UMMS Researcher Sheds New Light on Monarch Butterfly Migration

Landmark study illuminates the role of the butterfly’s circadian clock

May 23, 2003

WORCESTER, Mass.—In the fall, monarch butterflies travel up to 3,000 miles from  North America to a small region in central Mexico to spend the winter. Because of their brief lifespan, only one of every fourth or fifth generation makes the trip, indicating that the urge to migrate is instinctive for the monarchs, not learned.

While scientists believe that monarchs use the sun to navigate, they had known far less about how the butterflies adjust their direction each day as the sun’s position in the sky changes.  In a study published May 23, 2003 in the journal Science, a team at University of Massachusetts Medical School led by Steven M. Reppert, MD, the Higgins Family Professor of Neuroscience and professor and chair of neurobiology, has helped answer that question. “Monarchs have a genetic program to undergo this marvelous long-term flight in the fall,” Dr. Reppert said. “Now we have shown the requirement of the circadian clock for monarch butterfly migration.”

Reppert and his colleagues studied the effects of manipulating the daily light and dark cycles on monarchs subsequently studied inside a specially designed flight simulator. After being housed under a light/dark cycle in the laboratory that was close to the fall outdoor lighting cycle (light from 7:00 a.m. to 7:00 p.m.) migrant butterflies exposed to outdoor sun oriented to the southwest, toward Mexico. Butterflies housed under an earlier cycle (light from 1:00 a.m. to 1:00 p.m.) flew to the southeast.

When the butterflies were exposed to constant  light, they flew directly toward the sun, presumably having lost their sense of time.  

By charting the flight data of the monarchs in the study, Reppert’s team gathered what they believe is direct evidence of the essential role of the insects’ internal circadian clock in celestial navigation. “When the clock is disrupted, monarchs are unable to orient toward Mexico. Without proper navigation, their migration to the south wouldn’t occur, and that generation of butterflies would not survive,” Reppert said.

Reppert’s team also found that while ultra-violet light is important for sun compass navigation, some other wavelength of light was required for entraining (or calibrating) the butterflies’ internal circadian clocks. This difference may provide a means for untangling the two biological processes. “The light input pathways are quite distinct, so tracking those pathways in may eventually lead us to the cellular level where this clock-compass interaction is occurring,” Reppert said.

Research in other animals has identified a number of genes that make up the circadian clock, as their expression oscillates in a daily cycle. The clock is entrained to the daily light cycle via specialized by special light-sensitive cells, called photoreceptors. Reppert’s team found that a common clock gene, known as “period,” is also part of the monarch circadian clock. Constant light disrupted the cycling of this gene’s expression. It also affected the time of day butterflies emerged from their chrysalises, known to be a marker of circadian time-keeping in other insects.

Understanding how the circadian clock assists the sun compass in the relatively simple navigation by monarchs could provide a model for studying navigation by other animals, Reppert said, citing both foragers such as honeybees and desert ants, as well as long distance migrators such as songbirds.  “Increasing knowledge of the genetic makeup of the monarch circadian clock will help tease apart the entire migratory process, a process that remains one of the great mysteries of biology,” Reppert said.

Dr. Reppert is recognized nationally as a leading expert on circadian rhythms. His lab undertakes basic scientific research aimed at understanding the cellular and molecular mechanisms of circadian clocks, which in turn holds great promise for developing innovative treatment strategies for a wide range of clinical disorders.

The University of Massachusetts Medical School is one of the fastest growing academic health centers in the country and has built a reputation as a world-class research institution, consistently producing noteworthy advances in clinical and basic research.  The Medical School attracts more than $143 million in research funding annually, 80 percent of which comes from federal funding sources.  Research funding enables UMMS scientists to explore human disease from the molecular level to large-scale clinical trials.  Basic and clinical research leads to new approaches for diagnosis, treatment and prevention of disease. Visit www.umassmed.edu for additional information.

Contact: Michael Cohen, 508-856-2000