As winter turns to spring, the days grow imperceptibly longer. These subtle cues may be lost on us, but birds somehow keep tabs, monitoring the passing of the seasons so that they know when to mate. Now, after decades of searching, researchers report that they have finally identified the protein that enables birds to know when spring is here.
In the bird world, timing is everything. Rooks, for example, must breed in February or March to ensure that the soil will be moist enough for them to easily dig out worms to feed their babies. Goldfinches, on the other hand, breed several months later, because they feed on seeds that aren't available early on. "It's the increasing day length in early spring that triggers all these reproductive responses," says Russell Foster, a neuroscientist at the University of Oxford in the United Kingdom and lead author of the new study.
But how do birds sense these longer days? Experiments in the 1930s showed that the key lies not in a bird's eye, as one might expect, but in the hypothalamus, a region deep in the brain. To demonstrate this, a French scientist inserted thin glass rods into the hypothalamus of blinded ducks in order to artificially illuminate only that part of the brain. Exposing the ducks' brains to springlike day lengths prompted testicular growth. Winter day lengths had no effect.
In many ways, it makes sense that a seasonal sensor would reside deep in the brain, Foster says. Bird's skulls are so thin and their brains are so small that light can penetrate to cells in the interior. Furthermore, the sensor is "close to all those bits in the hypothalamus that are regulating the reproductive system," he says.
Although these early experiments hinted at the approximate location of the light sensors, for decades no one could pinpoint exactly what they were. Fresh clues came in 1997 when Foster's team discovered a new light-sensitive protein in the eyes of salmon and named it vertebrate ancient opsin (VA opsin). Because the protein also appeared in fish brains, the researchers "got terribly excited," Foster says. "We thought, 'Great! This is going to be the brain photoreceptor of birds and reptiles and amphibians and fish.'" But over the next decade, researchers failed to find any evidence of VA opsin in other vertebrates. Foster and colleagues didn't give up, however, and their persistence has paid off.
To locate the elusive protein in birds, the researchers sifted the chicken genome for DNA sequences similar to the VA opsin gene in zebrafish. Eventually they stumbled across a region on chromosome 6 that was 70% identical. Even more promising, the gene was expressed in chicken eye and brain tissue.
But Foster and colleagues still didn't know whether the chicken version of the VA opsin gene coded for a light-sensitive molecule. So they engineered mouse nerve cells to produce the gene's protein. And indeed, the cells became photosensitive, generating electrical signals in response to light. To determine exactly where these photosensitive cells are located, the researchers manufactured antibodies that would bind to VA opsin. The antibodies "bound beautifully" to nerve cells in the hypothalamus in chicken and quail brains, Foster says. "[That's] exactly the region of the brain where all of the early studies had suggested there had to be a photoreceptor," says Foster, whose team reports its results online this week in Current Biology.
Peter Sharp, an avian biologist at the University of Edinburgh in the United Kingdom, says the paper makes a good case. The study shows that VA opsin is "a very strong candidate for this deep brain photoreceptor," he says. But Sharp is not quite ready to declare the mystery solved:. "The big problem is they haven't got the final proof. They need to be able to show if they remove that protein in some way, then the birds will cease to be able to tell when day length is lengthening and spring is coming."