One gene change changes freeloader to model citizen
and World Science staff
Scientists have found that a one-letter change of DNA can transform a microbe with a habit of cheating its peers into a model citizen that cooperates with its bacterial comrades.
Researchers have long been trying to untangle the evolutionary roots of cooperation and altruism. It’s a confounding problem, since evolutionary theory, at least superficially, suggests cheaters should triumph and do-gooders should die out.
To attack the question, biologists have sought to find the simplest possible examples of cases in which cooperation evolves, in order to study what fundamentally drives the process. In seeking out such a model, a scientist can hardly hope for better than one in which cooperation arises from the smallest possible type of genetic change, in the simplest type of organism—bacteria.
Gregory Velicer of the Max-Planck Institute for Developmental Biology in Tubingen, Germany, and colleagues reported finding just that in strains of the soil-dwelling bacterium Myxococcus xanthus. This shows how simple genetic changes can radically alter complex forms of social behavior, the researchers said.
During hard times, groups of M. xanthus join forces to form structures called fruiting bodies, in which they pool resources and produce hardy reproductive compartments called spores. Relatively few individuals survive, but it lets the group wait out bad times and produce offspring that will emerge later.
This system is open to cheating by strains that do not bother to form these aggregations, but nonetheless reap the benefits of the nutrients provided. Although “cheater” strains of bacteria exist naturally in various species, Velicer’s team in this case produced freeloaders artificially, by putting some bacteria for 1,000 generations into easy conditions. There they “forgot” how to build the fruiting bodies, losing the genes that enabled them to do so.
When Velicer and colleagues mixed the cheating and socially responsible strains together under alternating conditions of stress and plenty, they noticed a surprising result: the strain that ultimately evolved was a cooperative strain that produced more surviving cells than either of the two original strains.
“The new cooperator evolved from the cheater and not from the original cooperator,” Velicer wrote in an email. But “the new cooperator did not evolve a general ‘niceness’ to everyone. Cells of the new cooperator cooperate fully only with their own kind.”
When the researchers sequenced the genes of the strains, they found that the cheaters’ newfound social conscience was due to a single-letter change in the DNA code.
The researchers, reporting their findings in this week’s issue of the research journal Nature, christened the new strain Phoenix, because it rescued the bacterial population from the ashes of almost certain doom at the hands of the cheaters.
Kevin Foster of Harvard University, in an accompanying commentary in the journal, wrote that the experiment suggests that in a society disintegrating because of cheaters, evolution will favor mutations that bring back teamwork.
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