Cryptochrome and magnetosensitivity in Drosophila

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Many animals use the Earth's magnetic field as a compass and map to aid migration. However, the precise biological origin of animal magnetoreception remains unclear. It is proposed that for an animal to make use of 'geomagnetic' information, there must be something that initially detects the Earth's magnetic field (a 'receptor' or 'sensor') and means by which this information is communicated to critical molecules ('responders') in neurons. Changes triggered in the central nervous system (CNS) result in the animal responding to the magnetic field.Among the proposed primary magnetosensors is the protein CRYPTOCHROME (CRY). However, neither CRY, nor any other proposed magnetoreceptor, has been conclusively shown to directly produce a magnetically-induced response in the activity of the CNS under real-world conditions. In the laboratory, we have used cellular (electrical activity of central neurons) and whole organism (locomotor behaviour) assays to demonstrate a substantial and reproducible CRY-dependent magnetic field effect in the fruit fly, Drosophila melanogaster. Strikingly, we have shown that this effect persists when just a small fragment of CRY is present. This seriously undermines the established view that only a determined biophysical reaction that requires full-length CRY is necessary and sufficient to make CRY a magnetosensor. Instead, we hypothesize that cells have additional modalities at their disposal to sense magnetic fields. For instance, we have evidence that Flavin Adenine Dinucleotide (FAD), an organic molecule present in all cells, and which CRY binds to, is per se (as a free molecule) receptive to magnetic fields. Therefore, one of our hypotheses is that the magnetic response mediated by the aforementioned small CRY fragment, might reflect an amplification of magnetic sensing by free FAD. The implication is that somehow, free FAD becomes 'coupled' to the small CRY fragment. Alternatively, the 'sensor' could be a different protein able to bind to the CRY fragment. What we think is key to CRY's role, is its ability to bring the 'sensor' near the cellular 'responders', which requires amino acid motifs carried by the small CRY fragment. We will test these hypotheses using genetics and biochemistry to set the scene and then by conducting neurobiological and behavioural investigations. A positive outcome will have a significant impact to the field by establishing a new way of thinking about magnetoreception in animal cells.