Each fall, millions of monarch butterflies wing it from their birthplaces in eastern North America to a small winter retreat in the mountains of central Mexico. Scientists have long puzzled at how the insects navigate the unfamiliar journey, which can span up to 3000 kilometers. Now researchers have exposed the cogs and gears that make a monarch's biological clock tick, timekeeping that they say is necessary for successful southward navigation.
Scientists believe monarchs follow the sun like a compass. But this is easier said than done. Because the sun appears to rise and fall each day, the insects must constantly adjust to its movements, lest they quickly lose their way. In 2003, Steven Reppert, a neurobiologist at the University of Massachusetts Medical School in Worcester, showed that butterflies use an internal timepiece to help them fly in the right direction (ScienceNOW, 22 May 2003).
In a pair of new studies, Reppert and his colleagues suggest that the connection between the clock and the compass relies on light-absorbing proteins called cryptochromes. First discovered in plants, cryptochromes help synchronize the internal clock--or circadian rhythm--in flies and mice (Science, 27 November 1998, p. 1628).
After assembling a database of expressed sequence tags--short DNA sequences that can be used to identify a gene expressed in a cell at a given time--Reppert and his colleagues located two families of cryptochromes in the monarch. That's unusual, Reppert notes, because other creatures studied have only one type of cryptochrome. Further molecular analysis revealed that monarchs have both a flylike cryptochrome (CRY1), which absorbs light to set the internal clock, and a mouselike cryptochrome (CRY2), which keeps the clock running. "This changes how we think molecular clocks can be put together," Reppert says. What's more, he notes, the monarch's clock may represent a timepiece that is ancestral to both flies and mice, which would imply that these creatures each lost one of the cryptochrome genes. If that's the case, studying monarch clock mechanics could provide important clues to how such timepieces evolved.
Reppert and colleagues also found that CRY2 levels oscillate daily in the part of the monarch brain responsible for following the sun like a compass. The protein might help the clock talk to the compass and therefore give the butterflies their bearings during the long journey. The team presents the database findings tomorrow in PLoS ONE and describes the role of cryptochromes in the butterfly clock today in PLoS Biology.
Gene Robinson, a neuroscientist at the University of Illinois, Urbana-Champaign, says the study provides a basis for the proposal that cryptochrome connects the circadian clock to the sun compass. Still, Orley “Chip” Taylor Jr., an insect ecologist at the University of Kansas, Lawrence, says the work does little to explain navigation. To make it to Mexico, monarchs coming from New York, Texas, or Iowa, for example, would have to follow different bearings and begin their journeys at different times. This is hard to reconcile with a specieswide clock, he notes. "We are a long way from understanding that," Taylor says.