Cryptochromes are a ubiquitous group of blue-light absorbing flavoproteins that in the mammalian retina have an important role in the circadian clock. In birds, cryptochrome 1a (Cry1a), localized in the UV/violet-sensitive S1 cone photoreceptors, is proposed to be the retinal receptor molecule of the light-dependent magnetic compass. The retinal localization of mammalian Cry1, homologue to avian Cry1a, is unknown, and it is open whether mammalian Cry1 is also involved in magnetic field sensing. To constrain the possible role of retinal Cry1, we immunohistochemically analysed 90 mammalian species across 48 families in 16 orders, using an antiserum against the Cry1 C-terminus that in birds labels only the photo-activated conformation. In the Carnivora families Canidae, Mustelidae and Ursidae, and in some Primates, Cry1 was consistently labeled in the outer segment of the shortwave-sensitive S1 cones. This finding would be compatible with a magnetoreceptive function of Cry1 in these taxa. In all other taxa, Cry1 was not detected by the antiserum that likely also in mammals labels the photo-activated conformation, although Western blots showed Cry1 in mouse retinal cell nuclei. We speculate that in the mouse and the other negative-tested mammals Cry1 is involved in circadian functions as a non-light-responsive protein.

Department of Biosciences Goethe University Frankfurt am Main Max von Laue Str 13 60438 Frankfurt am Main Germany

Department of Earth and Environmental Sciences Ludwig Maximilians University Munich Theresienstr 41 80333 Munich Germany Institute for Biology and Environmental Sciences IBU University of Oldenburg 26111 Oldenburg Germany

Department of General Zoology Faculty of Biology University of Duisburg Essen Universitätsstr 5 45141 Essen Germany Department of Game Management and Wildlife Biology Faculty of Forestry and Wood Sciences Czech University of Life Sciences Kamýcká 129 165 21 Praha 6 Suchdol Czech Republic

Department of Hematology Oncology and Developmental Biochemistry University Medicine Justus von Liebig Weg 11 37077 Göttingen Germany

Max Planck Institute for Brain Research Max von Laue Str 4 60438 Frankfurt am Main Germany Department of Earth and Environmental Sciences Ludwig Maximilians University Munich Theresienstr 41 80333 Munich Germany Ernst Strüngmann Insitute for Neuroscience Deutschordenstr 46 60528 Frankfurt am Main Germany

Max Planck Institute for Brain Research Max von Laue Str 4 60438 Frankfurt am Main Germany Ernst Strüngmann Insitute for Neuroscience Deutschordenstr 46 60528 Frankfurt am Main Germany

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