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Image: Firefly video
Click on the video button to watch a Tufts University laboratory demonstration of how nitric oxide affects the brightness of a firefly's lantern.
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Science

It’s a sultry summer evening, and the fireflies are engaging in a little old fashioned courting as they flit through the dusk, using the brightly glowing lanterns in their abdomens to flash a kind of lovers’ Morse code. Even among the fireflies, communication between the sexes is crucial: Courtship depends on these precisely timed and patterned flashes. So how do fireflies achieve such control over their on/off switches? A unique experiment in Friday’s issue of the journal Science suggests that nitric oxide gas may be the key to these bursts of brilliance.

Nitric oxide, best known as a common pollutant found in car exhaust, took on new scientific significance in 1998 when three researchers won the Nobel Prize in Medicine for discovering that nitric oxide can also transmit signals between cells in the heart and blood vessels. This was the first clue that a gas could act as a signaling molecule in an organism. Since then, scientists have shown that nitric oxide is used to control blood pressure, fight off invading bacteria, and communicate between nerve cells.

Since nitric oxide is also an important signaler in insects as well, the Science researchers, led by Barry A. Trimmer of Tufts University, wondered if the gas might play some role in firefly flashing.

Anatomy of a flasher
Researchers already know what the firefly lantern looks like on the inside, and they understand the basics behind the lantern’s flash. The firefly lantern consists of several layers of reflector cells and a single layer wallpapered in thousands of cells called photocytes, arranged in rings around the ends of branching air tubes. In the photocytes, small organs called peroxisomes contain a light-producing brew of the protein luciferin, the enzyme luciferase, and the molecular fuel ATP.

When oxygen is added to this mix, it touches off a chemical reaction that eventually releases energy in the form of light. If you ever had a bratty brother who squashed fireflies during those summer bug hunts, you probably saw this phenomena up close: the smashed lantern’s contents were suddenly exposed to the rush of oxygen in the air, causing a short-lived but very intense glow.

Triggering this reaction in a working lantern is a little more complicated. Scientists think that cellular energy factories called mitochondria, densely packed all around the peroxisomes, act as gatekeepers that control oxygen’s access to the light-producing organs. To light the lantern, however, the firefly’s nervous system sends a signal to cells at the very tip of the air tubes threaded throughout the lantern. So how does this “flash now” message make its way from the airway cells, through the gatekeepers, and finally arrive at the peroxisomes themselves?

Message to mitochondria
Suspecting that nitric oxide might be the mysterious messenger, the Science researchers devised an experiment to test their hunch. They placed individual fireflies inside tiny observation chambers supplied with oxygen, and introduced nitric oxide gas into the chambers. The nitric oxide caused almost immediate flashing, and most of the fireflies kept their lanterns on at a steady glow. With the help of a reactive stain applied to dissected firefly lanterns, Trimmer and colleagues confirmed that the cells at the air tube ends and the mitochondria are hotspots of nitric oxide activity.

Image: Nitric oxide activity
A photomicrograph of a firefly's light-producing lantern clearly shows a flowerlike pattern of reactive stains in the cells around each air tube, indicating that nitric oxide was present.
These experiments helped the researchers figure out how nitric oxide fits into the light-producing reaction. They think that the nerve cell’s “flash now” signal triggers cells around the air tubes to produce nitric oxide. The nitric oxide gas then makes its way to the gatekeeper mitochondria, where it shuts off the mitochondria’s oxygen intake.

Since mitochondria are “quite famous as oxygen-consuming power plants,” says Science co-author June Aprille of Tufts University, the sudden “smothering” of the mitochondria by nitric oxide frees up a lot of oxygen — oxygen that can now enter the peroxisomes and ignite their illuminating contents.

“So amazingly enough, it’s a temporary cut in the power supply that probably triggers the firefly flash,” says Trimmer.

The research team thinks that the lantern winks out when the nitric oxide is degraded, but the exact details of this “off” switch are still a mystery. Scientists have shown that white light can reverse the stifling effects of nitric oxide on a key respiratory enzyme in mitochondria, so the lantern’s flash may contribute to its own off signal, the Science authors suggest.

© 2013 American Association for the Advancement of Science

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