With their distinctive orange wings, remarkable long-distance migration and widespread presence, monarch butterflies are some of the most recognizable butterflies in the world. A team of researchers, led by scientists at the University of Chicago and including UGA ecologist Sonia Altizer, has published a study in Nature that reveals unexpected answers to the origins of monarchs and the genetic basis of their best-known traits.
“The results of this study shift our whole thinking about these butterflies,” said the paper’s senior author Marcus Kronforst of the University of Chicago.
The scientists analyzed the genomes of nearly 100 monarchs from around the world to explore questions about the monarch’s evolutionary history and characteristic traits—where it originated, what genes determine its migratory behavior and what causes its bright orange warning coloration. What they learned surprised them and underscored the importance of efforts to conserve North America’s migratory monarchs, whose numbers are declining.
They started their study by looking at the monarch’s evolutionary history. Analyses were based in part on butterflies from Altizer’s collection.
“The monarch is an outlier,” said Altizer, an associate dean of the Odum School of Ecology and the UGA Athletic Association Professor in Ecology. “Other species in the same genus and in the same subfamily as monarchs are generally tropical butterflies that have fairly restricted geographic ranges, while monarchs have a very broad modern geographic range and can migrate tremendous distances. Yet nobody understood the historical connection between these different monarch populations around the globe.”
They expected the genetic analysis to show that, like Neotropical birds, monarchs originated in the tropics and later expanded into North America and became migratory. Instead, they found just the opposite.
“For monarchs, the migratory North American population appears to be ancestral to all other modern populations,” Altizer said. “It appears to have given rise to the current populations in the New World tropics and in Europe and in the Pacific.”
The researchers also wanted to discover the genetic basis for migratory behavior. The component of the monarch genome that appeared to show the strongest link to long-distance migration was a gene that affects the structure and function of flight muscles—a gene very different from one that predicts dispersal propensity in other butterflies.
Finally, they looked for the genetic basis for the warning coloration of monarch wings by comparing the DNA of a small group of rare white monarchs from Hawaii with the more common orange ones found elsewhere. Again, their results ran counter to expectations. Instead of genes that affect the production of orange and red pigment causing the difference, it was a gene that affects the ability of the pigment to spread across the wing.