The earliest monarch butterflies arose in North America and were migratory, contrary to what scientists believed. Over time, the butterflies evolved populations in other locations, some of which stay put year-round, scientists conclude October 1 in Nature.
Because many of the monarch’s closest butterfly relatives live in the tropics and do not migrate, “the thought was that the butterflies [came] from South and Central America and became migratory from resident populations,” says Tyler Flockhart, a conservation biologist who concentrates on monarchs at the University of Guelph in Canada. “But that doesn’t seem to be the case.”
Monarchs, Danaus plexippus, are famous for their annual migration from the United States and Canada to Mexico and back. But nonmigratory populations of the same species live in the tropics of Central and South America and in various other sites across the Atlantic and Pacific.To determine how these populations relate to each other, Marcus Kronforst, an evolutionary biologist at the University of Chicago, and his colleagues sequenced the genomes of 80 monarchs from various locations as well as nine genomes from closely related species. By examining differences in the genomes, the researchers built an evolutionary tree.
Surprisingly, the North American population ended up closest to the bottom of the evolutionary tree, indicating that they are most closely related to the common ancestors of all monarchs. Three populations independently lost migratory behavior: one that ended up in South and Central America, one across the Pacific and once across the Atlantic. “The butterflies originated in North America and spread out in these three waves,” Kronforst says.
Kronforst and his colleagues identified approximately 500 genes that changed in the monarchs that stopped migrating. One of the genes stood out. “Every time the butterflies lose migration, this one gene changes entirely, and it changes exactly the same way,” he says. “Those other 499 genes change a little bit; this one is very different.”
That gene plays a role in muscle function. When the researchers examined monarch tissue samples, they saw that the gene was less active in monarchs that migrated than in nonmigratory ones. The researchers also found that migratory monarchs had lower metabolisms. Together these findings suggest that the muscle gene allows the migratory monarchs to fly more efficiently and burn less oxygen.
Kronforst and his team were surprised that they didn’t find changes in genes involved in complex behavior, which is necessary for migration.
Kronforst now wants to confirm how the muscle gene that his team identified functions in the butterfly body. In North America, migratory monarch numbers are declining due to deforestation, extreme weather and herbicides killing the milkweed plants the butterflies lay eggs on. Researchers want to decipher the mechanics and evolution of migration before it vanishes.
“There’s no other insect that has migration like this,” says Kronforst, noting monarchs’ ability to travel vast distances across multiple generations and return to the same small area. “If it disappears there’s nothing else like it.”
Editor’s Note: This article was updated October 6, 2014 to correct the number of monarch genomes sequenced in the study.