By Carol Clark
A major study that sequenced the genomes of monarch butterflies from around the world identified a gene related to muscle function that appears central to the monarch’s spectacular annual mass migration across North America.
The findings, published in Nature October 1, also traced the evolutionary origins of monarchs to North America, instead of South America as was previously hypothesized, and identified a gene related to the butterfly’s distinctive orange-and-black coloration.
“Our findings add several interesting twists to our understanding of these iconic insects,” says Jaap de Roode, an evolutionary ecologist at Emory and a co-author of the paper.
Evolutionary geneticist Marcus Kronforst from the University of Chicago led the sequencing of the genomes of 101 monarchs, which involved scientists from five other universities, in addition to Emory. De Roode, who runs one of a handful of labs in the world focused on monarch butterflies, assisted in the design of the project and the analysis of the results.
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.”
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.”
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.
In 2010, de Roode’s lab discovered that monarchs use a toxic chemical in certain species of milkweed to rid themselves of harmful parasites, providing some of the best evidence that animals use medication.
“Monarchs are a fascinating system to study,” he says.
“There is so much that we don’t yet know about these insects, and we may never know because they are now in decline.”
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: 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,” de Roode says. “It’s an amazing natural phenomenon in danger of disappearing.”
De Roode and his students helped create the Jimmy and Rosalynn Carter Pollinator Garden at The Carter Center, which opened October 1 in honor of President Carter’s 90th birthday. The garden is filled with milkweed and flowers and plants native to Georgia and is part of the Rosalynn Carter Butterfly Trail, developed by the former first lady to draw attention to the plight of the monarchs.
Credits: Top and bottom photos via ThinkStock; center photo by Jaap de Roode.
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