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Geomagnetic Field Effects on Living Systems

 

initially few, in some cases up to several kilometers, in directions parallel and/or perpendicular to the

bearing of the local MF intensity (Dennis et al., 2007). Tis behavior occurred irrespective of the home­

ward direction and signifcantly more ofen than what was expected by random chance (Dennis et al.,

2007). It is suggested that pigeons when homing detect and respond to spatial variation in the GMF

(Dennis et al., 2007).

A potential complication for all strategies of magnetic map navigation similar to GPS is that the GMF

is not static but instead changes gradually over time. Tis change in feld elements, known as “secular

variation” (Skiles, 1985), means that the MF existing at a given location will not necessarily remain

exactly the same over the life span of a long-lived animal. Similarly, the pattern of isolines throughout

a given geographic region gradually changes. Although the GMF changes over time, strong selective

pressure presumably acts to ensure a continuous match between the responses of animals and the felds

that mark critical locations in migratory routes at any point in time (Lohmann and Lohmann, 1998;

Lohmann et al., 1999, 2001).

W. Wiltschko and Wiltschko (1972) discovered the inclination compass (as described above) in

European robin and speculated with great insight that on the whole, this magnetic compass represents

a highly fexible direction-fnding system. W. Wiltschko and Wiltschko (1972) estimated that its ability

to adjust to a varying intensity range makes it independent of any secular variation in total intensity,

and the fact that it does not use the polarity of the MFs, so-called “polarity compass,” means that it is

not afected by the GMF reversals that have taken place several times since the phylogenetic origin of

birds (Runcorn, 1969).

Behavioral studies in European robins (Zapka et al., 2009) strongly suggest that a forebrain region

named “Cluster N” (Mouritsen et al., 2005), which receives input from the eyes via the thalamofugal

visual pathway (Heyers et al., 2007), is involved in processing magnetic compass information. Several

studies on the migratory birds’ brains showed that bilateral lesions of Cluster N disable magnetic orien­

tation, and therefore, Cluster N is assumed to be a light-processing forebrain region (Möller et al., 2004;

Mouritsen et al., 2004; Zapka et al., 2009). Moreover, it is presumed that it is the cryptochrome (CRY)

of the retina that meets the conditions under which the electron transfer reaction occurs at the photore­

ceptors on the retina sphere (Ritz et al., 2000).

Te vast majority of mobile animal species have magnetoreception in the form of inclination com­

pass. Insects including butterfies and fies may use a CRY-based chemical magnetic sensor in their

antennae. Birds may sense MFs using magnetosensory cells in the inner ear (frst described by Harada

et al., 2001) and beak (frst described by Beason and Semm, 1987) with an iron-based mechanism, and in

eyes with a CRY-based RPM (Ritz et al., 2009). In particular, in birds, it is assumed that the inclination

compass is based primarily on RPM (Wiltschko et al., 2005). However, the exact identity of the magneti­

cally sensitive RP in CRY or “CRY-based RPM” is currently unknown. Presumably, the infuence of the

MF in some way afects the concentration of a CRY signaling state that, in turn, results in a neurophysi­

ological response. However, there exists very little evidence of the signal transduction mechanism that

might link magnetically sensitive chemistry in CRY to an organism response. In this context, Marley

et al. (2014) examined the MF efect on seizure response in Drosophila larvae. Embryos were exposed to

light for 100 ms every second between 11 and 19 hours afer egg laying, but exposed to the MF through­

out embryogenesis. Afer hatching, larvae were transferred to vials and maintained in complete dark­

ness and in the absence of any applied MF until ~3 days later when wall climbing third instar larvae

were tested for seizure-like behavior (Marley et al., 2014). Tey revealed that 100 mT static MF (SMF)

signifcantly increased the efect of blue light (470 nm) on seizure severity in larvae compared to light

pulses alone (Marley et al., 2014). Te MF efect on seizure duration was shown to be Drosophila mela­

nogaster CRY (DmCRY)-dependent: being abolished in a cry03 null (cry-/-) background and rescued

by transgenic expression of UAS-cry in a cry null (Marley et al., 2014). Prolongation of seizure duration

was also prevented by prior ingestion of typical antiepileptic drugs (e.g., phenytoin and gabapentin),

consistent with an efect on neuronal activity (Marley et al., 2014). Exposing the embryos to a 100 mT MF

in the frst instance has a range of benefts over the mT feld exposures immediately relevant to animal