LAUSR

Original form Arguably the greatest mystery in sensory biology is magnetoreception — how animals sense Earth’s magnetic field and use it as a compass to determine their spatial orientation. Animals as varied as birds, sea turtles, fishes, crustaceans and insects depend on this field for both shortand long-range navigation. The identity of the biological tissue responsible for sensing the field’s direction, and the sensory mechanism that underpins this type of navigation, have remained an enigma. In migratory birds, the main contenders are magnetically sensitive proteins called crypto chromes, which are located in the retina. However, proof has been lacking that these proteins truly possess the magnetic sensitivity and physical properties needed to detect Earth’s extremely weak magnetic field. On page 535, Xu et al. provide this proof in vitro, bringing us tantaliz ingly close to solving the mystery of magnetoreception. There are currently two main hypotheses for how animals sense Earth’s magnetic field (as well as some alternative hypoth eses put forward in the past few years). One proposes that, as an animal changes direction, crystals of the oxidized-iron compound magnetite (Fe3O4), located in its body and aligned with the field, exert a rotational force — called torque — on mechanoreceptors with which they are in physical contact. This might thereby signal changes in body alignment through the opening and closing of mechano receptor ion channels. The other main hypothesis (Fig. 1) proposes that, when cryptochrome proteins absorb photons of light and become ‘photoexcited’, they form magnetically sensitive chemical intermediates known as radical pairs. Variations in the yield of their reaction product (the form of the cryptochrome that contains a radical molecule called FADH) are thought to signal the animal’s direction with respect to Earth’s magnetic field. These two proposed mechanisms are not mutually exclusive — indeed, migratory birds might possess both, using magnetite for their ‘magnetic-map’ sense (the ability to sense magnetic characteristics associated with a given location on Earth’s surface) and cryptochromes for their magnetic-compass sense (which offers the animal a way to sense its direction relative to magnetic north). Cryptochromes are found both in animals and in plants, and are a type of protein known as a flavoprotein. Cryptochromes bind non-covalently to a molecule called a chromophore, such as FAD, which absorbs photons of blue light when in its fully oxidized state. In animals, cryptochrome proteins termed CRY1 and CRY2 are involved in the regulation of daily (circadian) rhythms, and their expression levels in tissue typically cycle over the course of 24 hours. By contrast, Sensory biology