Animals as diverse as birds, sea turtles, bats, and bees can sense the earth's magnetic field and use it as a guide in navigation. A new study identifies the molecule that may hold the key to this special ability. Véronique LaCapra brings us this story.

Movement in the Earth's iron core produces a magnetic field around the planet. Humans can use the magnetized needle of a compass to navigate. But some animals can perceive this magnetic field directly to orient themselves to their surroundings.

The exact mechanism underlying this remarkable magneto-sensitivity has remained a mystery.

To investigate this phenomenon, scientists needed a test species. "One of the best animals to use is the fruit fly, because you can manipulate genetically various aspects of the fruit fly's biology," says Dr. Steven Reppert, a neurobiologist at the University of Massachusetts Medical School.

To confirm that fruit flies were able to sense a magnetic field, the researchers developed an elegant experimental apparatus: a T-shaped maze. Picture the letter "T" made out of hollow tubes, with a metal coil at the end of each horizontal arm. By running an electric current through the coils, the scientists could generate a magnetic field in either arm of the "T".

To run the tests, the researchers put fruit flies into the maze, starting at the bottom. When the flies reached the top of the T, Reppert says, "They could either go to the left or to the right." One way took them toward the magnetic field, the other, away from it. 

Next the scientists trained the flies by putting some sugar at the end of the magnetized arm of the T-maze. Then they took the sugar away, and tested whether the flies would still go towards the magnetic field, no matter which arm it was in, expecting a sweet reward.

"What we wanted to do was to see whether the flies would associate the food reward with the magnetic field," explains Reppert. "And in fact the flies did, and it gave us a much more prominent response of the flies to the magnetic field."

Now that Reppert and his colleagues were sure the flies could detect a magnetic field, they wanted to figure out how they were doing it.  Previous research had suggested that animals may use special light receptors, called cryptochromes. "Cryptochromes are proteins that function as blue light photoreceptors in the fruit fly."

In ordinary light conditions, the cryptochromes in a fruit fly's eye and brain are exposed to the full spectrum of light, from blue to green to red. The blue part of the light spectrum activates the cryptochromes, causing them to undergo specific chemical responses that the fruit flies need for their biological clock to function.

To test whether the cryptochromes also play a role in the fruit fly's ability to sense magnetic fields, the researchers put a light filter over the T-maze. "So the flies still could see red light and green light," Reppert describes, "but the blue and ultraviolet shorter wavelengths were blocked. And in that instance, the magneto-receptive response was totally gone."

Reppert and his colleagues had shown that the flies needed blue light to detect a magnetic field. But the researchers still had to make a definitive link between magneto-sensitivity and cryptochromes.

To do this they used fruit flies with genetic mutations that effectively disabled the cryptochrome gene. "And then we could ask the question," says Reppert, "since the gene is no longer working, what happens to the magneto-sensitive response? The prediction was it would go away, and indeed it did."

Migratory birds, sea turtles, and other animals that use the earth's magnetic field to navigate also have cryptochromes, albeit different ones from those found in fruit flies.

Reppert says that the next challenge will be to investigate whether the cryptochromes of other species play a similar role to those of the fruit fly in sensing magnetic fields. "We have the possibility in the fly to actually take any animal's cryptochrome, to put it in as a trans-gene, and to ask the question in the fly, can it function as a magneto-sensitive molecule, and we're very excited about that potential."

Reppert's current findings were published in the journal Nature.