July 2009 Featured Paper

"Nitric Oxide and the Control of Firefly Flashing" by Barry A. Trimmer, et al., Science, June 29, 2001, Vol. 292, 2486 - 2488.
Read the Original Paper (free registration required).

Synopsis

"Bioluminescent flashing is essential for firefly reproduction, yet the specific molecular mechanisms that control light production are not well understood." This study suggests that the chemical nitric oxide (NO) plays an important role in the control of the firefly flash.

Oxygen and Light Production

The firefly lantern contains light-producing cells called photocytes. Within these photocyte cells is a chemical called luciferin. With the help of the enzyme luciferase, luciferin breaks down into another chemical and, in the process, gives off energy in the form of light. However, this happens only in the presence of oxygen. When oxygen is not present in the cells, the reaction doesn't happen and no light is produced. It is believed that the regulation of oxygen in these cells is what regulates the flash. The question is, what regulates the presence or absence of oxygen?

The oxygen for this light production comes from the trachea — the breathing system of insects. The trachea continually branches into finer and finer tubes until they can supply oxygen to each cell in the insect's body. In the case of the light-producing photocytes, the oxygen passes into these cells from the trachea but is not available for the luciferin light reaction because it is immediately consumed by the mitochondria in the cell. With no available oxygen, there is no light. In order for light to be produced, the mitochondria must receive some signal to stop consuming oxygen — thereby making it available for the luciferin light reaction.

The nerves that control the firefly flash do not extend to these photocytes and cannot control whether the mitochondria are consuming oxygen. Rather, these nerves end at the ends of the trachea. Therefore, there must be some chemical that can pass from the end of the nerves, through the cell wall of the photocytes, and cause the mitochondria to stop consuming the oxygen, thus making it available for the light reaction.

Experimentation

Experiments in this study suggest that the gas nitric oxide (NO) is the chemical responsible for regulating the mitochondria, thus controlling the availability of oxygen needed for the light reaction. NO is an important chemical signal in the central nervous system and it can easily pass through cell walls. When fireflies were placed in a chamber with a steady flow of NO, they began flashing immediately, and many fireflies exhibited continuous light production. Other experiments showed that when NO was removed from the cells, no flashing occurred, even though oxygen was present.

Results

It is well understood that oxygen is the immediate trigger for light production, but this study suggests that NO plays a key role controlling the presence of oxygen in the cells by turning off the mitochondria's ability to absorb the oxygen, thus making it available for the light reaction.


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