The most detailed look yet at the monarch butterfly’s built-in clock suggests it’s an ancient model.
The molecular mechanism that keeps the day-night rhythm in the butterfly’s brain shares features with the fruit fly and mouse clocks, says Steven Reppert of the University of Massachusetts Medical School in Worcester. These three well-studied clocks depend on similar genes, which orchestrate a feedback loop: A substance builds up to a concentration that shuts down its own production. Once the concentration drops, the loop begins again.
The monarch’s clock, though, includes an unusual pair of cryptochrome proteins that help regulate this loop. One cryptochrome resembles the fruit fly’s. It changes behavior when bathed in blue light and keeps the clock in sync with passing days. The other butterfly cryptochrome works like the mouse’s, with no light sensitivity but a strong braking effect on the clock gears. Ancestral clocks probably had both kinds, says Reppert. Fruit fly ancestors lost one and mouse ancestors the other.
In studying the cryptochromes, Reppert’s team detected nightly rises of the mouselike one in the butterfly’s neural connection between the brain’s clock and the navigation center. Butterflies orient by the sun during their migrations of hundreds of miles, so Reppert says the brain must have a way to keep a bearing as the sun moves throughout the day. That second cryptochrome, then, might be the signal that connects the clock and the compass, he and his colleagues note in the January PLoS Biology.