The yield of the different states has been demonstrated to be influenced by strong magnetic fields, and based on this, it was hypothesized that a molecule that formed such radicals in different yields depending on the magnetic field alignment could be the basis of a magnetoreceptor (Schulten, Swenberg & Weller, 1978) (Fig. 3). It was subsequently

discovered that magnetic compass orientation is dependent on the wavelength of light (Wiltschko et al., 1993; Wiltschko & Wiltschko, 2006) and so the model see more was modified to suggest that the molecule involved in the radical pair process was photoreceptive and that a photon of light would instigate this reaction (Ritz, Adem & Schulten, 2000). Evidence that the magnetic compass was lateralized via the right eye to the left brain hemisphere suggested that the magnetic field was perceived through

the eyes [Wiltschko et al., 2002b; although see Hein et al. (2011) for evidence of no lateralization]. A study involving ZENK, an immediate early gene, which is expressed in neurones, indicated that an area of the brain called cluster-N, responsible for night vision, was active during migratory restlessness (Mouritsen et al., 2005). A subsequent study in which this area of the brain was lesioned indicated that migratory robins could no longer use their magnetic compass (Zapka et al., 2009). Thus, migratory songbirds appear to possess a magnetoreceptor Roscovitine mediated by the visual system, which is based on a photoreceptive molecule. Evidence that this is due to a radical pair mechanism comes from an experiment based on the prediction that the interaction between a radical pair and the magnetic field could be disrupted by a weak electromagnetic

field in the radio spectrum (1.315 MHz, the so called Larmor frequency). It was indeed the case that migratory robins could no longer orient in an emlen funnel when such a field was applied (Ritz et al., 2004). The molecule involved has been proposed to be a cryptochrome (Ritz et al., 2000). This is a blue light receptor and appears to form long-lived radical pairs, which would be necessary for it to work as a magnetoreceptor 5-Fluoracil price (Liedvogel et al., 2007). Four different cryptochromes have been found in the eyes of migratory birds, Cry 1a, (Moller et al., 2004; Mouritsen et al., 2004; Niessner et al., 2011), Cry 1b (Moller et al., 2004), Cry 2 (Mouritsen et al., 2004) and Cry 4 (Mouritsen et al., 2004). In terms of Fig. 3, it is thought that the radical pair comprises a flavosemiquinone radical and a terminal residue of a conserved triad of tryptophan residues (a flavin–tryptophan radical pair) (Biskup et al., 2009; Maeda et al., 2012). Based on our understanding of how a similar reaction occurs in plants, the flavosemiquinone radical would appear to lead to the signalling state (Bouly et al., 2007).