Which way? Researchers tested magnetosensitivity in flies by setting up a magnetic field on one side of a chamber and observing which side the flies preferred.

Robert Gegear

Guided by the Light

Who needs a GPS when you have, well, whatever migratory birds and sea turtles have? For centuries, humans have marveled at the ability of these and other animals to navigate the globe, in some cases returning to the same breeding spot year after year from as far as 15,000 kilometers away. How they find their way remains a mystery, but new research suggests their prowess may depend on the ability to "see" Earth's magnetic field.

Researchers agree that however animals navigate, they use Earth's magnetic field as a guide. Theories about how animals detect these fields, a property called magnetosensitivity, generally fall into two camps. One group argues that tiny crystals of a magnetic mineral known as magnetite, which is found in the brains of some birds and in bacteria, are key. Other scientists say that animals carry photoreceptor molecules that enable them to actually "see" magnetic fields. How these molecules work is not clear, but some researchers think light might destabilize electrons in the photoreceptors, making them susceptible to Earth's magnetic pull.

Previous work has shown that animals must be able to respond to blue light to detect magnetic fields, so researchers have eyed cryptochrome, a protein that allows plants and animals to sense blue light, as a likely candidate for the magnetosensitivity photoreceptor. But it's hard to isolate the effects of a single protein in a complex organism. So a team of researchers from the University of Massachusetts Medical School in Worcester started small, forgoing migratory birds and other large animals for the humble fruit fly.

Flies normally have cryptochrome receptors, called Cry receptors. The researchers selected flies with the receptors and some bred to lack the proteins. The team then put each group in a T-shaped maze that had a magnetic field in one side of the T and no magnetic field in the other. The researchers then watched to see which side the flies gravitated toward under different light conditions.

The flies with Cry receptors flew toward the magnetic field when the maze was flooded with regular light. They showed a slightly weaker preference for the magnetic field when the light was filtered to allow in only some of the ultraviolet and blue light of the spectrum, and they had no preference at all when ultraviolet and blue light were absent. In contrast, flies without Cry receptors did not show a preference for the magnetic field under any light condition, providing some of the first experimental evidence that cryptochrome plays an important role in sensing magnetic fields, the researchers report online this week in Nature.

This result doesn't prove that cryptochrome is the actual receptor, but it's clearly essential for magnetosensitivity in the flies, says neuroscientist and co-author Steven Reppert. "This gets us to the point where we can ask more questions."

Kenneth Lohmann, a neurobiologist at the University of North Carolina, Chapel Hill, says the findings could lead to advances in his own work on the magnetosensitivity of sea turtles, which travel long distances to their home beaches to lay eggs. Although cryptochromes might not play exactly the same role in larger animals, the study bolsters the idea that animals use light to steer, he says.