Monarchs migrate and fly differently, but they meet and mate

Every year millions of monarch butterflies migrate across eastern North America to fly from the north to the United States-Canada border to winter in central Mexico, covering up to 3,000 miles. Meanwhile, on the other side of the Rocky Mountains, western monarchs generally fly 300 miles to the Pacific coast to winter in California. Eastern and Western monarchs were long believed to be genetically distinct populations.

However, a new study confirms that while eastern and western butterflies fly differently, genetically they are the same. The newspaper Molecular Ecology published the findings, led by evolutionary biologists at Emory University.

“It was surprising,” says Jaap de Roode, an Emory biology professor and lead author of the study. His laboratory is one of the few in the world that studies monarch butterflies.

“One would expect organisms with different behaviors and ecologies to show some genetic differences,” says de Roode. “But we found that you can’t genetically distinguish between western and eastern butterflies.”

The current paper builds on previous work by Roode’s lab that found similarities between 11 genetic markers for eastern and western monarchs, as well as other more limited genetic studies and observational and follow-up data.

“This is the first genome-wide comparison of eastern and western monarchs to try to better understand their behavioral differences,” says Venkat Talla, current study first author and Emory postdoctoral fellow in the lab.

Talla analyzed more than 20 million DNA mutations in 43 monarch genomes and found no evidence of genomic differentiation between eastern and western monarchs. Instead, she found identical levels of genetic diversity.

“Our work shows that eastern and western monarchs are uniting and exchanging genetic material to a much greater extent than previously believed,” says Talla. “And it adds to the evidence that differences in their environments are likely to determine differences in their migration patterns.”

Co-author Amanda Pierce, who led the previous study on 11 genetic markers, launched the project while a graduate student at De Roode Laboratory.

“Monarch butterflies are so fragile and so light, and yet they can travel thousands of miles,” says Pierce. “They are beautiful creatures and a great model system for understanding unique and innate behaviors. We know that migration is ingrained in their genetic wiring in some way.”

After the monarchs leave their hibernation sites, they fly north and lay eggs. The caterpillars turn into butterflies and then fly further, mating and laying another generation of eggs. The process repeats for several generations until finally, as the days shorten and temperatures cool, monarchs emerge from their pupae and begin to fly south. This migratory generation does not waste energy in raising or laying eggs, saving everything for the long journey.

“For every butterfly that comes to California or Mexico, that is their first trip there,” Pierce marvels.

Previous work had identified a propensity for eastern and western monarchs to have slight differences in the shapes of their wings. For the current article, the researchers wanted to identify any variations in their flight styles.

They collected eastern monarchs from a migratory stopover site in Saint Marks, Florida, and western monarchs from one of their overwintering sites near Oceano, California. Pierce conducted flight tests with the butterflies by tying them to a mill that restricted their flight patterns to circles with a circumference of approximately 25 feet. The tests were performed in a laboratory under controlled light and temperature conditions that mimicked hibernation sites. Artificial flowers were arranged around the circumference of the flying mills.

“The idea was to try to give them an appearance of a ‘natural’ environment to help motivate and guide them,” Pierce explains.

The butterflies were released unharmed from the flight mills after short tests.

The results showed that eastern monarchs would choose to fly longer distances while western monarchs flew shorter distances but with stronger velocity bursts. “The western monarch’s most powerful flight trait is essentially a sprinter,” Pierce says, “while eastern monarchs display a flight trait more akin to marathoners.”

Pierce graduated from Emory and now works as a geneticist for the Environmental Protection Agency in Washington, DC

Talla, who specializes in bioinformatics, grew up in India, where the rich diversity of wildlife inspired him to become an evolutionary biologist. He moved to Sweden to obtain his doctorate, where he studied the genomics of the European White Wood Butterfly. Although all wood targets appear visually identical, they are actually three different species.

“One of the big questions I’m interested in answering is how an individual species ends up becoming multiple species.” Size says. “I want to understand all the processes involved in that evolution.”

He took the opportunity to join De Roode Laboratory. “Monarchs have always been at the top of my list of butterflies that I wanted to study because of their incredible migrations,” says Talla. “They are a fascinating species.”

Last November, he joined De Roode on a laboratory trip to the eastern monarch hibernation site, in and alongside the Monarch Butterfly Biosphere Reserve in central Mexico. Tens to hundreds of millions of monarchs cover the trees and the landscape during winter. “It’s an amazing sight,” says Talla. “It makes you wonder how everyone knows how to get there.”

Previous monitoring and observation studies had shown that at least some western monarchs fly south to Mexico rather than west to California. Full genome analysis suggests that more than a few western monarchs may be making the journey to Mexico, where they mingle with eastern monarchs. And when butterflies leave Mexico, some can fly west instead of east.

“Evidence from multiple directions is coming together to support the same view,” says de Roode.

The findings may aid in the conservation of monarchs. Due to a combination of habitat loss, climate change, and lack of nectare flowers, the number of eastern and western monarchs has declined in recent decades, and westerners show the most precipitous drop. The United States Fish and Wildlife Service is considering whether butterflies need special protections.

“If environmental factors are the only ones driving the differences between eastern and western monarchs, we may be able to help the western population by transplanting some of the eastern ones to the west,” says de Roode.

De Roode’s lab now plans to investigate what exactly in butterflies’ environments triggers different expressions of their genes.