A protein found in robins’ eyes has all the hallmarks of a magnetoreceptor and could help birds navigate using the Earth’s magnetic fields, according to a study published today (June 23) in Nature. The research, an intensive in vitro analysis of robin cryptochrome 4 (Cry4), revealed that the protein is magnetically sensitive and fulfills several predictions of one of the leading quantum-based theories for how avian magnetoreception might work.
The authors of the study argue that their findings support Cry4 as the likely receptor for birds’ still largely mysterious magnetic sense. But some other researchers who spoke to The Scientist say that while the results are extremely useful for understanding cryptochromes, a family of proteins often studied in circadian rhythms, the paper omits some scientific context for its findings and doesn’t necessarily support Cry4 as the elusive magnetoreceptor.
“It’s a very important step to show that this cryptochrome 4 actually can perceive light and then become magnetically sensitive,” says Rachel Muheim, a zoologist and magnetoreception researcher at Lund University in Sweden who was not involved in this work. She says that in her view Cry4 has already emerged in the wider literature as the most likely candidate in avian navigation, but “there are a number of experiments that can’t be explained” by the mechanism the new study focuses on and that aren’t addressed in the paper, she adds. “There’s a lot of unanswered questions.”
Many animals use the Earth’s magnetic fields to help them navigate, but scientists have long debated the biological mechanisms underpinning this sixth sense, with repeated accusations of irreproducible findings and sometimes heated debates between different research groups.
The cryptochrome theory of magnetoreception is based on quantum mechanics. Cryptochromes are light-sensitive flavoproteins found in the retinas of birds and several other groups of animals, and they’re known to form a pair of radicals—molecules with unpaired electrons—when exposed to light. These electrons have correlated spins, and theoretical and in vitro work indicate that their states can be influenced by magnetic fields, leading to the hypothesis that cryptochrome proteins could provide the basis for animal magnetoreception.
At least four cryptochromes (Cry1a, Cry1b, Cry2, and Cry4) have been found in birds’ eyes to date. While some groups have focused their attention on Cry1a, University of Oxford chemist Peter Hore, a coauthor on the new study, tells The Scientist that he and his colleagues at Oxford and at the University of Oldenburg in Germany view Cry4 as the more likely candidate. Unlike some of the other avian cryptochromes, Hore says, Cry4 binds to a particular molecule needed to help it absorb light and undergo the sort of photochemistry necessary to sense magnetic fields.
In our view, the evidence is overwhelmingly pointing to Cry4 being the hottest candidate.—Henrik Mouritsen, University of Oldenburg
“Also, the other cryptochromes in birds show a 24-hour rhythm in their expression, consistent with their involvement in circadian regulation, whereas the Cry4 doesn’t—but it does show a seasonal variation, which the others don’t,” he adds. “That could be consistent with the need to migrate in spring and autumn.”
To explore the idea further, the team isolated Cry4 from European robins (Erithacus rubecula) that migrate at night when skies are dim, though not completely dark, and studied the protein in vitro using various techniques, including computer simulations and several types of spectroscopy that can measure how magnetic fields affect protein photochemistry. The researchers also created mutant versions of the protein to figure out the role of individual amino acids in Cry4’s sensitivity to magnetic fields.
They showed that Cry4 generates radical pairs in a light-dependent reaction by having electrons hop along a string of tryptophan amino acids, and that these pairs are highly sensitive to magnetic fields. What’s more, compared to robin Cry4, the same cryptochrome isolated from chickens and pigeons—two nonmigratory birds—were a lot less sensitive to magnetic fields, suggesting that robin Cry4 might be particularly specialized for magnetoreception, the authors write in their paper.
David Keays of Ludwig Maximilians University in Munich who was not involved in the work says that the findings do seem to support Cry4’s role in magnetoreception, and adds that the study is consistent with his own group’s research on the protein’s sensing of magnetic fields. He notes that the magnetic fields used in the study were stronger than those generated by the Earth. “The authors see a clear magnetic effect when applying 10-30mT fields to CRY4 in test tubes,” he adds in an email. “[W]hether the same applies to Earth strength fields (50uT) in a migrating bird is an open question.”
Alex Jones, a specialist in photochemistry and magnetic field effects at the UK’s National Physical Laboratory who was not involved in the study, says that the data and the authors’ interpretation of the results as they relate to the protein’s behavior are “very solid.” Regarding the strength of the magnetic fields used, he adds that “the radical pair mechanism predicts effects at both weaker and stronger fields, and starting from exposure to stronger fields makes sense from an experimental point of view.”
Debate about existing behavioral data
Margaret Ahmad, a photobiologist at Sorbonne Université in Paris who helped discover cryptochromes in the early 1990s while working on Arabidopsis plants and was not involved in the current work, says that while the researchers “did a tremendous job” of providing valuable data on Cry4, she finds their conclusions about avian navigation unsupported and contrary to existing data from experiments in animal behavior.
“It’s not scientifically admissible to relate [this Cry4 mechanism] to magnetosensing in view of the behavioral evidence that contradicts any role” for it in avian magnetoreception, says Ahmad, who has previously debated with Hore’s group in the literature about the functions of other cryptochrome proteins in plants.
One of the points of contention is the fact that the particular radical pair mechanism described in the new paper is known not to work under green light. Yet Muheim and others have reported from behavioral experiments that robins and other birds are in fact capable of orienting just fine under green light, although there is an ongoing discussion in the field about whether these findings could partly be due to the light conditions birds were exposed to before being tested.
Hore says that it’s unclear how Cry4 fits into this literature at the moment. “I’m not sure what to make of that,” he says. “Maybe the photochemistry is more complicated than we think—it could certainly be different in vivo.”
Ahmad also points to research by veteran magnetoreception researchers Roswitha Wiltschko and Wolfgang Wiltschko, who carried out orientation experiments using flickering lights and changing magnetic fields. The pair concluded that robins’ magnetosensor worked even in complete darkness. This typically isn’t the sort of condition that robins would navigate under, but the findings led the researchers to propose that it’s a different, light-independent radical pair reaction in cryptochrome that’s important for magnetoreception.
My personal sense is that we’re a little beyond just looking at one end of this, at just the physical chemistry, or just the birds.—Thorsten Ritz, University of California, Irvine
These points weren’t discussed in the paper, Ahmad notes. “Why would you ignore this [data in your paper]? You have to fit your model into the existing behavioral data,” she says. “If I wasn’t in the field at all, I would get the idea that ‘Ah! The bird magnetoreceptor, we’ve got it!’”
The Wiltschkos, who are based at Goethe University Frankfurt and have collaborated with Ahmad and Hore on various occasions, have argued that Cry4’s position in the eye—associated with oil droplets that block the types of light needed to activate the protein—also make Cry4 an unlikely candidate. In an email sent to The Scientist, Roswitha Wiltschko highlights many of the same points as Ahmad, and notes that she favors Cry1a, “which is found in [photoreceptors that] contain clear oil droplet that let all wavelengths pass,” as the likely magnetoreceptor in birds.
She also adds that “the avian magnetic compass is not at all restricted to migratory birds. [In] particular homing pigeons and chickens have been demonstrated to use a magnetic compass; hence we would not expect them to be poorer equipped.”
The University of Oldenburg’s Henrik Mouritsen, a coauthor on the new study, says that his group has tried to replicate a number of different studies of magnetoreception by the Wiltschkos and other groups without success. “If we cannot replicate certain claims, we cannot really refer to those things as facts, because we cannot see that—and we have also published that we cannot see that.”
He agrees that the team’s current study doesn’t provide conclusive proof that Cry4 is the magnetoreceptor in robins, but adds that in combination with the rest of the literature (excluding papers his group has concluded are irreproducible), “in our view, the evidence is overwhelmingly pointing to Cry4 being the hottest candidate.”
He emphasizes that the protein’s behavior in vitro matches up very well with theoretical predictions about magnetoreception. “I would say that it is astounding if what we measure in this molecule is just a coincidence,” he adds. “[I] think that the claim that what we have measured is unlikely to have anything to do with magnetic sensing in birds is . . . unwarranted.”
A better link between in vitro and in vivo
Thorsten Ritz, a physicist at the University of California, Irvine, who helped develop the original cryptochrome radical pair model of avian magnetoreception more than 20 years ago and was one of the reviewers on the new paper, which was submitted in mid-2019, tells The Scientist that while detailed descriptions of the proteins that are potentially involved are extremely important, “my personal sense is that we’re a little beyond just looking at one end of this, at just the physical chemistry, or just the birds—that’s how we started 50 years ago in this field.” Researchers should be working on developing new experimental systems that allow them to study everything from sensing a magnetic field through to some kind of phenotypic effect, he adds.
Hore says that although it’s challenging to do in vivo experiments with purported magnetoreceptors, it might be possible to inhibit Cry4 function in birds’ eyes and study the importance of the protein’s magnetosensitivity in vivo. He adds that the team is now working with mutant cryptochrome proteins to understand what makes robin Cry4 particularly sensitive to magnetic fields compared to chicken or pigeon cryptochrome.
They’re also interested in the role of a tyrosine amino acid that sits at the end of the tryptophan chain. “I think that could be quite important for the mechanism of sensing and signaling,” says Hore. “I’d like to find a way to make the in vitro experiments mimic the situation in vivo more closely to see if we can get evidence for the involvement of that tyrosine.”