Abstract
The radical-pair hypothesis of magnetoreception has gained a lot of momentum, since the flavoprotein cryptochrome was postulated as a structural candidate to host magnetically sensitive chemical reactions. Here, we first discuss behavioral tests using radio-frequency magnetic fields (0.1–10 MHz) to specifically disturb a radical-pair-based avian magnetic compass sense. While disorienting effects of broadband RF magnetic fields have been replicated independently in two competing labs, the effects of monochromatic RF magnetic fields administered at the electronic Larmor frequency (~1.3 MHz) are disparate. We give technical recommendations for future RF experiments. We then focus on two candidate magnetoreceptor proteins in birds, Cry1a and Cry1b, two splice variants of the same gene (Cry1). Immunohistochemical studies have identified Cry1a in the outer segments of the ultraviolet/violet-sensitive cone photoreceptors and Cry1b in the cytosol of retinal ganglion cells. The identification of the host neurons of these cryptochromes and their subcellular expression patterns presents an important advance, but much work lies ahead to gain some functional understanding. In particular, interaction partners of cryptochrome Cry1a and Cry1b remain to be identified. A candidate partner for Cry4 was previously suggested, but awaits independent replication.
Highlights
Birds use their magnetic compass sense to extract directional information from the Earth’s magnetic field (Wiltschko and Wiltschko 1972)
We focus on the basic principles of functionality of the magnetic compass in birds and on experiments that have been designed to test for the underlying biophysical mechanism
The magnetic compass in birds is the so-called inclination compass (Wiltschko and Wiltschko 1972), which detects the spatial orientation of the field lines but not their polarity
Summary
Birds use their magnetic compass sense to extract directional information from the Earth’s magnetic field (Wiltschko and Wiltschko 1972). In a spin-correlated radical pair where each unpaired electron experiences different hyperfine interactions, the two spins precess at different frequencies and change their relative orientation, oscillating between singlet and triplet states This natural interconversion between singlet and triplet is modulated by an external magnetic field (Schulten et al 1978). Birds turned out to be disoriented in the Larmor condition down to RF amplitudes of 15 nT, while 150 nT administered at non-Larmor frequencies had no effect These results have been interpreted in terms of a radical pair in which one of the radicals is devoid of hyperfine interactions and is only sensitive to the external magnetic field (Ritz et al 2009). The studies differed in various experimental details (coil design, RF equipment, illumination), bird testing procedures, and analysis protocols All these differences make it difficult to identify the key factor responsible for the presence or absence of disorientation effects under the local Larmor condition. Trails per bird in each condition Rayleigh test (second order, RF cond) MWW: RF vs ctrl
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