Light Sensing in Birds: On the Trail of a 70-Year-Old Mystery
by Rebecca Hirsch | Today's Science, September 2009
© 2009 Today's Science
All life has evolved under shifting patterns of light and darkness. Every autumn the days grow short, and in spring they grow long again. Living things from bacteria to humans sense the shifts in daylight and respond with changes in behavior.
One familiar example is the behavior of birds in springtime. Come spring, birds sing, mate, build nests and hatch chicks. By mating in the spring, birds ensure that their young are born in a season when food is plentiful.
Exactly how birds mark the arrival of spring has long been a mystery. In the 1930s scientists discovered that birds detect daylight deep within their brains. For years, researchers have hunted for that light sensor, but without success.
Now research teams in England and Spain think they may have found it. Publishing in the August 25, 2009 edition of Current Biology, the teams identified a light-sensing protein that resides in the exact region of the brain where birds sense day length. The research presents tantalizing evidence that the protein is the elusive sensor that tells birds spring has arrived.
Tracking Seasonal Sensors
Birds sense the arrival of spring not by warm weather or springtime flowers, but by the lengthening days. Longer days produce hormone fluxes in birds' bodies, leading to changes in the reproductive organs that prepare the birds for mating. By synchronizing their mating to day length, birds ensure that their chicks hatch at a time when it food is abundant.
Decades ago researchers puzzled over how birds sensed daylight. One obvious possibility was the eye, an organ already known to sense and respond to light. But experiments with blindfolded birds showed that the eye was not needed for birds to sense day length.
Another possibility was the brain. Light can shine through birds' skulls and penetrate deeply into their brains. In the 1930s researchers placed fine glass rods deep within the brains of ducks and illuminated the brains with artificial light, simulating day light. Shining spring-like day lengths tricked the birds' bodies into responding as if it were springtime. Shorter winter-like days had no such effect. Researchers narrowed the light sensing region to an area deep within the brain known as the hypothalamus.
And that is where the trail went cold. Although researchers knew the light sensor was located in the hypothalamus, for decades they searched for the light sensor, always coming up empty.
Clues from Fish
Then, a few years ago, experiments in fish provided fresh clues. While doing research on salmon, biologist Russell Foster and his colleagues discovered a new light-sensing protein, one that belonged to a family of proteins called opsins. Opsins were known to reside in the eye and receive light signals needed for vision.
As expected, Foster's team discovered that the opsin from salmon was present in fish eyes. But the team discovered something else. The salmon opsin was also present in fish brains, specifically in the hypothalamus.
That discovery made Foster and his colleagues take notice. Could a similar opsin be the elusive light sensor in birds?
Closing in on the Sensor
In the current study, Foster and his colleagues set out to look for opsin in birds. They weren't looking for just any opsin; they wanted to find one that was similar to the opsin found in the hypothalamus of fish.
The researchers turned their attention to chickens, the first bird to have its complete genome sequenced. The researchers began combing through the chicken genome and found a stretch of DNA that looked promising. It encoded a protein whose amino acids were 70% identical to the opsin from fish. What's more, the gene was active in chicken eyes and brains.
The gene certainly looked like a good candidate for the light sensor, but the researchers would need to test whether the gene made a protein that could actually sense and respond to light. To do this the scientists grew neural cells from mice in the laboratory. They engineered one set of cells with the opsin gene from chickens. A second set of cells did not contain the gene and served as a control. Researchers shined light on both sets of cells then looked to see how the cells reacted. Cells with the opsin gene responded with a signal cascade that rippled through the cell. Cells without the gene gave no response to light.
The result was clear: the chicken protein not only looked like a photoreceptor, it acted like one.
Lighting up the Brain
Even with these promising results, the case was far from closed. For the bird opsin to be the springtime light sensor, it would need to be present in the exact region of the hypothalamus known to respond to day length. So the researchers turned their attention to finding out where in bird brains the protein resided.
The team used two tools to help them find where in bird brain's the protein was located. The first tool was an antibody, a short protein that attaches like velcro to proteins. Different antibodies stick to different proteins. Foster and his colleagues created antibodies that attached themselves only to the opsin protein. The second tool was a dye that attaches itself to antibodies and glows when fluorescent light is shined on it.
Armed with these tools, the researchers did their experiment. They made thin tissue sections of chicken hypothalamus. Next, they swished the sections in an antibody bath, allowing the antibodies to attach themselves to opsin in the brain tissue. Then they dipped each section in fluorescent dye, allowing the dye to grab onto the antibodies. Finally, they shined fluorescent light on the sections to reveal the protein's location.
The result? Jackpot! The fluorescent dye lit up the exact part of the hypothalamus known to be the light-sensing part of the brain.
So, did the researchers crack the case? Did they prove that opsin is the seasonal sensor in birds?
Not exactly. Foster and his colleagues have certainly made a compelling argument that opsin may be the light sensor, but the researchers haven't proven their case. In order to do that, they would need to show that opsin is required for birds to sense spring. One way to do that would be to remove or disable opsin in live birds, then show that the birds can no longer sense the season. It is a difficult experiment, but an important one. If researchers can pull it off and show that birds without opsin lack the ability to sense day length, then a 70-year-old mystery would truly be solved.
Journal Abstracts and Articles
(Researchers' own descriptions of their work, summary or full-text, on scientific journal websites).
Halford, Stephanie et al. "VA Opsin-Based Photoreceptors in the Hypothalamus of Birds." Current Biology vol. 19, pp. 1396-1402, August 25, 2009.
Bibliography
Willyard, Cassandra. "Birds' Light Meter Discovered" ScienceNOW Daily News (August 7, 2009) [accessed August 14, 2009]: http://sciencenow.sciencemag.org/ cgi/ content/ full/2009/807/3.
"Scientists Explain Why Birds Get Sex on the Brain in the Spring." University of Oxford (August 7, 2009) [accessed August 14, 2009]: http://www.ox.ac.uk/media/news_stories/ 2009/090807.html.
© Today's Science, 2009
© 2009 Today's Science
All life has evolved under shifting patterns of light and darkness. Every autumn the days grow short, and in spring they grow long again. Living things from bacteria to humans sense the shifts in daylight and respond with changes in behavior.
One familiar example is the behavior of birds in springtime. Come spring, birds sing, mate, build nests and hatch chicks. By mating in the spring, birds ensure that their young are born in a season when food is plentiful.
Exactly how birds mark the arrival of spring has long been a mystery. In the 1930s scientists discovered that birds detect daylight deep within their brains. For years, researchers have hunted for that light sensor, but without success.
Now research teams in England and Spain think they may have found it. Publishing in the August 25, 2009 edition of Current Biology, the teams identified a light-sensing protein that resides in the exact region of the brain where birds sense day length. The research presents tantalizing evidence that the protein is the elusive sensor that tells birds spring has arrived.
Tracking Seasonal Sensors
Birds sense the arrival of spring not by warm weather or springtime flowers, but by the lengthening days. Longer days produce hormone fluxes in birds' bodies, leading to changes in the reproductive organs that prepare the birds for mating. By synchronizing their mating to day length, birds ensure that their chicks hatch at a time when it food is abundant.
Decades ago researchers puzzled over how birds sensed daylight. One obvious possibility was the eye, an organ already known to sense and respond to light. But experiments with blindfolded birds showed that the eye was not needed for birds to sense day length.
Another possibility was the brain. Light can shine through birds' skulls and penetrate deeply into their brains. In the 1930s researchers placed fine glass rods deep within the brains of ducks and illuminated the brains with artificial light, simulating day light. Shining spring-like day lengths tricked the birds' bodies into responding as if it were springtime. Shorter winter-like days had no such effect. Researchers narrowed the light sensing region to an area deep within the brain known as the hypothalamus.
And that is where the trail went cold. Although researchers knew the light sensor was located in the hypothalamus, for decades they searched for the light sensor, always coming up empty.
Clues from Fish
Then, a few years ago, experiments in fish provided fresh clues. While doing research on salmon, biologist Russell Foster and his colleagues discovered a new light-sensing protein, one that belonged to a family of proteins called opsins. Opsins were known to reside in the eye and receive light signals needed for vision.
As expected, Foster's team discovered that the opsin from salmon was present in fish eyes. But the team discovered something else. The salmon opsin was also present in fish brains, specifically in the hypothalamus.
That discovery made Foster and his colleagues take notice. Could a similar opsin be the elusive light sensor in birds?
Closing in on the Sensor
In the current study, Foster and his colleagues set out to look for opsin in birds. They weren't looking for just any opsin; they wanted to find one that was similar to the opsin found in the hypothalamus of fish.
The researchers turned their attention to chickens, the first bird to have its complete genome sequenced. The researchers began combing through the chicken genome and found a stretch of DNA that looked promising. It encoded a protein whose amino acids were 70% identical to the opsin from fish. What's more, the gene was active in chicken eyes and brains.
The gene certainly looked like a good candidate for the light sensor, but the researchers would need to test whether the gene made a protein that could actually sense and respond to light. To do this the scientists grew neural cells from mice in the laboratory. They engineered one set of cells with the opsin gene from chickens. A second set of cells did not contain the gene and served as a control. Researchers shined light on both sets of cells then looked to see how the cells reacted. Cells with the opsin gene responded with a signal cascade that rippled through the cell. Cells without the gene gave no response to light.
The result was clear: the chicken protein not only looked like a photoreceptor, it acted like one.
Lighting up the Brain
Even with these promising results, the case was far from closed. For the bird opsin to be the springtime light sensor, it would need to be present in the exact region of the hypothalamus known to respond to day length. So the researchers turned their attention to finding out where in bird brains the protein resided.
The team used two tools to help them find where in bird brain's the protein was located. The first tool was an antibody, a short protein that attaches like velcro to proteins. Different antibodies stick to different proteins. Foster and his colleagues created antibodies that attached themselves only to the opsin protein. The second tool was a dye that attaches itself to antibodies and glows when fluorescent light is shined on it.
Armed with these tools, the researchers did their experiment. They made thin tissue sections of chicken hypothalamus. Next, they swished the sections in an antibody bath, allowing the antibodies to attach themselves to opsin in the brain tissue. Then they dipped each section in fluorescent dye, allowing the dye to grab onto the antibodies. Finally, they shined fluorescent light on the sections to reveal the protein's location.
The result? Jackpot! The fluorescent dye lit up the exact part of the hypothalamus known to be the light-sensing part of the brain.
So, did the researchers crack the case? Did they prove that opsin is the seasonal sensor in birds?
Not exactly. Foster and his colleagues have certainly made a compelling argument that opsin may be the light sensor, but the researchers haven't proven their case. In order to do that, they would need to show that opsin is required for birds to sense spring. One way to do that would be to remove or disable opsin in live birds, then show that the birds can no longer sense the season. It is a difficult experiment, but an important one. If researchers can pull it off and show that birds without opsin lack the ability to sense day length, then a 70-year-old mystery would truly be solved.
Journal Abstracts and Articles
(Researchers' own descriptions of their work, summary or full-text, on scientific journal websites).
Halford, Stephanie et al. "VA Opsin-Based Photoreceptors in the Hypothalamus of Birds." Current Biology vol. 19, pp. 1396-1402, August 25, 2009.
Bibliography
Willyard, Cassandra. "Birds' Light Meter Discovered" ScienceNOW Daily News (August 7, 2009) [accessed August 14, 2009]: http://sciencenow.sciencemag.org/ cgi/ content/ full/2009/807/3.
"Scientists Explain Why Birds Get Sex on the Brain in the Spring." University of Oxford (August 7, 2009) [accessed August 14, 2009]: http://www.ox.ac.uk/media/news_stories/ 2009/090807.html.
© Today's Science, 2009