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Evolving resistance to cheaters

This week's paper, "Cheater-resistance is not futile" was published in Nature. It describes experimental evolution of the social amoeba Dictyostelium, whose propensity to cheat other members of its species was discussed by Will Ratcliff in a recent guest post titled "Sneaky slime molds."

When two Dictyostelium strains are mixed in a reproductive structure, cheaters contribute fewer cells to the stalk that holds up the reproductive spores. Could the presence of such cheaters select for cheater-resistance genes, just as the presence of owls or hawks selects for mouse genes that make their coats match the soil color?

To find out, the researchers started with a genetically diverse population (the raw material for natural selection) and added a cheater. By definition, the cheater would tend to increase in frequency over cycles of reproduction, but they prevented that by using a cheater they could kill after it had had its effect on the relative reproduction of the other strains.

They did this several times, and each time one or two cheater-resistant genotypes took over the selected population. For example, starting with a population that made less than 40% of the spores when mixed 50:50 with the cheater, they evolved a population that made about 50%. (The cheater-resistant strain didn't push its advantage, apparently!)

In at least some cases, they identified the specific mutation that let their mutant hold its own against the cheater. The gene "has no annotated homologues in other organisms", so it's probably not a universal anticheater gene. In fact, it didn't even work against all Dictyostelium cheaters. Still, it would be interesting to know how it works at the molecular level.

Meanwhile, cooperators take heart. It's possible to keep cheaters under control, without becoming a cheater yourself.

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