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Firefly courtship is built on bioluminescence, but scientists are unsure what controls the glow. Fireflies are flashers, and their abdominal lantern, or light-producing organ--permeated by nerve cells and equipped with a trachea--is made up of cells containing luciferin, a protein that emits light in the presence of oxygen. Just inside the: walls of these cells, called photocytes, are masses of mitochondria--the cells' power, houses--which need oxygen to do their work. Normally they grab hold of any incoming oxy, gen. Until recently, no one knew how oxygen could pass deep into the photocytes, where the luciferin is, and cause the cells to glow.

Barry Trimmer, of Tuffs University, and colleagues became interested in the process after noticing similarities between caterpillars' brain cells, which use nitric oxide (NO) as a signal and the cells controlling the fireflies' lantern. They found that exposing live fireflies to NO gas caused the lantern to glow continuously. Many insects synthesize NO for use as a neurotransmitter; whether this molecule signals the fireflies' photocytes to glow wasn't dear. Even when the insects' nervous system was removed and the lantern exposed to NO, the organ still glowed--indicating that NO gas was not causing nerves to fire. Introduction of octopamine, a known firefly neurotransmitter, caused flashing, which was stopped by adding a substance that absorbed the NO. The scientists speculate that NO bonds more readily to the mitochondria than oxygen does; unable to bond there, oxygen drifts to the organelles that house the light-emitting luciferin. As for why the flash stops, they propose that either the increased oxygen accelerates degradation of NO or that the flash of tight may reverse the bonding of the gas to the mitochondria. ("Nitric Oxide and the Control of Firefly Flashing" and "NO Helps Make Fireflies Flash," Science 292, 2001)

COPYRIGHT 2001 Natural History Magazine, Inc.
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