The horizontal magnetic dance of the honeybee is compatible with a single-domain ferromagnetic magnetoreceptor

Numerical tests of magnetoreception models assisted with behavioral experiments on American cockroaches

Many animals display sensitivity to external magnetic field, but it is only in the simplest organisms that the sensing mechanism is understood. Here we report on behavioural experiments where American cockroaches (Periplaneta americana) were subjected to periodically rotated external magnetic fields with a period of 10 min. The insects show increased activity when placed in a periodically rotated Earth-strength field, whereas this effect is diminished in a twelve times stronger periodically rotated field. We analyse established models of magnetoreception, the magnetite model and the radical pair model, in light of this adaptation result. A broad class of magnetite models, based on single-domain particles found in insects and assumption that better alignment of magnetic grains towards the external field yields better sensing and higher insect activity, is shown to be excluded by the measured data. The radical-pair model explains the data if we assume that contrast in the chemical yield on the order of one in a thousand is perceivable by the animal, and that there also exists a threshold value for detection, attained in an Earth-strength field but not in the stronger field.

Multi-modal imaging and analysis in the search for iron-based magnetoreceptors in the honeybee Apis mellifera

The honeybee Apis mellifera is one of many animal species for which empirical evidence of a magnetic sense has been provided. The underlying mechanisms postulated for magnetoreception in bees are varied, but most point towards the abdomen as the most likely anatomical region for its location, partly owing to the large accumulation of iron in trophocyte cells that comprise the honeybee fat body. Using a multi-modal imaging and analysis approach, we have investigated iron in the honeybee, with a particular focus on the abdomen and the utility of such techniques as applied to magnetoreception. Abdominal iron is shown to accumulate rapidly, reaching near maximum levels only 5 days after emerging from the comb and is associated with the accumulation of iron within the fat body. While fat body iron could be visualized, no regions of interest, other than perhaps the fat body itself, were identified as potential sites for magnetoreceptive cells. If an iron-based magnetoreceptor exists within the honeybee abdomen the large accumulation of iron in the fat body is likely to impede its discovery.

Linking magnetite in the abdomen of honey bees to a magnetoreceptive function

Previous studies of magnetoreception in honey bees, Apis mellifera, focused on the identification of magnetic material, its formation, the location of the receptor and potential underlying sensory mechanisms, but never directly linked magnetic material to a magnetoreceptive function. In our study, we demonstrate that ferromagnetic material consistent with magnetite plays an integral role in the bees' magnetoreceptor. Subjecting lyophilized and pelletized bee tagmata to analyses by a superconducting quantum interference device generated a distinct hysteresis loop for the abdomen but not for the thorax or the head of bees, indicating the presence of ferromagnetic material in the bee abdomen. Magnetic remanence of abdomen pellets produced from bees that were, or were not, exposed to the 2.2-kOe field of a magnet while alive differed, indicating that magnet exposure altered the magnetization of this magnetite in live bees. In behavioural two-choice field experiments, bees briefly exposed to the same magnet, but not sham-treated control bees, failed to sense a custom-generated magnetic anomaly, indicating that magnet exposure had rendered the bees' magnetoreceptor dysfunctional. Our data support the conclusion that honey bees possess a magnetite-based magnetoreceptor located in the abdomen.

Spontaneous magnetic orientation in larval Drosophila shares properties with learned magnetic compass responses in adult flies and mice

We provide evidence for spontaneous quadramodal magnetic orientation in a larval insect. Second instar Berlin, Canton-S and Oregon-R × Canton-S strains of Drosophila melanogaster exhibited quadramodal orientation with clusters of bearings along the four anti-cardinal compass directions (i.e. 45, 135, 225 and 315 deg). In double-blind experiments, Canton-S Drosophila larvae also exhibited quadramodal orientation in the presence of an earth-strength magnetic field, while this response was abolished when the horizontal component of the magnetic field was cancelled, indicating that the quadramodal behavior is dependent on magnetic cues, and that the spontaneous alignment response may reflect properties of the underlying magnetoreception mechanism. In addition, a re-analysis of data from studies of learned magnetic compass orientation by adult Drosophila melanogaster and C57BL/6 mice revealed patterns of response similar to those exhibited by larval flies, suggesting that a common magnetoreception mechanism may underlie these behaviors. Therefore, characterizing the mechanism(s) of magnetoreception in flies may hold the key to understanding the magnetic sense in a wide array of terrestrial organisms.

Molecular mechanisms of compartmentalization and biomineralization in magnetotactic bacteria

Magnetotactic bacteria (MB) are remarkable organisms with the ability to exploit the earth's magnetic field for navigational purposes. To do this, they build specialized compartments called magnetosomes that consist of a lipid membrane and a crystalline magnetic mineral. These organisms have the potential to serve as models for the study of compartmentalization as well as biomineralization in bacteria. Additionally, they offer the opportunity to design applications that take advantage of the particular properties of magnetosomes. In recent years, a sustained effort to identify the molecular basis of this process has resulted in a clearer understanding of the magnetosome formation and biomineralization. Here, I present an overview of MB and explore the possible molecular mechanisms of membrane remodeling, protein sorting, cytoskeletal organization, iron transport, and biomineralization that lead to the formation of a functional magnetosome organelle.

A Candidate Magnetic Sense Organ in the Yellowfin Tuna, Thunnus albacares

Single-domain magnetite crystals have been isolated and characterized from tissue located in a sinus within the dermethmoid bone of the skull of the yellowfin tuna, Thunnus albacares. Their chemical composition, narrow size distribution, and distinctive crystal morphology indicate that these crystals are biochemical precipitates. Experiments on the interaction between particles reveal the organization of the particles in situ and suggest a possible form for candidate magnetoreceptor organelles. The consistent localization of such particles with similar arrangement within the dermethmoids of this and other pelagic fishes suggests that the ethmoid region is a possible location for a vertebrate magnetic sense organ.

Magnetic orientation and the magnetic sense in arthropods

The physical properties of the earth's magnetic field are summarized with the aim of emphasizing their significance as cues that can be exploited in orientational tasks. Past work has revealed magnetic orientation in vertebrates as well as invertebrates, including arthropods. The key finding to date has been that, as opposed to many vertebrates, the magnetic compass of arthropods responds to the polarity, rather than to the inclination of the earth's magnetic field. As in the case of vertebrates, the debate over how arthropods detect magnetic fields has yet to be resolved. Currently, evidence has been reported in support of a detection system based on magnetite crystals together with a variety of detection systems based on events occurring at the molecular level. Interactions between the magnetic and other compasses in orientation experiments suggest the existence of an area in the brain where spatial orientation information from magnetic and other stimuli converges. The slow advance of our knowledge on magnetic orientation in arthropods, as opposed to the much better understanding of magnetic orientation in vertebrates, arises from difficulties in identifying the appropriate behavioural contexts in which arthropods respond to magnetic fields in both laboratory and field situations. Arthropods thus present challenges not only in demonstrating magnetic orientation, but also in elucidating the sensory mechanisms involved in the perception of magnetic fields.

Destruction of human immunodeficiency-infected cells by ferrofluid particles manipulated by an external magnetic field: mechanical disruption and selective introduction of cytotoxic or antiretroviral substances into target cells

Submagnetic domain magnetic fluid particles of approximately 10 nm average diameter complexed with CD4 or monoclonal antibody and then injected into the patient, will localize to the cell membrane of the target cell. These ferrofluid particles will interact with an externally applied rotating magnetic field of rapidly changing polarity. Under these conditions, the ferrofluid particles will be drawn into a circular path and an axial spin will be induced as each particle aligns itself with the magnetic force lines. A portion of these magnetic fluid particles will be drawn into the target cell membrane and into the cytoplasm causing brief perforations of the cell membrane of the target cells. If enough mechanical damage is done to the plasma membrane or to the intracellular structures, cell lysis may result, but in any case the brief disruptions of the target cell membrane can be used to selectively introduce membrane impermeant cytotoxic or antiretroviral substances into the target cell while relatively sparing normal cells.

Magnetite in human tissues: a mechanism for the biological effects of weak ELF magnetic fields

Due to the apparent lack of a biophysical mechanism, the question of whether weak, low-frequency magnetic fields are able to influence living organisms has long been one of the most controversial subjects in any field of science. However, two developments during the past decade have changed this perception dramatically, the first being the discovery that many organisms, including humans, biochemically precipitate the ferrimagnetic mineral magnetite (Fe3O4). In the magnetotactic bacteria, the geomagnetic response is based on either biogenic magnetite or greigite (Fe3S4), and reasonably good evidence exists that this is also the case in higher animals such as the honey bee. Second, the development of simple behavioral conditioning experiments for training honey bees to discriminate magnetic fields demonstrates conclusively that at least one terrestrial animal is capable of detecting earth-strength magnetic fields through a sensory process. In turn, the existence of this ability implies the presence of specialized receptors which interact at the cellular level with weak magnetic fields in a fashion exceeding thermal noise. A simple calculation shows that magnetosomes moving in response to earth-strength ELF fields are capable of opening trans-membrane ion channels, in a fashion similar to those predicted by ionic resonance models. Hence, the presence of trace levels of biogenic magnetite in virtually all human tissues examined suggests that similar biophysical processes may explain a variety of weak field ELF bioeffects.

Relationship of the vestibular hair cells to magnetic particles in the otolith of the guitarfish sacculus

Magnetite particles were systematically localized within the otolithic mass as a conspicuous curved band in the most ventral region of the guitarfish sacculus. The magnetite band overlaid two separated segments of neuroepithelium in the macula, a long segment oriented parallel to the longitudinal body axis, and a short segment oriented in a transverse plane. The exogenous magnetite particles differed from the endogenous otoconia both in their capacity of orienting to magnetic fields, and their difference in mass due to the higher atomic weight of iron. In addition to the normal gravistatic function of the sacculus, two additional receptor functions are hypothesized based upon the differences between the endogenous and exogenous otoconia. A geomagnetic field could induce magnetite displacements detectable by the hair cells for purposes of geomagnetic orientation. Alternatively, the greater atomic weights of magnetite, relative to that of otoconia, could result in gravitational and linear acceleration, which differed in different regions of the macula.