Cryptochrome-dependent magnetoreception in a heteropteran insect continues even after 24 h in darkness

Sensitivity to magnetic fields is dependent on the intensity and color of light in several animal species. The light-dependent magnetoreception working model points to cryptochrome (Cry) as a protein cooperating with its co-factor flavin, which possibly becomes magnetically susceptible upon excitation by light. The type of Cry involved and what pair of magnetosensitive radicals are responsible is still elusive. Therefore, we developed a conditioning assay for the firebug Pyrrhocoris apterus, an insect species that possesses only the mammalian cryptochrome (Cry II). Here, using the engineered Cry II null mutant, we show that: (i) vertebrate-like Cry II is an essential component of the magnetoreception response, and (ii) magnetic conditioning continues even after 25 h in darkness. The light-dependent and dark-persisting magnetoreception based on Cry II may inspire new perspectives in magnetoreception and cryptochrome research.

How do honeybees use their magnetic compass? Can they see the North?

While seeking food sources and routes back to their hive, bees make use of their advanced nervous and sensory capacities, which underlie a diverse behavioral repertoire. One of several honeybee senses that is both exceptional and intriguing is magnetoreception - the ability to perceive the omnipresent magnetic field (MF) of the Earth. The mechanism by which animals sense MFs has remained fascinating as well as elusive because of the intricacies involved, which makes it one of the grand challenges for neural and sensory biology. However, investigations in recent years have brought substantial progress to our understanding of how such magneto-receptor(s) may work. Some terrestrial animals (birds) are reported to be equipped even with a dual perception system: one based on diminutive magnetic particles - in line with the original model which has also always been hypothesized for bees - and the other one, as the more recent model describes, based on a sensitivity of some photochemical reactions to MF (radical-pair or chemical mechanism). The latter model postulates a close link to vision and supposes that the animals can see the position of the geomagnetic North as a visible pattern superimposed on the picture of the environment. In recent years, a growing body of evidence has shown that radical-pair magnetoreception might also be used by insects. It is realistic to expect that such evidence will inspire a re-examination and extension or confirmation of established views on the honeybee magnetic-compass mechanism. However, the problem of bee magnetoreception will not be solved at the moment that a receptor is discovered. On the contrary, the meaning of magnetoreception in insect life and its involvement in the orchestration of other senses is yet to be fully understood. The crucial question to be addressed in the near future is whether the compass abilities of the honeybee could suffer from radio frequency (RF) smog accompanying modern civilization and whether the fitness of this dominant pollinator might be affected by RF fields. The goal of this review is to provide an overview of the path that the behavioral research on honeybee magnetoreception has taken and to discuss it in the context of contemporary data obtained on other insects.

Radio frequency magnetic fields disrupt magnetoreception in American cockroach

The sense that allows birds to orient themselves by the Earth's magnetic field can be disabled by an oscillating magnetic field whose intensity is just a fraction of the geomagnetic field intensity and whose oscillations fall into the medium or high frequency radio wave bands. This remarkable phenomenon points very clearly at one of two existing alternative magnetoreception mechanisms in terrestrial animals, i.e. the mechanism based on the radical pair reactions of specific photosensitive molecules. As the first such study in invertebrates, our work offers evidence that geomagnetic field reception in American cockroach is sensitive to a weak radio frequency field. Furthermore, we show that the 'deafening' effect at Larmor frequency 1.2 MHz is stronger than at different frequencies. The parameter studied was the rise in locomotor activity of cockroaches induced by periodic changes in the geomagnetic North positions by 60 deg. The onset of the disruptive effect of a 1.2 MHz field was found between 12 nT and 18 nT whereas the threshold of a doubled frequency field 2.4 MHz fell between 18 nT and 44 nT. A 7 MHz field showed no impact even in maximal 44 nT magnetic flux density. The results indicate resonance effects rather than non-specific bias of procedure itself and suggest that insects may be equipped with the same magnetoreception system as the birds.

Effect of light wavelength spectrum on magnetic compass orientation in Tenebrio molitor

In many animal species, geomagnetic compass sensitivity has been demonstrated to depend on spectral composition of light to which moving animals are exposed. Besides a loss of magnetic orientation, cases of a shift in the compass direction by 90 degrees following a change in the colour of light have also been described. This hitherto unclear phenomenon can be explained either as a change in motivation or as a side effect of a light-dependent reception mechanism. Among the invertebrates, the 90 degrees shift has only been described in Drosophila. In this paper, another evidence of the phenomenon is reported. Learned compass orientation in the Tenebrio molitor was tested. If animals were trained to remember the magnetic position of a source of shortwave UV light and then tested in a circular arena in diffuse light of the same wavelength, they oriented according to the learned magnetic direction. If, however, they were tested in blue-green light after UV light training, their magnetic orientation shifted by 90 degrees CW. This result is being discussed as one of a few cases of 90 degrees shift reported to date, and as an argument corroborating the hypothesis of a close connection between photoreception and magnetoreception in insects.

Ablation of antennae does not disrupt magnetoreceptive behavioural reaction of the American cockroach to periodically rotated geomagnetic field

Neither a mode of function nor an exact anatomical localisation of the animal magnetoreceptor have been identified in any organism. Insects' antennae are organs specialized as unique neural input structures for a number of sensory modalities and have also been suggested to play a certain role in magnetoreception. In the present study, we used the American cockroach Periplaneta americana and tested the impact of amputation of both its antennae on the spontaneous magnetosensitive behaviour. By means of a full-laboratory assay we registered a non-specific unlearned movement reaction to the changing magnetic environment within the frame of the natural time and intensity parameters of the field. We report no loss of the magnetoreceptive behaviour in antennaeless cockroaches. Our finding narrows the spectrum of the insects' magnetite-rich nerve structures which might potentially be involved in magnetoreception.

Identification of Photosystem I and Photosystem II enriched regions of thylakoid membrane by optical microimaging of cryo-fluorescence emission spectra and of variable fluorescence

Oxygenic photosynthesis of higher plants requires linear electron transport that is driven by serially operating Photosystem II and Photosystem I reaction centers. It is widely accepted that distribution of these two types of reaction centers in the thylakoid membrane is heterogeneous. Here, we describe two optical microscopic techniques that can be combined to reveal the heterogeneity. By imaging micro-spectroscopy at liquid nitrogen temperature, we resolved the heterogeneity of the chloroplast thylakoid membrane by distinct spectral signatures of fluorescence emitted by the two photosystems. With another microscope, we measured changes in the fluorescence emission yield that are induced by actinic light at room temperature. Fluorescence yield of Photosystem II reaction centers varies strongly with light-induced changes of its photochemical yield. Consequently, application of moderate background irradiance induces changes in the Photosystem II fluorescence yield whereas no such modulation occurs in Photosystem I. This contrasting feature was used to identify regions in thylakoid membranes that are enriched in active Photosystem II.

Laboratory behavioural assay of insect magnetoreception: magnetosensitivity of Periplaneta americana

A relatively simple all-laboratory behavioural assay of insect magnetoreception has been developed. We found non-conditioned reactions of American cockroach to the periodical shifts of the geomagnetic field. The movement activity of animals individually placed into Petri dishes was scored as a number of body turns. Test groups were exposed to a 90-min interval with the horizontal component of the geomagnetic field periodically rotated by 60 degrees back and forth with 5 min periodicity. The number of body turns was compared with the preceding and following intervals and with the corresponding interval of the control group kept in the natural field. We obtained a significant increase in activity when changes in field were applied. Interestingly, the period of increased activity did not coincide precisely with the 90 min stimulation interval. The onset of animal restlessness was delayed by tens of minutes and persisted correspondingly after the stimulation stopped. A respective evaluation criterion was suggested and verified. Owing to its simplicity and minimal manipulation of the insects, together with low demands on the memory and motivation state of animals, the approach potentially may be used as a laboratory diagnostic tool indicating magnetoreception in insect neurophysiology research.

Magnetic orientation in the mealworm beetle Tenebrio and the effect of light

There is evidence for both light-dependent and light-independent mechanisms of magnetoreception of terrestrial animals. One example of a light-independent mechanism frequently cited is the magnetic compass of the mealworm beetle (Tenebrio molitor). We found that magnetoreception of the mealworm beetle per se is a replicable phenomenon but that, in contrast to earlier findings, Tenebrio only exhibited consistent magnetic compass orientation when light was present. The problem of whether the loss of orientation is due to a light-dependent magnetoreception mechanism or is instead an effect of motivation change is discussed.

The influence of a static, homogenous magnetic field (B=320mT) on extracardiac pulsations of Tenebrio molitor pupae (Coleoptera: Tenebrionidae)

While investigating and describing interactions among living organisms and magnetic fields (MFs) it is imperative to lay great emphasis on independent reproducibility of published experimental results. Mutual confrontation of existing theoretical models with reliable data obtained under comparable conditions can aid gradual mapping of this hitherto badly organized and understood discipline of biology. The objective of our experiment, based on analysing extracardiac pulsations of the pupae of Tenebrio molitor under the influence of a MF, was to verify published data on allegedly accelerated development induced by a MF employing a different procedure. The obtained data are in agreement with a hypothesis of increased pupal metabolism during the period of MF activity. Furthermore, some dependence on the age of the pupae cannot be ruled out.

Hole burning study of excited state structure and energy transfer dynamics of bacteriochlorophyll c in chlorosomes of green sulphur photosynthetic bacteria

Results of low temperature fluorescence and spectral hole burning experiments with whole cells and isolated chlorosomes of the green sulfur bacterium Chlorobium limicola containing BChl c are reported. At least two spectral forms of BChl c (short-wavelength and long-wavelength absorbing BChl c) were identified in the second derivative fluorescence spectra. The widths of persistent holes burned in the fluorescence spectrum of BChl c are determined by excited state lifetimes due to fast energy transfer. Different excited state lifetimes for both BChl c forms were observed. A site distribution function of the lowest excited state of chlorosomal BChl c was revealed. The excited state lifetimes are strongly influenced by redox conditions of the solution. At anaerobic conditions the lifetime of 5.3 ps corresponds to the rate of energy transfer between BChl c clusters. This time shortens to 2.6 ps at aerobic conditions. The shortening may be caused by introducing a quencher. Spectral bands observed in the fluorescence of isolated chlorosomes were attributed to monomeric and lower state aggregates of BChl c. These forms are not functionally connected with the chlorosome.