Abstract
The magnetic compass of birds seems to be based on light-dependent radical-pair processes in the eyes. Cryptochromes are currently the only candidate proteins known in vertebrates that may serve as the primary radical-pair-based magnetoreceptor molecules. Previous immunohistochemical studies have suggested that cryptochrome 1a (Cry1a) is localised in the photoreceptor outer segments of the ultraviolet/violet (UV/V) cones, and it has been claimed that differences in Cry1a antibody staining intensities show that Cry1a is activated by light and that this should make Cry1a the most likely magnetoreceptive candidate molecule. Here, we present an independent study of Cry1a distribution within retinae of several bird species, ranging from non-migratory domestic chicken and rock pigeon to night-migratory passerines, using both the previously used antibody and two newly generated antibodies, one against the same epitope as the originally used antibody and one against a different epitope of Cry1a. We confirm the UV/V cone outer segment localisation of Cry1a in all the tested bird species. In some stainings, we found Cry1a immunoreactivity as a distinct punctate pattern throughout the whole length of the UV/V cone outer segments. These dots with a diameter of around 170 nm might suggest that many Cry1a molecules accumulate in distinct spots in the UV/V cone outer segments. However, we did not see any notable difference in Cry1a immunoreactivity between light- and dark-adapted retinae. We find no evidence whatsoever that a C-terminal antibody against Cry1a labels only a light-activated form of the Cry1a protein.
Highlights
Fifty years ago, it was shown that European robins use a magnetic compass for navigation (Wiltschko1968)
(antibody cryptochrome 1a (Cry1a)-Frankfurt from Nießner et al 2011) and Fig. 3E using Fiji. In their 2011 study, Nießner and colleagues found Cry1a immunoreactivity in the retinae of the ev European robin (Erithacus rubecula) and the domestic chicken (Gallus gallus domesticus) exclusively in the outer segments of the UV/V cones [Nießner et al 2011; polyclonal antibody directed against a iew peptide at amino acid position 600-618]
When this study was published, we already had generated a polyclonal anti-Cry1a antibody that is specific to a 25 aa long
Summary
It was shown that European robins use a magnetic compass for navigation (Wiltschko1968). Cluster N, a part of the avian visual wulst, processes magnetic compass information (Zapka et al.2009, 2010; Mouritsen et al 2016) originating from both eyes (Hein et al 2010, 2011; Engels et al.2012). This strongly indicates that magnetic compass detection in night-migratory songbirds is lightdependent and processed in the birds’ visual system. Of the various hypotheses that have been suggested, the most promising is a physical, quantum-mechanical mechanism. This socalled radical pair theory suggests that photon absorption in specialised magnetoreceptor molecules
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