Abstract
Cryptochromes, blue-light absorbing proteins involved in the circadian clock, have been proposed to be the receptor molecules of the avian magnetic compass. In birds, several cryptochromes occur: Cryptochrome 2, Cryptochrome 4 and two splice products of Cryptochrome 1, Cry1a and Cry1b. With an antibody not distinguishing between the two splice products, Cryptochrome 1 had been detected in the retinal ganglion cells of garden warblers during migration. A recent study located Cry1a in the outer segments of UV/V-cones in the retina of domestic chickens and European robins, another migratory species. Here we report the presence of cryptochrome 1b (eCry1b) in retinal ganglion cells and displaced ganglion cells of European Robins, Erithacus rubecula. Immuno-histochemistry at the light microscopic and electron microscopic level showed eCry1b in the cell plasma, free in the cytosol as well as bound to membranes. This is supported by immuno-blotting. However, this applies only to robins in the migratory state. After the end of the migratory phase, the amount of eCry1b was markedly reduced and hardly detectable. In robins, the amount of eCry1b in the retinal ganglion cells varies with season: it appears to be strongly expressed only during the migratory period when the birds show nocturnal migratory restlessness. Since the avian magnetic compass does not seem to be restricted to the migratory phase, this seasonal variation makes a role of eCry1b in magnetoreception rather unlikely. Rather, it could be involved in physiological processes controlling migratory restlessness and thus enabling birds to perform their nocturnal flights.
Highlights
Cryptochromes are blue-light absorbing proteins found in bacteria, plants and animals
The Radical Pair Model of magnetoreception [3] suggests that the singlet/triplet ratio, which depends on the alignment of the radical pair in the ambient magnetic field, leads to an activation pattern on the retina that is centrally symmetric to the field lines and can provide the birds with information on magnetic directions
In birds in migratory state, we found a considerable amount of eCry1b in all ganglion cells including the displaced ganglion cells (Fig 1a)
Summary
Cryptochromes are blue-light absorbing proteins found in bacteria, plants and animals They play an important role in circadian clocks, in part as photoreceptors for entrainment; in plants, they control developmental processes like hypocotyl elongation and photoperiodic induction of flowering In birds, they have been proposed as receptor molecules for the magnetic compass, forming the crucial radical pairs that interact with the magnetic field. The present study is devoted to the question where eCry1b is located in the retina of European Robins
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