Scientists Unlock Secret to Monarch Migration

Monarch butterfly

Photo: Malgorzata Milaszewska / Wikimedia Commons

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The circadian clock of a monarch butterfly is closer in complexity to a human's than a fruitfly's, suggesting that it is the key to the insect's extraordinary migration.

The monarch butterfly has one of the most extraordinary migrations on earth. Starting in Mexico, where adult butterflies have spent the winter, they fly north and fan out across North America. There, they die, but not before mating and laying eggs. Several generations can live and die during the summer before one turns south again in the late summer and early fall for the long migration back to Mexico, where they depend on a 70-square-mile pine grove to survive.

University of Massachusetts Medical School Professor and Chair of Neurobiology Steven M. Reppert, MD, had previously "demonstrated that the butterflies use a time-compensated sun compass and daylight cues to help them navigate to the pine groves," according to information provided by the open-access journal, the Public Library of Science. "His studies have shown that time compensation is provided by the butterfly’s circadian clock, which allows the monarch to continually correct its flight direction to maintain a fixed flight bearing even as the sun moves across the sky."

Now, in new research published by the journal, he and colleagues describe the surprising complexity of the gene that governs that circadian clock, a mechanism never before described in any insect or mammal.

Biologically, circadian clocks are loops of light-sensitive proteins, descended evolutionarily from plants, that are produced and destroyed over the course of a 24-hour cycle. Insect and mammalian clocks differ, in part, by how CRY, a cryptochrome protein, functions: In flies, it allows light to trigger the process, whereas in mice it acts to enable the resetting of the loop without any trigger from light. The monarch has both the insect-like CRY and another that is more similar to those found in mammals, and it may provide the communication between the light-sensitive trigger and the clock itself, the scientists found.

In short, they identified the biological mechanism that allows butterflies to navigate the flight to and from Mexico, even as the sun moves across the sky.

“This is a very interesting realignment of how one thinks about insect clock models. There was no reason to suspect that the butterfly clock would be different from that of Drosophila (fruit fly). That it is different has already told us something about how circadian clocks have evolved,” Reppert said in a prepared statement. “What we have in the butterfly is an astounding clock mechanism, one that is more similar to our own circadian clock and less similar to the clock of the fly! The presence and function of two distinct CRYs suggest that the monarch’s is an ancestral clock; a clock that, over the course of evolution, has changed differently in other insects and mammals.”

The next project: Sequence the entire monarch butterfly genome.

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