Although the monarch butterfly (Danaus plexippus) can be found in many parts of the world, only the North American species makes the remarkable annual en masse migration from as far as Canada to spend winter in Mexico.

What prompts the Monarch butterfly to undertake the arduous 5,900 mile round-trip journey in swarms of millions lies on its genetics, findings of a new study suggest.

Marcus Kronforst, from the University of Chicago, and colleagues sequenced the genome of 101 butterflies from different parts of the globe including migratory and non-migratory monarchs as well as butterfly species that are closely related. They also compared the genetic blueprint of the monarchs that migrate and those that do not and discovered that one gene stood out from hundreds of other genes associated with the butterfly's muscle, development and neural functions, the collagen IV alpha-1.

Not all Monarch butterflies are built to migrate and travel long distances, the researchers found, and it turned out that this ability boils down to the collagen IV alpha-1, which plays an essential role in flight muscle function and formation. The gene helps form collagen, a protein found in the connective tissues of animals, and reduces the energy expenditures in the muscles of the butterflies that travel long distances.

"Migration is regarded as a complex behavior, but every time that the butterflies have lost migration, they change in exactly the same way, in this one gene involved in flight muscle efficiency," Kronforst explained. "In populations that have lost migration, efficiency goes down, suggesting there is a benefit to flying fast and hard when they don't need to migrate."

Kronforst and colleagues likewise found that the migratory monarchs use less oxygen and have lower flight metabolic rates than those that do not migrate and this likely boost their capability for long distance flights.

"We find the strongest signatures of selection associated with migration centre on flight muscle function, resulting in greater flight efficiency among migratory monarchs," the researchers wrote in their study, which was published in the journal Nature on Oct. 1.

Although monarch butterflies, which have wingspans that measure about 4 inches and are distinctly known for their iconic black and orange wings, are not in danger of extinction, there are concerns that their phenomenal migration is at risk. Environmentalists said that severe weather poses problems to the migratory butterflies for the last two years and that deforestation is destroying their overwintering habitat.

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