Abstract
The radical pair model proposes that the avian magnetic compass is based on radical pair processes in the eye, with cryptochrome, a flavoprotein, suggested as receptor molecule. Cryptochrome 1a (Cry1a) is localized at the discs of the outer segments of the UV/violet cones of European robins and chickens. Here, we show the activation characteristics of a bird cryptochrome in vivo under natural conditions. We exposed chickens for 30 min to different light regimes and analysed the amount of Cry1a labelled with an antiserum against an epitope at the C-terminus of this protein. The staining after exposure to sunlight and to darkness indicated that the antiserum labels only an illuminated, activated form of Cry1a. Exposure to narrow-bandwidth lights of various wavelengths revealed activated Cry1a at UV, blue and turquoise light. With green and yellow, the amount of activated Cry1a was reduced, and with red, as in the dark, no activated Cry1a was labelled. Activated Cry1a is thus found at all those wavelengths at which birds can orient using their magnetic inclination compass, supporting the role of Cry1a as receptor molecule. The observation that activated Cry1a and well-oriented behaviour occur at 565 nm green light, a wavelength not absorbed by the fully oxidized form of cryptochrome, suggests that a state other than the previously suggested Trp•/FAD• radical pair formed during photoreduction is crucial for detecting magnetic directions.
Highlights
Birds use the geomagnetic field for directional orientation
The observation that activated Cryptochrome 1a (Cry1a) and well-oriented behaviour occur at 565 nm green light, a wavelength not absorbed by the fully oxidized form of cryptochrome, suggests that a state other than the previously suggested Trp†/FAD† radical pair formed during photoreduction is crucial for detecting magnetic directions
The avian magnetic compass was first described for European robins, Erithacus rubecula, passerine migrants [1], and has since been demonstrated in numerous other bird species, including non-migrants like domestic chickens, Gallus gallus [2]
Summary
The avian magnetic compass was first described for European robins, Erithacus rubecula, passerine migrants [1], and has since been demonstrated in numerous other bird species, including non-migrants like domestic chickens, Gallus gallus [2] (for review, see [3]). This compass appears to be based on a radical pair mechanism [4,5]: absorption of a photon leads by electron transfer to the formation of a pair of radicals which occur in singlet and triplet states. We exposed chickens to different lighting protocols and used the antiserum to look for differences in the amount of immunolabelled Cry1a in the retina
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