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 .
《The genetics of monarch butterfly migration and warning colouration , Published on Journal《Nature》on 1st October , 2014 .