The monarch butterfly is beginning to reveal the genetic secrets of its remarkable migration.
Every year, in autumn, monarchs migrate thousands of miles from their summer home in the eastern US to their wintering grounds in central Mexico. We know the migration is genetically programmed, rather than learned, because the southbound migrants are not the same as those that flew north in the spring – they are several generations removed.
The butterflies rely on an internal sun “compass” to guide them on their journey, corrected for the time of day, so they maintain a constant bearing as the sun moves across the sky. Until now, though, no one knew how the circadian clock and the compass are linked in the insects’ brains.
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Steven Reppert, a neurobiologist at the University of Massachusetts Medical School in Worcester, and his colleagues studied the daily cycle of molecular changes in the pars lateralis, the region of the butterfly brain that contains the circadian clock. One of the key elements in the clock, they found, is a protein called cry2, which inhibits its own production in a negative feedback loop, so the concentration rises and falls in a 24-hour cycle.
Levels of cry2 also rise and fall in a daily rhythm in another part of the brain known as the central complex, which houses the monarch’s sun compass (PLoS Biology, ). In all likelihood, this means that cry2 is the molecular link between clock and compass. “That’s exciting, because there has to be a connection between the two,” says Reppert. “This is the first molecular evidence that there is direct crosstalk.”
This link helps the butterflies’ sun compass compensate for the sun’s changing position so that they can fly a straight course, however it still does not explain how they find a wintering ground last seen by their grandparents or great-grandparents. To address this, Reppert’s team has developed a library containing most of the butterfly’s genes (PLoS One, ).
Comparing active genes in migratory versus non-migratory butterflies should help reveal how a monarch manages its epic journey.
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