Missing link in firefly bioluminescence revealed: NO regulation of photocyte respiration.

Summary Sexual communication in most species of fireflies is a male‐female dialogue of precisely timed flashes of bioluminescent light. The biochemical reactions underlying firefly bioluminescence have been known for 30 years and are now exploited in biomedical assays and other commercial applications. Several aspects of flash regulation are also understood: flash rhythm is controlled by a central pattern generator, and individual flashes are neurally triggered, with octopamine serving as the transmitter. The molecular oxygen needed by the biochemical reactants is delivered by a network of tracheal arborizations extending throughout the light organ (lantern). However, the actual means by which oxygen quickly reaches the reactants packaged within specialized photocytes and the specific event(s) triggered by neural action have not been identified; termination of axons away from the photocytes has exacerbated the latter problem. A recent paper (1) by a consortium of cell and evolutionary biologists, however, reports that nitric oxide (NO), manufactured and released in response to neuronal discharge, is the missing link by which neural action in the firefly lantern yields a sudden flash of light. BioEssays 23:992‐995, 2001. fl 2001 John Wiley & Sons, Inc. Bioluminescence and bioluminescent signaling Bioluminescence is generated by different forms of a chemical process, chemiluminescence, and has evolved independently on over 30 occasions in organisms distributed among four kingdoms. (2) It occurs in nine metazoan phyla and is most prevalent in marine species. The bioluminescence in some fishes and cephalopods is produced by symbiotic bacteria, but in most bioluminescent animals, is generated intrinsically. In fireflies (Coleoptera: Lampyridae), the process occurs in a specialized organ comprising the two terminal abdominal segments. (3,4) Chemically, bioluminescent processes are significant because the light radiated does not depend either on high temperature (incandescence) of the radiating substance or on its prior absorption of light (fluorescence). The chemical reactions that generate bioluminescence are all based on oxidation of a substrate, generically termed luciferin, that requires catalysis by an enzyme, luciferase. (5) Molecular oxygen typically serves as the oxidant. Additional substances or factors, e.g., ATP, are generally needed to complete the reaction. Comparative studies show that luciferins are heatstable compounds of relatively low molecular mass. (2) Possibly derived from pigment molecules, they possess the critical property of emitting a photon when oxidized to an electronically activated state. The luciferin of various firefly species is luciferyl adenylate, a benzothiazole that is oxidized and then modified to a carbonyl compound that emits light while still enzyme-bound. Luciferases are more variable (molecular masses range from 21,000-420,000) than luciferins among bioluminescent organisms and tend to be speciesspecific. (2) Their structure and activity suggest that they are derived from mixed-function oxidases operating in basic metabolic processes. The specific structure and configuration of an organism’s luciferase may influence the energy of the photon released, and hence the spectrum of its bioluminescence. Hues of bioluminescence among firefly species range from 550 to 580 nm. The vast majority of bioluminescent organisms simply glow