New research traces the evolutionary origins of monarchs to North America, instead of South America as was previously hypothesized, and identifies a gene related to the butterfly’s distinctive orange-and-black appearance.
In Nature, the team of scientists reports sequencing the genomes of 101 monarch butterflies from around the world. “Our findings add several interesting twists to our understanding of these iconic insects,” says Jaap de Roode, an evolutionary ecologist at Emory University and a coauthor of the paper.
Although the monarch isn’t in danger of extinction, its famous mass migration across North America appears to be in peril.
Monarch butterflies populations have plummeted from an estimated 180 million-900 million in 1996-1997 to an estimated 6.7 million-33 million this past year. “The whole migrating population could be gone over the next decade,” says de Roode. “It’s an amazing natural phenomenon in danger of disappearing.”
Monarch larvae feed on milkweed, which is becoming rarer due to pesticide use and development of land. As a result, the number of monarchs making the annual migration from Canada and the United States to Mexico has dropped dramatically.
An ‘upside down’ migration theory
The researchers traced the ancestral lineage of monarchs to a migratory population that likely originated in the southern United States or Mexico.
The evolutionary tree created from the sequencing showed that the monarch’s current worldwide distribution appears to stem from three separate dispersal events—to Central and South America; across the Atlantic; and across the Pacific. In all three cases, the butterfly independently lost its migratory behavior.
The monarch’s North American origin runs counter to a long-standing hypothesis that the butterfly originated from a non-migratory tropical species.
“Previously, it was widely thought that after spreading from the tropics through North America, the evolution of migration enabled monarchs to fly south and survive the winter,” de Roode says. “It turns out that we had that upside down.”
‘Efficient’ monarch butterflies
To better understand the genetic basis for the butterfly’s migratory behavior, the researchers compared the genomes of migratory and non-migratory monarch populations from around the world.
A disparity between the two groups among genes related to muscle function stood out, including one in particular: collagen IV alpha-1. The migratory butterflies expressed greatly reduced levels of this gene, which is involved in muscle function.
Humans have a similar gene that is associated with a muscle disease known as myopathy, de Roode notes.
“The data clearly show that muscles are the main thing that enables monarchs to migrate over thousands of miles,” he says. “We also found that the migrating monarchs have much more efficient metabolisms.”
The team discovered that migratory monarchs consumed less oxygen and had significantly lowered flight metabolic rates, which likely increases their ability to fly long distances compared to non-migratory butterflies.
“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,” says study leader Marcus Kronforst, assistant professor of ecology and evolution at the University of Chicago.
“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.”
Mutant white monarchs
By comparing the genome of mutant white monarchs, found in Hawaii, with other monarchs the researchers identified a gene clearly correlated with the butterfly’s beautiful orange color.
A similar gene found in mice, myosin 5a, is associated with a dilute phenotype: Instead of the black fur, mice with a mutation in this gene are light brown or beige.
“Myosin 5a is a gene that regulates transport of pigment within a cell, moving color to a hair shaft in the case of mice, or apparently to scales in the case of monarchs,” de Roode says. The gene has never before been implicated in insect coloration, he adds.
Researchers from Stanford University and the University of Queensland also contributed to the study.