Researchers have uncovered key genetic mechanisms underlying one of the most impressive feats of animal migration on Earth: the autumnal voyage of monarch butterflies from eastern North America to distant Mexican fir forests.

In this week's issue of PLoS Biology, neurobiologist Steven Reppert and colleagues from the University of Massachusetts Medical School and the Czech Academy of Sciences identify the function of regulatory genes essential to the butterflies' ability to navigate thousands of kilometers to their Mexican wintering sites. The genes code for molecular components of the insects' circadian clock, which allows them plot accurate migratory courses by synchronizing alterations in their flight paths with passing time as the sun streaks across the sky.

"What we wanted to do was to understand, at a fundamental level, how this clock works," Reppert told The Scientist.

The researchers found that monarchs have both the Drosophila and the vertebrate versions of so-called cryptochrome genes, which are known to play important roles in the circadian clocks of animals. It was previously thought that insects have one type of cryptochrome genes (cry1) while vertebrates have another (cry2). "It was a very surprising finding," said Reppert. "We didn't know that this second family of cryptochromes existed in insects."

Reppert suggested that monarchs have retained both cry genes while Drosophila and mice have lost one or the other family. Reppert said that his findings have "helped us in understanding the evolution of clocks."

Adriana Briscoe, a University of California, Irvine biologist who studies the evolution of color vision in butterflies, agreed. "[These findings] bring tremendous clarity to the field because people were comparing apples to oranges when they were comparing mammalian cryptochromes and Drosophila cryptochromes," said Briscoe, who was not involved with the study. In this study Reppert and his colleagues showed, for the first time, that cry1 codes for photoreceptor proteins that sense blue light as cry proteins do in Drosophila, while cry2 codes for proteins that repress functional transcription in the circadian clock mechanism as cry proteins do in mammals.

In previous work, Reppert and his team showed that pigment-producing genes in the monarch eye communicate with the butterfly's circadian clock. As part of the new study, Reppert and his team also found, in an area of the monarch brain called the central complex, a definitive molecular and cellular link between the circadian clock and the monarch's ability to navigate using the sun. Briscoe said that Reppert's study was "really going to overturn a lot of views we had about the specific components of circadian clocks."