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2001 > October
Scientists crack firefly communication code
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 so precisely.
An interdisciplinary team of researchers from Tufts University and Brigham & Women’s Hospital have joined forces to solve this mystery of nature. They found that a deceptively simple molecule, nitric oxide, plays a key role in controlling the firefly flash. Their work was reported in the June 28 issue of the journal Science.
“We knew about the chemistry that makes fireflies light up,” says Barry Trimmer, lead author of the study and an insect neurobiologist at Tufts. “But we now have the missing piece of the puzzle that explains how fireflies are able to throw the switch on and off.”
In humans, nitric oxide (NO) acts as a messenger throughout the body, controlling blood flow as well as mediating learning and memory. According to Thomas Michel, a cardiologist at Brigham & Women’s and study co-author, NO is “the smallest molecule known to carry messages between cells, and it is a gas.”
Many species, 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, “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 develop an elaborate courtship system based on flash communication.
“Fireflies are very romantic,” says Lewis, “because their whole adult life is spent courting.” There are hundreds of different firefly species, she says, and each uses 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.
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 a 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 fireflies into a tiny custom-designed chamber and exposed them to oxygen and nitric oxide gas. To the surprise of the scientists, whenever the fireflies were exposed to nitric oxide, they glowed or flashed almost continuously. When the nitric oxide was turned off, they stopped.
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 to see where the NO-producing enzyme might be located. The enzyme was discovered in a strategic location—right next 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 that are sequestered in the photocyte interior.
The situation changes dramatically when a nerve signal arrives and stimulates cells to begin producing NO. The 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 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 happens in a fraction of a second.
For more information about the firefly research, visit the web site www.ase.tufts.edu/biology/Firefly.