New study sheds light on fireflies research team unlocks secrets of the firefly flash
Light fades from the summer sky, and the spectacular light show put on by thousands of fireflies begins. While these remarkable bioluminescent insects have inspired poets and delighted children for centuries, it has been a long-standing mystery as to exactly how they manage to blink their lights so precisely.
An interdisciplinary team of researchers from Tufts University and Brigham and Women's Hospital joined forces to solve this intriguing puzzle. This week's issue of the journal Science reports their discovery--that a deceptively simple molecule, nitric oxide, unexpectedly plays a key role in controlling the firefly flash.
"We knew about the chemistry that makes fireflies light up," says Barry Trimmer, lead author and insect neurobiologist at Tufts, "but we now have the missing piece of the puzzle that explains how they are able to throw the switch on and off."
In humans, nitric oxide (NO) acts as a messenger throughout the body, controlling blood flow in the body as well as mediating learning and memory. According to Thomas Michel, a cardiologist at Brigham and Women's Hospital and co-author, NO is unique: "It's the smallest molecule known to carry messages between cells, and it is a gas."
In addition to shedding new light on how it is that fireflies control their flashing, this work hails a new role for nitric oxide. While this chemical has recently gained fame for carrying messages between cells, most notably for treating erectile dysfunction, it now has a new role in helping to carry messages between individual fireflies.
Many animals, including jellyfish and bacteria, are capable of the seemingly magical feat of converting chemical energy into a bioluminescent glow. "However," notes co-author Sara Lewis, an evolutionary ecologist at Tufts University, "the firefly's talent for producing precisely-timed, rapid bursts of light is quite rare." This ability has allowed fireflies (which are actually beetles, not flies) to evolve an elaborate courtship system based on flash communication, a kind of Personals Listing just for fireflies.
"Fireflies are very romantic beasts," says Lewis, "because their whole adult life is spent courting." There are hundreds of different firefly species, she explains, each using a different flash code for identification. Male and female flash patterns also differ, but in both sexes flashes are generated by a light-producing abdominal lantern.
"How do they do that?" is a frequent question asked by firefly watchers young and old. The firefly lantern contains thousands of specialized cells known as photocytes. In flashing fireflies, these photocytes are subdivided into an inner region rich in organelles containing luceriferin and luciferase, chemicals that react to generate light when oxygen is available. The photocyte edges are densely packed with mitochondria, "which are quite famous as the oxygen-consuming power plants of almost all cells," says June Aprille, a cell biologist at Tufts and also member of the research team.
Remarkably, when NO is around, mitochondrial oxygen use stops cold. Scientists have known that flashes start off when the firefly's nervous system sends a signal to the abdominal lantern. The big unanswered question was: what happens inside the lantern between the nerve signal and light production?
The research team put live fireflies into a tiny custom-designed chamber and exposed them to oxygen and nitric oxide gas. The NO gas doses were similar to those used in treating heart and lung diseases in human patients. To the surprise of the scientists, whenever the fireflies were exposed to nitric oxide they glowed or flashed almost continuously, and they stopped once the nitric oxide was turned off.
The researchers also discovered that the flashes normally stimulated by adding the neurotransmitter octopamine to firefly lanterns could be completely blocked by NO-absorbing chemicals. They then looked at microscopic structures inside the firefly lantern using special stains and antibodies to see where the NO-producing enzyme might be located. The enzyme was discovered in a strategic location - right next door to the light-generating apparatus in the photocytes.
Taken all together, these findings suggest how the flash control machinery may work. Normally, the firefly's lantern is turned off and the photocytes are dark. Oxygen moving into the photocytes is continuously consumed by millions of tiny, respiring mitochondria. These mitochondria not only provide energy for all cellular activities (including flash production), but in firefly photocytes they also act as gatekeepers for oxygen entry into the cell interior. Mitochondrial respiration prevents oxygen from reaching the light-producing chemicals which are sequestered in the photocyte interior.
The situation changes dramatically when a nerve signal arrives and stimulates cells to begin producing NO. This gas quickly reaches the mitochondria clustered just inside the nearby photocytes, and brings the process of mitochondrial respiration to a screeching halt. No longer used up by the mitochondrial gatekeepers, oxygen can now pass freely through the photocyte to activate light production.
"So, amazingly enough," says Barry Trimmer, "it's a temporary cut in the power supply that probably triggers the firefly flash." As the NO signal decays, the mitochondria power up and begin consuming oxygen again. This turns the lantern off again--all of this happening in a fraction of a second.
Contact: Sara Lewis at 617-627-3548, sara.lewis@tufts.edu or Barry Trimmer at 617-627-3924, barry.trimmer@tufts.edu
The project's just-launched Web site can be found at: http://www.tufts.edu/home.php. (It is password protected and case sensitive. User ID is "firefly" and password is "biology")
Sidebar:
Firefly research: cross-disciplinary collaboration among colleagues, between spouses
A long-standing mystery about how fireflies control their flash was successfully unlocked by a research team made up of scientists from diverse life-science disciplines. The two-year project also brought two husband and wife scientists back together for their first collaborative research project in over 25 years.
The project's initial catalyst was Barry Trimmer, a widely published insect neurophysiologist at Tufts University in Medford, Massachusetts. Of his collaborators, says Trimmer, "each one brought a really unique perspective to the project." One co-author, Thomas Michel, is a cardiologist at Brigham and Women's Hospital known for his work on nitric oxide and cardiovascular signaling. Two other colleagues at Tufts University were June Aprille, an accomplished mitochondrial expert, and Sara Lewis, an evolutionary ecologist known for her previous work on firefly behavior. "Working together, we could think about the same problem from completely different angles," says Trimmer, "and it was this multi-faceted attack that ultimately helped us solve the mystery."
In addition to the research group's successful synergism, co-authors Michel and Lewis are a husband and wife team. "Since we're in such different research fields", notes Michel, who also teaches at Harvard Medical School, "we hadn't had a chance to work together scientifically since the mid-1970s." The two scientists last collaborated on a research project when they were Harvard undergraduates examining processes affecting the water quality of Squam Lake in New Hampshire. "When the chance to work on this firefly project came up" says Lewis, 'both of us were intrigued by the question--and excited about collaborating on it. It was all very serendipitous and a lot of fun."
Michel says the firefly project has brought their whole family together in scientific discussions. "This went beyond our usual debriefing at the dinner table," he notes. "Both of our children, aged nine and 13, helped out by catching fireflies and have become interested in learning about the insects' biology."
Contact information for the authors: Barry Trimmer: 617-627-3924 or barry.trimmer@tufts.edu, Sara Lewis: 617-627-3548, or slewis1@tufts.edu, Thomas Michel: 617-732-7376, or michel@calvin.bwh.harvard.edu, June Aprille: 617-627- 3323 or june.aprille@tufts.edu. The project's just-launched Web site: http://ase.tufts.edu/biology/Firefly
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