer parts than we imagine to be necessary. Still, if even a small fraction of these genes are essential to cooperation, that could help explain why multicellular life took so long to evolve.

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