Radical-pair-based magnetoreception in birds: radio-frequency experiments and the role of cryptochrome
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.