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Scientists discover molecule behind birds’ magnetic sense:

“Some birds, notably migratory species, are able to detect the Earth’s magnetic field and use it to navigate. New results from a team of Franco-German researchers suggest that light-sensitive molecules called cryptochromes could be the key to the birds’ magnetic sense.

They did not suggest it – they tested a 10-year old hypothesis.

Cryptochromes are photoreceptors which are sensitive to blue light, and they are involved in a number of processes linked to the circadian cycle, such as growth and development.

Caution: cryptochromes have different functions in different organisms. They are very closely related to photolyases, molecules involved in DNA repair. They are photopigments in plants, but have no circadian function in them. They are involved in circadian phototransduction in insects, but are not pigments and are not clock genes in them. They are core circadian genes in vertebrates, but are not pigments in them. So, we have to be careful when dealing with such a jack-of-all-trades.

Birds’ ability to detect magnetic fields is affected by light; this ‘sixth sense’ only works properly in the presence of blue or green light, while light of other wavelengths disrupts the magnetic sense.

Do you know how much I hate the phrase “sixth sense’?

The scientists realised that the cryptochromes could well be involved in the perception of the magnetic field, as they have all the physical and chemical properties needed, notably the absorption of blue and green light and the formation of ‘radical pairs’ – molecules which respond to magnetic fields. Crucially, the retina of birds’ eyes is rich in cryptochromes.

Unable to test their hypothesis on migratory birds, the researchers turned to a laboratory plant, Arabidopsis thaliana, with similar properties. It is known that the activation of their cryptochromes by blue light influences the behaviour of these plants; for example it inhibits the growth of the hypocotyle (stem).

This is creative, but poses a problem that I mentioned above – in different environments (i.e. inside the bodies of different organisms with different genomes), cryptochromes assume different functions.

To determine whether the magnetic field influences the function of the cryptochromes, researchers from France’s National Centre for Scientific Research (CNRS) and universities in Frankfurt and Marbourg grew the plants in the presence of blue and red light and magnetic fields of varying strengths. They found that increasing the magnetic field only increases the inhibition of the growth of the hypocotyle in the presence of blue light. When red light is used, the plant uses other photoreceptors called phytochromes, and the growth of the hypocotyle is not affected by changes in the magnetic field. Furthermore, mutant plants which have no cryptochromes are also insensitive to changes in the magnetic field.

This is a nice piece in the puzzle, but nothing conclusive yet, of course.

The study shows for the first time that in plants, the work of the cryptochromes is affected by magnetic fields and suggests that the mechanisms of magnetic field perception in plants, and by extension in migratory birds, use the same photosensitive molecules. The researchers also suggest that, as cryptochromes have been strongly conserved throughout evolution, all biological organisms could have the ability to detect magnetic fields, even if they do not use them.”

The phrase “and by extension” worries me for the reasons I noted above.

As for all organisms detecting magnetic fields – yes, decades of research show that most can, from bacteria to, perhaps, even humans. However, this does not mean that cryptochromes are the magnetosensory molecules in all of them, or even that the radical-pair model of magnetoreception applies to all organisms.

It is well established that many organisms do not require the presence of blue-green light in order to orient by he magnetic fields. It is also known that many organisms, from bacteria through salmon to pigeons, possess miniscule crystals of feromagnetite. In bacteria, those form a chain running through the posterior medial line of the cell. In salmon and pigeons, they are embedded inside cell membranes of the dendrites of the trigeminal nerve.

So, cryptochromes may be involved in some way in magnetic sense of some organisms. Extrapolating any broader (i.e., it is the only mechanism; cryptochromes are the main element of the mechanism; this mechanism works in all organisms) is unlikely to be correct. So, the press release is hypoing the work beyond what it really shows. It is good. Actually, it is really cool. But the press release soured me on it.

For an excellent (and quite current) review of the topic, see this review (pdf) and for a moer lay-audience oriented, also quite current article, see this article on The Science Creative Quarterly.

Comments

  1. #1 PhysioProf
    September 12, 2006

    “They are involved in circadian phototransduction in insects, but are not pigments and are not clock genes in them.”

    Actually, the cryptochrome story in insects is now thought to be a little more complicated than this.

    First, the presence of only a single crytochrome–as in Drosophila–is the exception, and it has been proposed that one of the two insect cryptochrome isoforms functions as a photoreceptor, the other as a transcriptional repressor (as in mammals).
    Curr Biol. 2005 Dec 6;15(23):R953-4.

    Second, there is now some evidence that Drosophila cry can indeed function as a trancriptional repressor in the context of circadian transcriptional negative feedback loops.
    Curr Biol. 2006 Mar 7;16(5):441-9.

  2. #2 coturnix
    September 12, 2006

    I tried to keep it simple for the sake of this particular topic. I have written a post a few months ago about the complexities of cryptochromes in Drosophila.

  3. #3 PhysioProf
    September 13, 2006

    Cool. I will take a look at that post. Cryptochrome is a fascinating molecule.

    Have you blogged on melanopsin? That is also a fascinating photoreceptor protein.

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