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Bioelectromagnetism

 

orientation programs, but no map (Mouritsen, 2018). For example, in the case of juvenile whitethroats,

Sylvia communis, it is reported that magnetic information is not necessary for establishing the appropri­

ate migratory direction when natural celestial cues are available in the pre-migratory period (Rabøl and

Torup, 2006). Tus, it remains unclear exactly which combination of sensory parameters including

celestial cues triggers the start and stop of the frst natural migration (Mouritsen, 2018).

R. Wiltschko and Wiltschko (2021) have also noticed and reviewed that specifc magnetic condi­

tions or changes in conditions may act as “signposts,” triggering specifc responses. Tese are spontane­

ous, obviously innate responses, in contrast to the navigational processes based on the learned “map”

(Wiltschko and Wiltschko, 2021). Although not many examples have been discovered yet, these exam­

ples show that the GMF is involved in a variety of phenomena where it is advantageous that certain

things happen at specifc locations (Wiltschko and Wiltschko, 2021).

Appropriate vertical movement is critical for the survival of fying animals. Although negative geo­

taxis (moving away from the ground surface) driven by gravity has been extensively studied, much

less is understood concerning a static regulatory mechanism for inducing positive geotaxis (moving

toward the ground surface). Using the Drosophila melanogaster as an above-mentioned model organ­

ism, Bae et al. (2016) showed that the GMF induces positive geotaxis and antagonizes negative gravi­

taxis. Remarkably, the GMF acts as a sensory cue for an appetite-driven associative learning behavior

through the GMF-induced positive geotaxis. Tis GMF-induced positive geotaxis requires the three

geotaxis genes, such as cry, pyx, and pdf, and the corresponding neurons residing in Johnston’s organ of

the fy’s antennae (Bae et al., 2016).

It has been shown that in principle it is possible to obtain MF efects for MFs as weak as the GMF of

~50 μT (Maeda et al., 2008; Henbest et al., 2008). It is gradually becoming known that a wide variety of

animal species perceive and respond to MFs, but the mechanisms of their magnetic sense are not clari­

fed yet in detail. One plausible hypothesis about the magnetic sense of animals is that a very small mag­

netic material called “magnetite” in the animal’s body works like a compass needle (frst described by

Kirschvink and Gould, 1981). As typical examples, magnetites, which are present in the nose of rainbow

trout (Walker et al., 1997) and the upper beak of pigeons (Fleissner et al., 2003, 2007; Falkenberg et al.,

2010), have been considered to be responsible for magnetic reception. Moreover, in addition to magne­

tites, another type of iron minerals, i.e., “maghemites” were also found in subcellular compartments

within sensory dendrites of the upper beak of several bird species (Solov’yov and Greiner, 2007, 2009).

Tus, the iron minerals in the beak were found in the form of crystalline maghemite (γ-Fe2O3) platelets

arranged in chains inside the dendrite, and assemblies of magnetite (Fe3O4) nanoparticles attached to

the cell membrane (Solov’yov and Greiner, 2007, 2009). Here, maghemite can be considered as an Fe(II)­

defcient magnetite (Cornell and Schwertmann, 2003). As one of the mechanisms for iron mineral-based

magnetoreception, it was suggested that in an external MF, the maghemite platelets become magnetized

and enhance the local MF in the cell by orders of magnitude (Fleissner et al., 2007). Tus the magnetite

clusters will experience an attractive (repulsive) force inducing their displacement, what might induce

primary receptor potential via strain-sensitive membrane channels leading to a certain orientation

efect (Solov’yov and Greiner, 2007, 2009).

However, it has been found that magnetite-containing cells are a type of “macrophage” (macrophage

is known to have a vital role in host defense and iron homeostasis) (Wang and Pantopoulos, 2011), and

although they are rich in iron ions, they do not project onto nerve cells (Treiber et al., 2012). However, it

is unknown whether the magnetite-containing macrophages are involved in magnetic sensing.

With regard to the transduction link between nanoparticles of iron oxides (most frequently magne­

tite, Fe3O4) and nerve cells, a number of distinguished researchers proposed the following transduction

mechanism (Cadiou and McNaughton, 2010; Winklhofer and Kirschvink, 2010; Eder et al., 2012; Vácha,

2020). Te strength of the Fe Oxide Particles model lies in its simplicity which is underlined by its

simplicity with other known cellular mechanisms sensitive to mechanical forces and tensions. If micro­

scopic particles with either a permanent or induced magnetic dipole are exposed to the surrounding

GMF, mechanical forces act on them.