Monarchs cluster together for warmth in a Mexican overwintering site. (Credit: Jaap de Roode)
Monarchs cluster together for warmth in a Mexican overwintering site. (Credit: Jaap de Roode)

The Monarch butterfly is best-known for its annual 5,000-kilometer migration each autumn from North America to Mexico, where they blanket forests with undulating blankets of orange and black wings. But the North American monarch is in trouble.  Its numbers have dramatically dropped because of loss of habitat and the decline of its primary food source, milkweed.  

Researchers say understanding monarch migration could help promote conservation efforts, and a new study in the journal Nature describes a single gene that appears to be responsible for the migrating behavior - and the lack of it.  
Some Monarch species -- those found in South and Central America, in the Pacific and in Europe and North Africa -- don’t migrate.   

Comparing species

The study compares genomes of 100 of those monarch species.  What researchers had expected -- that the butterfly originated as a non-migratory species in tropical South America, came north and then evolved migration -- was not what they saw in the genetic history, says co-author Marcus Kronforst, assistant professor in the Department of Ecology and Evolution at the University of Chicago.  

“We found that the North American populations appear to be the ancestral populations, and that the butterfly was probably ancestrally migratory.  And then it dispersed out of North America into South and Central America, and lost migration.  And then it also independently dispersed across the Pacific and lost migration, and then independently, a third time, dispersed across the Atlantic and lost migration.”

Early evolution

A monarch family tree created from the genomic analysis tells the age of the different populations and how they were related to each other over evolutionary time, with the older populations - the migrating monarchs - at the base of the tree.  

As the researchers looked for genetic change across species to locate where and when migration stopped, Kronforst says one gene stood out.

“Basically in this one gene, a collagen gene, all three times that the butterflies left North America and lost migration, they changed at this one gene in exactly the same way,” he said.

Kronforst says that same gene is related to the monarch flight muscles.

“It looks like what we are seeing is that the North American migratory butterflies are just very efficient in how they fly," he said. "They have enhanced flight muscle efficiency, whereas the non-migratory butterflies actually appear to be pushed in the opposite direction by natural selection.  They appear to be flying fast and hard.”

Color gene identified

In scanning the genomic data, the researchers also identified a single gene associated with the butterfly’s signature orange color. Kronforst expects future studies will yield more discoveries about the iconic monarch, whose migration flight has filled North American skies for millions of years.