Evolution has a bizarre way of repeating itself. Consider the postman butterfly, named for its repetitive daily route from flower to flower. It's completely black, save for a bright red band on its upper wings and a white stripe on its lower wings. The small postman butterfly, on the other hand, is an entirely different species that looks nearly identical.
Now, a group of scientists seeking to understand the mechanism through which evolution produces near-doubles has figured out that the two species underwent substantially different genetic changes, only to look the same. Their study details a striking example of convergent evolution, which is when two species have similar traits even though their shared ancestor didn't have that trait—when the species diverged, it evolved in them separately.
“These butterflies, even though they're different species, they have diverged from each other not so long ago in evolutionary time,” said Carolina Concha, the study’s first author. “It’s surprising to me that they have this flexibility of being able to follow a different path to achieve the same result.”
The group’s findings were published on Thursday in the journal Current Biology.
Heliconius butterflies are known for their bright and diverse wing patterns. Scientists believe that some of these butterflies use their distinctive patterning to signal that they are toxic to potential predators. In some instances, non-toxic Heliconius butterflies adopt very similar patterning for protection. But both the postman and small postman are poisonous, which could mean they grew to resemble one another for a “strength in numbers” approach—if a predator learned to avoid one butterfly, it would avoid the other automatically.
To find out how evolution double-dipped, Concha and her 24 co-authors genetically engineered a dozen species of Heliconius butterflies to remove a gene called WntA that they believed would affect wing pattern and color. Not only did the butterflies without the gene have different colors and patterns on their wings than their normal relatives, the loss of the gene affected different species in different ways. Some butterflies’ wings stayed mostly the same, while others gained swaths of pink in place of black, or lost dots and stripes.
After genetic modification, the postman butterflies’ red stripe extended down the entirety of their upper wing, while the small postman butterflies saw the white stripe on their lower wings become more yellow. To Concha, this suggested that the two species were using the same gene in different ways—if they weren’t, one would expect the butterflies lacking the gene to look identical to one another.
After looking under a microscope that magnified the wings to 15,000 times their normal size, it was clear that the loss of the gene even affected the structure of the tiny scales that make up each wing.
“In each case, there was a shift in the structure of the scale, and so we interpreted that to mean that this gene is acting very early and that it has a very fundamental function in determining the identity of the scale,” Concha said.
In addition to the postman and small postman, the authors drew the same conclusion for two other co-mimetic pairs of butterflies that are not closely related but look identical.
Because of the fragility of Heliconius butterfly eggs as well as the advances in genetic editing that made the experiments possible, this study is an impressive step forward, said Michael Perry, an assistant professor of cell and developmental biology at UC San Diego who was not involved in the research.
“Eight or nine years ago, I would’ve never believed you if you said that it would be possible to make targeted mutations in 12 different species of Heliconius. In fact, I would say using the technology at the time, you’d be lucky if a graduate student spent their entire PhD just trying to knock out one gene and actually got a result in one species,” Perry said.
A knock-out experiment, in which scientists delete one gene from an organism and see what happens, is the gold standard in the field, according to Concha. The experiment took slightly over two years to carry out because of how painstaking it was to genetically engineer the butterflies, she added.
Identifying that the gene plays a role in wing patterns is only the first step in investigating how two different species can, through evolution, look identical over time. Concha said that she and her collaborators are now trying to determine the particular mechanism by which the gene influences butterflies’ wings.