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. Sequencing the genomes of monarch butterflies from around the world, a team of scientists has now made surprising new insights into the monarch's genetics. They identified a single gene that appears central to migration -- a behavior generally regarded as complex -- and another that controls pigmentation. The researchers also shed light on the evolutionary origins of the monarch. They report their findings Oct. 1 in Nature.

"The results of this study shift our whole thinking about these butterflies," said study senior author Marcus Kronforst, PhD, Neubauer Family Assistant Professor of Ecology and Evolution at the University of Chicago.

Every year, millions of monarchs fly from as far north as Canada to spend the winter in Mexico. Predominantly a North American species, the monarch also exists in South and Central America, and has relatively recently spread across the Atlantic and Pacific oceans. Only North American monarchs migrate.

A North American Original

To better understand monarch genetics and the basis for migratory behavior, Kronforst's team sequenced and compared the genomes of 101 butterflies, including migratory North American monarchs, non-migratory monarchs from around the world and a few closely related species.

The researchers analyzed the monarch's evolutionary origins using genetic comparisons. 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. The authors note that more work needs to be done to fully document the butterfly's evolutionary history.

"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," said study author Shuai Zhan, PhD, a professor in Shanghai Institutes for Biological Sciences and a former joint postdoctoral fellow at the University of Chicago and the University of Massachusetts Medical School.