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Arizona’s Sky Islands host ants that lost their wings when desert divided their habitat 10,000 years ago.

Arizona’s Sky Islands host ants that lost their wings when desert divided their habitat 10,000 years ago.

Marie-Julie Favé

Arizona’s virgin ant queens could shed light on the predictability of evolution

Before a warming world turned Arizona’s lower elevations into desert 10,000 years ago, they were carpeted in wildflowers and forests. Rising above it all were the five Sky Islands: 2400-meter-tall mountains that hosted unique wildlife, including ants whose winged virgin queens swarmed the forests to found new colonies. But when desert swallowed the lowlands, all that remained of the ants’ stomping grounds were the forested peaks. To cope, the now-isolated colonies evolved an identical strategy: The queens lost their wings. New research shows that these parallel adaptations could shed light on a longstanding debate on the predictability of evolution.

For a long time, biologists have wondered whether the evolutionary history of life on Earth would happen the same way if we hit the “rewind” button and started over. Some argue the course of evolution is largely predictable, but others say it is too dependent on random events to be repeatable.

To explore this issue, Marie-Julie Favé, a biologist at the University of Montreal in Canada, focused on the ants of Arizona’s Sky Islands. The insects—Monomorium emersoni—started as a single population, but they became separated into five main groups when the desert encroached on their habitat. To deal with the new climate, queens in all of the groups lost their wings. Flying queens can go farther than flightless ones and significantly increase the territory of their colonies. But flight carries a high risk of death—ants floating on the breeze are much likelier to be blown into the desert, where they will die. When the land around the Sky Islands turned to desert, flight lost its advantage.

“The Sky Islands are really a fascinating environment,” says Favé, who conducted the research while a graduate student at McGill University in Montreal. “Within half an hour, you can drive from the desert into the mountains whose forests look just like those in Quebec. It’s an enormous challenge for these ants to adapt to such a dramatic environmental gradient.”

To find out how they adapted, the researchers sequenced genes from all five populations. They found a significant genetic divergence between the ants on the northern and southern mountain ranges, which reflected their historical isolation. With this as their baseline, they set out to examine the genetic and developmental basis for wing loss in the queens from the five populations.

Thanks to extensive studies on wing development in fruit flies, the researchers were familiar with the network of genes that was likely to be involved in wing development in the ants. Using antibodies, they were able to see in the lab where the candidate genes were being expressed in the ants’ wing discs—small pouches of cells that later become adult wings. They identified the gene expression patterns for ants from all five populations, and found that some changes were repeated in each group, whereas others were unique.

For example, the expression of a key gene involved in cell differentiation varied in each of the populations. Other genetic changes repeated themselves in each group, presumably because there are a limited number of ways to make wingless queens. For example, four genes involved in wing patterning all had similarly changed patterns of expression. This is the first study in wild populations to show that adaptation to climate change is a “mosaic” of random and predictable genetic changes, says lead author Ehab Abouheif, an evolutionary developmental biologist at McGill University, in a paper published online today in BMC Evolutionary Biology.

The fact that the five populations evolved similar adaptations suggests that there is predictability to evolution, he says. But the randomness of evolutionary change is also apparent, because the separate ant populations are using slightly different genetic mechanisms to get to the same end.

Abouheif says the results could have profound implications for understanding the genetic and developmental basis of adaptation to climate change. Current models that predict animals’ responses to climate change focus on demography—migration and changes in population size—but do not consider how climate change might alter a species’s development, in particular its ability to disperse.

Greg Wray, a biologist at Duke University in Durham, North Carolina, who was not involved in the study, says the authors were smart to use wing development in the well-studied fruit fly to understand wing development in the less well-understood flying ants. He also noted that the researchers’ ability to study five instances of the same evolutionary experiment in wild populations is particularly unusual. “What makes the Sky Islands so interesting for this particular study is the possibility that you can get independent replicates,” or copies of the same evolutionary experiment, he says.