The Butterfly Clock

Steve Reppert at UMass Medical School in Worcester has just come out today with two very exciting new papers on the circadian clock of the monarch butterfly. They’re both published through PLoS so they’re free for the looking online. Check them out here and here. I wrote my MIT thesis on Reppert’s work so I must admit I have a soft spot for this stuff. Below, I’m bringing you an excerpt from said thesis, discussing the work that’s now culminated in these two papers.

[For quick background: Each fall, billions of monarch butterflies funnel from the Eastern US and Canada into a handful of tiny pine groves in central Mexico. As they’ve never made the trip before and they have no parents to lead the way, they must rely on genetic memory to get where they’re going. The mechanism they use to pull this off is called a time-compensated sun compass. They use the sun as a guidepost, but they must constantly recalibrate their internal compass to compensate for the fact that the sun appears to move across the sky throughout the day. The timepiece they use for this recalibration is the circadian clock. Okay, now for that excerpt.]

Instead of studying in meticulous detail the circadian clocks of every living being, scientists focus on representatives of particular groups.  For example, the mouse circadian clock is often used as a model for how mammalian clocks are built.  Similarly, the fruit fly clock has long been a stand-in for insect clocks in general.  Circadian biologists could safely assume that the monarch clock would resemble that of the fruit fly more than that of the mouse, because the monarch is more closely related to the fruit fly.  The fruit fly is much easier and cheaper to study than the monarch; its long history as a so-called model organism means that there are many well-established tools and procedures for working with it.  So it seemed like a reasonable, and practical, approximation.

In the fruit fly, as in most organisms, the clock resides in individual timekeeping cells.  It works by manufacturing and then destroying certain proteins in a feedback loop that takes about 24 hours to complete.  This feedback loop can sustain itself indefinitely, which is why the clock keeps working even in constant darkness.  When the fly encounters daylight, though, a specialized protein in the timekeeping cell absorbs the light; it tells the clock that the sun is out by feeding into the loop.  This specialized protein is CRY, the fruit fly version of the protein that illuminated the possible clock-compass connection.  CRY is how sunlight sets the fruit fly’s clock.

But Reppert wanted to figure out how the monarch’s time-compensated sun compass works, so he couldn’t rely on the fruit fly model—fruit flies don’t use a sun compass, time-compensated or otherwise.  He decided he needed to take a closer look at the monarch clockwork, to see how the butterfly clock works.