Elizabeth Pennisi in Science:
It’s well known that humans—and our antibiotics—are in an evolutionary arms race with the bacteria that make us sick. As fast as we rev up a strong defense, they change to evade those defenses, upping the ante for either our bodies or our drugs. This arms race has been called the Red Queen hypothesis after the Alice in Wonderland character who told Alice that in looking-glass land, she would have to run as fast as she could just to stay in the same place. But what about friendly alliances—such as those between us and the beneficial microbes that naturally inhabit our guts and other tissues and help us digest food and stave off other infections? Researchers have often assumed that once such a mutualistic partnership arose, both sides would be stable gene-wise—they would have no need to keep evolving to match each other. A decade ago, this idea was dubbed the Red King hypothesis. But a new study in ants finds that even partners known to be engaged in mutually beneficial associations have rapidly evolving genomes, apparently to keep those partnerships intact.
To find out how quickly friendly partners evolve, Corrie Moreau, an evolutionary biologist at the Field Museum of Natural History in Chicago, Illinois, and her graduate student Benjamin Rubin sequenced the genomes of seven ant species. Three survive by partnering with just one plant: the acacia tree, the Japanese knotweed, or a tropical tree called Tachigali. In the case of the acacia, Pseudomyrmex flavicornis defends the tree from elephants and other grazers in return for a special acacia-produced sugar and the hollow spines in which it nests. The duo also sequenced the genomes of three nonspecialist species—each closely related to one of the specialists—and one very distantly related ant species. All seven were in the Pseudomyrmex genus. Rubin, now at Princeton University, compared the genome of each mutualistic ant with that of its partner and the distant relative, using the three sequences to calculate a rate of evolution based on the number of DNA differences between the distant relative and the other ants. “What we expected to see was a slowdown in the rate of evolution in the mutualist,” Moreau says. But the opposite proved true. The genome of each mutualistic ant was evolving at a faster rate than that of the most closely related generalist, Rubin and Moreau report today in Nature Communications. Moreover, they discovered that changes were occurring in the same genes in all three mutualistic ant species compared with their nonmutualistic counterparts.