Each year, millions of monarch butterflies migrate to eastern North America, from where the US-Canadians fly over central Mexico to the overwinter – a distance of 3,000 miles. Meanwhile, on the other side of the Rocky Mountains, Western emperors typically travel 300 miles down to the Pacific coast to spend winter time in California. It was long believed that the eastern and western emperors were genetically distinct populations.
A new study, however, confirms that while eastern and western butterflies fly differently, they are genetically identical. The journal Molecular ecology Published findings under the leadership of evolutionary biologists at Emory University.
“It was surprising,” says Zap de Rood, Emory professor of biology and senior author of the study. His laboratory is one of the handful of people in the world who study monarch butterflies.
“You would expect organisms with different behaviors and ecology to show some genetic differences,” de Roode says. “But we found that you cannot genetically differentiate between western and eastern butterflies.”
The current paper builds on previous work by the De Rude Lab that found similarities between 11 genetic markers of eastern and western monarchs, as well as other more limited genetic studies and observation and tracking data.
Venkat Talla, the first author of the current study and Emory post-doctoral fellow in the lab, says, “This first is comparatively better than the eastern and western monarchies, so that their behavioral differences are better understood.” . “
Talla analyzed more than 20 million DNA mutations in 43 monarch genomes and found no evidence for genomic differentiation between eastern and western kings. Instead, they found similar levels of genetic diversity.
“Our work suggests that eastern and western emperors are mating together and that genetic material has been exchanged to a much greater extent than before,” Talla says. “And this adds to the evidence that it is the potential differences in their environments that shape the differences in their patterns of migration.”
Co-author Amanda Pearce, who led the first study on 11 genetic markers, initiated the project while she was a graduate student at the De Rude Lab.
“Monarch butterflies are so delicate and so light, and yet they can travel thousands of miles,” says Pierce. “They are beautiful organisms and a great model system for understanding unique, spontaneous behavior. We know that migration is somehow entangled in their genetic strands.”
The emperors fly north and lay eggs after they leave their overwintering sites. The caterpillars turn into butterflies and then move on, mating and laying another generation of eggs. This process is repeated for several generations, as the days get shorter and the temperature cools, the monarchy comes out of its bouquet and starts flying south. This migratory generation does not expend any energy on breeding or laying eggs, it saves everyone for a long journey.
“For every butterfly that makes it to California or Mexico, it’s their first trip there,” Pierce marvels.
Previous work identified a tendency for eastern and western emperors to have slight differences in the size of their wings. For the current paper, researchers wanted to identify any variations in their flight styles.
They collected eastern kings from a migratory halt site in St. Marks, Florida, and the Western Monarchy from one of their overwintering sites near Osho, California. Pierce drove flight trials with butterflies and tied them to a mill, which limited their flight pattern to circles with a circumference of about 25 feet. The tests were performed in a laboratory under controlled light and temperature conditions that mimic overwintering sites. Artificial flowers were arranged around the perimeter of the flying mills.
“The idea was to try to give them some glimpse of a ‘natural’ environment to inspire and help orient them,” Pierce explains.
The butterflies were freed from flight mills after short voyages performed.
The results showed that the eastern emperor would prefer to fly for longer distances, while the Western monarchy flew shorter distances, but with stronger bursts of speed. Pierce says, “The western emperor has a more powerful flight quality, like a sprinter, essentially,” while the Eastern monarchies show a flight characteristic similar 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 led him to become an evolutionary biologist. He moved to Sweden to get his PhD, where he studied the genomics of the European wood white butterfly. Although all wood whites are equally visible, they are actually three different species.
“One of the big questions I’m interested in answering is how does one individual species become more than one species?” Talla says. “I want to understand all the processes involved in that development.”
He jumped at the chance to join the De Rood Lab. Says Kalla, “The emperors have always topped my butterflies list, because I wanted to study because of their incredible stay.” “They are an attractive species.”
Last November, he joined D on a lab field trip to a monarchy Butterfly Biosphere Reserve in eastern Mexico and to an eastern monarchy site adjacent to it. Millions of kings blanket trees and landscapes during winter. “It’s a mind-blowing sight,” Talla says. “It makes you wonder how they all know how to get there.”
Previous tracking and observational studies had revealed that at least some Western monarchies fly south to Mexico rather than west to California. Full-genome analysis suggests that only a few of the Western emperors are traveling to Mexico, where they mate with the eastern emperors. And when the butterflies depart Mexico, some may fly west instead of east.
“Evidence from multiple directions is coming together to support the same view,” de Roode says.
The findings may help the emperor’s patronage. Due to a combination of habitat loss, climate change and lack of nectar flowers, westerners have declined the most in recent decades. The US Fish and Wildlife Service is currently considering whether butterflies require special protection.
“If environmental factors are all that exacerbate differences between Eastern and Western monarchs, it’s possible that we can help the Western population by transplanting some of the Eastern population into the West,” de Roode says.
The de Rood Lab is now planning to investigate what exactly are the different expressions of their genes in the environment of butterflies.
Eastern monarch butterfly population has come down to extinction
Venkat Talla et al, Genomic evidence for gene flow between monarchs with divergent migratory phenotypes and flight performance Molecular ecology (2020). DOI: 10.1111 / mec.15508
Provided by Emory University
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