By analyzing the genome sequences of 101 monarch butterflies (Danaus plexippus) from around the world, a team of genetic researchers headed by Dr Marcus Kronforst of the University of Chicago has identified a single gene that controls their striking coloration and another that appears central to long-distance migration of North American populations.
Monarch butterflies cluster in Santa Cruz, California. Image credit: Brocken Inaglory / CC BY-SA 3.0.
The monarch butterfly is one of the most iconic insects in the world, best known for its distinct orange and black wings and a spectacular annual mass migration across North America.
However, little has been known about the genes that underlie these famous traits, even as the insect’s storied migration appears to be in peril.
To characterize these genes and uncover the history of the monarch butterfly’s evolutionary origin and global dispersal, Dr Kronforst and his colleagues sequenced genomes of 101 monarchs from around the globe, including migratory North American monarchs, non-migratory monarchs from around the world and a few closely related species.
They traced the ancestral lineage of monarchs to a migratory population that likely originated in the southern U.S. or Mexico.
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, which later developed the ability to migrate.
While historical records have suggested that the monarch’s dispersal across the Pacific and Atlantic occurred in the 1800s, the analysis indicated the monarch actually crossed the oceans thousands of years ago.
Prof Shuai Zhan of Shanghai Institutes for Biological Sciences, who is the first author on the study published in the journal Nature, said: “in order to clearly resolve the history of monarch butterflies, we still need additional fossil, archaeological and genetic data, as well as more advanced technology, becoming available in the future.”
To study the genetic basis for migration, the team compared the genomes of migratory butterflies against the three non-migratory populations.
They identified more than 500 genes – most of which are involved in muscle, developmental and neural function – that differed to some degree. But a single gene disparity stood out.
Migratory butterflies expressed greatly reduced levels of collagen IV α-1, a gene involved in flight muscle formation and function.
The scientists 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.
They also discovered a single gene appears to function as a pigmentation switch. This gene, which codes for a protein of the myosin motor protein family, has never before been implicated in insect coloration. Its mutation likely disrupts the pigment transport to the wings, making the monarchs white. A related gene in mice, myosin 5a, has been shown to affect coat color in a similar way. The gene represents a new genetic pathway to explore insect coloration.
_____
Shuai Zhan et al. The genetics of monarch butterfly migration and warning colouration. Nature, published online October 01, 2014; doi: 10.1038/nature13812
