Monarch butterflies owe their long-distance migrations to one gene honed for efficient flight, according to a study released Wednesday. (Related: "Monarch Butterflies Struggle.")

The study of monarch genes also suggests that the butterflies began their evolutionary history as a migratory species that spread worldwide before a few groups settled down and eventually became separate homebody species.

In an epic migration, the monarch butterfly travels in great masses from Mexico to Manitoba every year. The range of that journey has stretched farther and farther north since the end of the last ice age, thanks to a gene that makes butterfly muscles more efficient, researchers suggest in a study published in the journal Nature.

"At first we thought migratory butterflies needed to bulk up with big muscles," says study senior author Marcus Kronforst of the University of Chicago. "What emerged is that natural selection is mighty powerful for flight efficiency." (Video: "Monarch Butterflies.")

The study also revealed that a single gene plays a big role in monarchs' signature orange-and-black coloration, and a flip of this genetic switch is responsible for the unusual white monarch butterflies of Oahu.

In the backdrop to the study, the iconic butterfly's numbers have been dropping precipitously, down to 33 million in 2013, a decline tied to a drop in the milkweed they depend on. (Related: "Monarch Butterfly's Reign Threatened by Milkweed Decline.")

Habitat loss and the destruction of native plants have been responsible for the rapid decline of the monarch butterfly, the most recognized butterfly in North America. To help protect these majestic insects as they migrate, citizens in the U.S. are resorting to a simple yet powerful tool: gardening.

Monarch Muscles

Millions of migratory monarchs travel north from Mexico in March, then head south in October, with four generations of the insects living and dying along the way. The insects have to make decisions about starting and stopping migration, so the study team expected genes related to behavior that would separate migratory monarchs from others.

Kronforst and colleagues compared the full genetic blueprints, or genomes, of 89 monarch butterflies, Danaus plexippus, and nine others from four nonmigratory families related to them, most notably South America's southern monarch butterfly.

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Monarch Butterfly's Genes Reveal the Key to Its Long-Distance Migratio

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