Aug 232008
 
ResearchBlogging.orgThe ability to sense and respond to magnetic fields is a fundamental aspect of behavior in many animals. While migratory birds famously use the earth’s magnetic field to navigate during, magnetic field responses occur in all manner of animals, from eels to invertebrates. Even the lowly fruit fly, best known as a reminder that you really should have taken the garbage out two days ago, can react to magnetism. While various explanations have been put forward in different species, magnetosensitivity remains fairly mysterious. In this week’s Nature, researchers from the University of Massachusetts Medical School show that the blue-light photoreceptor cryptochrome plays an essential role in allowing fruit flies to detect magnetic fields.

Cryptochrome (or Cry) inherited the ability to receive blue light along with its photolyase domain, which is homologous to a prokaryotic, light-dependent DNA repair protein. Cry proteins, which are present in all animals, do not perform any DNA repair work, but instead play a role in regulating the circadian rhythm. While it is not clear in all cases whether Cry’s ability to absorb blue light is biologically significant in clock regulation, it is known that fruit flies (Drosophila melanogaster) use Cry to synchronize their circadian clocks. Previous experiments had suggested that the ability of fruit flies to detect magnetic fields was somehow related to photoreception, and that short wavelengths (like those sensed by Cry) had different effects from longer ones.

Gegear et al. devised a relatively simple experiment to test the importance of Cry in Drosophila magnetosensing. They placed a T-junction in a box, with a magnetic coil on one side and a non-magnetic coil on the other. They released flies into the junction, with (trained) or without (naive) performing an earlier run where the magnetic field was associated with a sucrose reward. They shined a light into the box and used filters to investigate the role of specific wavelengths.

They discovered that several strains of Drosophila could be trained to go to the magnetic field, although the degree of preference and the nature of the naive response differed substantially between strains. Gegear et al. chose the strain that showed the greatest response in full-spectrum light (and displayed a tendency to avoid the magnetic field in the naive state) to perform the filter experiment. Cutting off all wavelengths of light shorter than 500 nm abolished both the naive and trained responses to the magnetic field in these flies, as did filtering out all wavelengths shorter than 420 nm. If only wavelengths shorter than 400 nm were cut off, some of the trained and naive response returned. Simply dimming the light was not enough to replicate the effect of filtering. These experiments indicate that magnetic sensitivity in these flies requires light in the blue to ultraviolet range.

In order to prove that cryptochrome specifically is necessary for this magnetic sensitivity, Gegear et al. took advantage of our tremendous knowledge of fly genetic manipulation to create mutant flies that did not have a functional Cry gene. No matter what wavelengths of light were used in the T-junction experiment, these flies did not respond to the magnetic field. Crossing these Cry-null mutants with normal flies restored magnetosensitivity. The authors also performed experiments to show that the circadian rhythm was not itself essential to magnetic response in the flies.

Because this is a genetic experiment, it cannot address the question of whether Cry is both the blue-light photoreceptor and the magnetosensor. Going just on what we have in this paper, it is also possible that Cry acts upstream of another magnetosensor protein or is part of its downstream signaling pathway. However, in light of research that shows the flavin photoreception in other cryptochromes induces the formation of magnetically-sensitive radicals, some of which I discussed last year, it certainly seems possible that Drosophila cryptochrome does the whole job itself. As I mentioned in the case of the previous article, though, there is not yet any understanding of a mechanism by which information about magnetic field could be transduced from Cry radicals into the nervous system.

Dorosophila Cry differs from other plant and animal Cry proteins in significant ways, so it’s unclear whether these results have any relevance for other organisms. However, the finding that Cry is essential to Drosophila magnetosensitivity suggests at least the possibility of parallel systems in migratory birds and other species that use magnetic fields.

Robert J. Gegear, Amy Casselman, Scott Waddell, Steven M. Reppert (2008). Cryptochrome mediates light-dependent magnetosensitivity in Drosophila Nature, 454 (7207), 1014-1018 DOI: 10.1038/nature07183

  One Response to “Guided by the (blue) light”

  1. That is fascinating. Aside from some protozoa, I never considered how higher organisms would sense magnetic fields. It seems like those researchers have found an elegant solution to the issue.

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