Movementin the Earth's iron core produces a magnetic field around the planet. Humanscan use the magnetized needle of a compass to navigate. But some animals canperceive this magnetic field directly to orient themselves to theirsurroundings.
Theexact mechanism underlying this remarkable magneto-sensitivity has remained amystery.
Toinvestigate this phenomenon, scientists needed a test species. "One of thebest animals to use is the fruit fly, because you can manipulate geneticallyvarious aspects of the fruit fly's biology," says , a neurobiologist at the University of Massachusetts Medical School.
Toconfirm that fruit flies were able to sense a magnetic field, the researchersdeveloped an elegant experimental apparatus: a T-shaped maze. Picture theletter "T" made out of hollow tubes, with a metal coil at the end ofeach horizontal arm. By running an electric current through the coils, thescientists could generate a magnetic field in either arm of the "T".
Torun the tests, the researchers put fruit flies into the maze, starting at thebottom. When the flies reached the top of the T, Reppert says, "They couldeither go to the left or to the right." One way took them toward themagnetic field, the other, away from it.
Nextthe scientists trained the flies by putting some sugar at the end of themagnetized arm of the T-maze. Then they took the sugar away, and tested whetherthe flies would still go towards the magnetic field, no matter which arm it wasin, expecting a sweet reward.
"Whatwe wanted to do was to see whether the flies would associate the food rewardwith the magnetic field," explains Reppert. "And in fact the fliesdid, and it gave us a much more prominent response of the flies to the magneticfield."
Nowthat Reppert and his colleagues were sure the flies could detect a magneticfield, they wanted to figure out how they were doing it.Previous research had suggested that animalsmay use special light receptors, called cryptochromes. "Cryptochromes areproteins that function as blue light photoreceptors in the fruit fly."
Inordinary light conditions, the cryptochromes in a fruit fly's eye and brain areexposed to the full spectrum of light, from blue to green to red. The blue partof the light spectrum activates the cryptochromes, causing them to undergospecific chemical responses that the fruit flies need for their biologicalclock to function.
Totest whether the cryptochromes also play a role in the fruit fly's ability tosense magnetic fields, the researchers put a light filter over the T-maze."So the flies still could see red light and green light," Reppertdescribes, "but the blue and ultraviolet shorter wavelengths were blocked.And in that instance, the magneto-receptive response was totally gone."
Reppertand his colleagues had shown that the flies needed blue light to detect amagnetic field. But the researchers still had to make a definitive link betweenmagneto-sensitivity and cryptochromes.
To dothis they used fruit flies with genetic mutations that effectively disabled thecryptochrome gene. "And then we could ask the question," saysReppert, "since the gene is no longer working, what happens to themagneto-sensitive response? The prediction was it would go away, and indeed itdid."
Migratorybirds, sea turtles, and other animals that use the earth's magnetic field tonavigate also have cryptochromes, albeit different ones from those found infruit flies.
Reppertsays that the next challenge will be to investigate whether the cryptochromesof other species play a similar role to those of the fruit fly in sensingmagnetic fields. "We have the possibility in the fly to actually take anyanimal's cryptochrome, to put it in as a trans-gene, and to ask the question inthe fly, can it function as a magneto-sensitive molecule, and we're veryexcited about that potential."
Reppert's current findings were published in the journal .