© 2004 by The Society for Integrative and Comparative Biology
Fireflies at One Hundred Plus: A New Look at Flash Control1
1 Department of Biological Sciences, The University at Albany, Albany, New York 12222
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 SYNOPSIS INTRODUCTIONMORPHOLOGY OF THE PHOTURIS...OXYGEN AS A POSSIBLE...PROBLEMS WITH THE OXYGEN...PEROXIDE AS THE AGENT...References |
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The mysterious process by which fireflies can control their flashing has inspired over a century of careful observation but has remained elusive. Many studies have implicated oxygen as the controlling element in the photochemical reaction, and the discovery of nitric oxide synthetase (NOS) in the lantern has suggested that nitric oxide (NO) may control oxygen access to the light-emitting photocytes, thereby triggering the flash. However, there are several drawbacks to oxygen as a controlling agent, and in view of the prominence of peroxisomes in lantern morphology and biochemistry, we suggest that it is hydrogen peroxide that triggers the flash, and we present a model by which this may take place.
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INTRODUCTION |
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SYNOPSISINTRODUCTIONMORPHOLOGY OF THE PHOTURIS...OXYGEN AS A POSSIBLE...PROBLEMS WITH THE OXYGEN...PEROXIDE AS THE AGENT...References |
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Any discussion of the biology of fireflies inevitably includes the mystery of their light production. Over the years a model of the biochemistry of firefly light production has evolved (see Wilson and Hastings, 1998
, for review), a model that has identified oxygen as the major control factor. In simple terms, the model states that firefly luciferin (the substrate) complexes with firefly luciferase (the enzyme) and ATP in the presence of Mg2+ to form an "active intermediate" that needs only oxygen to complete the photochemical reaction. With the addition of the oxygen, the active intermediate forms a cyclic peroxide that decomposes and in the process emits light. When biochemists mix these chemicals together, they typically produce a steady glow unless oxygen is added last, in which case one gets a flash that resembles that of the in vivo flash (DeLuca and McElroy, 1974). As a group, fireflies are more versatile: most can modulate their light to a greater or lesser extent (see Buck [1948] for a thorough overview of firefly anatomy and physiology), and some can actually flash, turning the lantern sharply on and off (Fig. 1). This singular ability has provoked over a century of careful morphological and physiological search for the elusive control switch, the event that pushes the photochemical reaction to completion. This paper reviews current models of flash control and adds to them a new twist: we review the biochemistry of the light reaction and ask if, rather than oxygen, it might be hydrogen peroxide that actually triggers the flash.
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Various lines of evidence (reviewed in Ghiradella, 1998
) have certainly supported the idea that flash control is essentially oxygen control. Stop-flow experiments (DeLuca and McElroy, 1974) demonstrated that the addition of oxygen to the preformed intermediate is sufficient to produce light with a speed (on the order of 60 msec) close to the rise time of lantern light. Within the light-emitting cells, or photocytes, the luciferin and luciferase are contained within peroxisomes (present in huge numbers), and not surprisingly, it is the peroxisome fraction of the cell that lights up. Peroxisomes are subcellular organelles noted for their enzymatic oxidation reactions, which are coupled to the formation of hydrogen peroxide from oxygen. They also contain catalase, which reduces hydrogen peroxide to water (either with hydrogens from coupled reactions, or without other substrates by simultaneously oxidizing and reducing the peroxide to oxygen and water). The lantern nerves, which appear to be modified spiracular nerves, do not synapse directly on the photocytes, but rather in the tracheal system, which in insects delivers air directly to the tissues. In short, the circumstantial evidence has pointed to oxygen as a major player in flash control. However, for a long time we have had no clear picture as to how molecular oxygen could be controlled or gated by a biological system.
Two recent studies have provided us with new possibilities. To understand these we must first review briefly the unique morphology of a typical flashing lantern, taking that of an adult Photuris as our model. We will concentrate on those structures known or believed to be involved in the flash and its control; readers wishing more comprehensive views of lantern morphology are urged to consult Buck (1948) for general lantern anatomy, Smith (1963) for an excellent general description of lantern ultrastructure in Photuris, and Ghiradella (1998) for a more recent review of lantern morphology and biochemistry and a detailed look at the tracheal system and its environs.
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MORPHOLOGY OF THE PHOTURIS LANTERN |
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Each lantern is a flat slab of tissue that consists of a dorsal and a ventral layer. The dorsal layer consists of large cells packed with granules whose nature is as yet undetermined (but which have been rumored by various authors to be urate, possibly because urate is a common feature of fat body from which the lantern appears to be derived). The ventral, or photogenic layer is the source of the light and is our concern here. Figure 2 presents a view in frontal section of the ventral layer of an adult Photuris lantern. The photocytes are arranged in "rosettes" around channels or "cylinders," each of which contains branches of the tracheal system and of the lantern nerve. Each photocyte abuts on at least two cylinders, so that it receives tracheal and nervous inputs at both ends. Those regions of the photocyte that front on the cylinders or on the tracheal extensions (see below) are distinctly different from the more interior parts of the cell and are called the "photocyte differentiated zones" (Fig. 3). They contain virtually all of the cell's mitochondria, as well as so-called "differentiated zone granules" (DZ granules), an as yet uncharacterized organelle that is also found deeper in the cell. The most striking feature of the cell's interior is the high concentration of peroxisomes, which virtually fill the cell to the exclusion of anything else other than the nucleus. As mentioned above, these organelles contain the enzyme and substrate for the light reaction, and it is this fraction of the cell that actually emits light.
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The lantern tracheal system is also highly specialized. All insect tracheal systems are essentially extensions of the outer exoskeleton, which invaginates into the interior, carrying with it its epithelial layer, which latter encases the tubes thus formed. Typical tracheae branch repeatedly until each branch is so fine that it is ensheathed in a single epithelial cell. This "terminal" trachea then gives rise to a spray of delicate tracheoles, all encased in the finger-like branches of a single tracheolar cell. The tracheoles are the capillaries of the system: they are permeable and are usually delicate and collapsible, despite the presence of taenidia, cuticular threads that usually wrap around respiratory tubes and presumably provide mechanical support. It is the tracheoles (and the attendant branches of their tracheolar cells) that front on and penetrate between the tissue cells to ensure that these have close access to gas exchange.
The firefly lantern tracheation is highly modified (Figs. 3, 4). It is perhaps the most extensive of any known tissue. The last of the tracheal epithelial cells (just upstream of the tracheolar cell) is now called the tracheal end cell (TEC); it is greatly enlarged and its luminal surface thrown into deep crypts lined with mitochondria. This is classic ion pump morphology, characteristic of tissues that move water by shifting such ions as sodium (see Smith, 1968, for review), and we will proceed on the assumption that this is its function, although this hypothesis has not been tested or confirmed. The TEC wraps around the tracheolar cell, which also has a hint of ion pump morphology, and the nerve ending is located between the two, embracing and (in our images at least) synapsing on the tracheolar cell. The transmitter is octopamine, common in invertebrate systems (Oertel and Case, 1976; Nathanson, 1979).
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The tracheoles, with their attendant cell branches, penetrate far between the photocytes and end blindly near the ends of their opposite numbers from the neighboring cylinder. As mentioned above, where the tracheal system abuts on or runs past a photocyte, it is accompanied by differentiated zone in the latter. Paradoxically, the intercellular gap between tracheal system and photocyte is large and seems to be reinforced by fibrous material, perhaps elements of basement membrane.
The cuticular elements of the lantern tracheal system are also highly specialized (Fig. 4). The tracheae are more or less standard, but between each of their terminal branches and its tracheoles lies an unusually featureless segment of tracheal tubing, the tracheal twig, which is lacking in taenidia and appears particularly flimsy and collapsible in whole mount. The tracheoles, in contrast, are uniquely elaborate, stiff and reinforced.
The lantern tracheal end cell, tracheolar cell and nerve ending, together with their enclosed plumbing, are collectively referred to as the "end organ." It seems that these specializations, together with the compartmentalized photocyte, are necessary for a lantern to flash, since only flashing lanterns have them all. We find a "control" lantern in the Photuris larva, which produces a glow that slowly rises and falls. (For a review of larval lantern ultrastructure and physiology, see Oertel et al., 1975.) The larval photocyte also has peroxisomes, mitochondria (and DZ granules), but these are not segregated from one another in different cellular compartments, nor is the tracheal system particularly specialized. The nerve synapses directly on the photocyte, rather than on the tracheolar cell. The biochemistry of the light reaction seems similar in larval and adult organs (Oertel et al., 1975), but these are not derived from one another and in fact coexist briefly at the time of eclosion, so it is not known to what degree they are homologous or how far conclusions drawn from one can be generalized to the other.
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OXYGEN AS A POSSIBLE AGENT OF FLASH CONTROL |
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The general acceptance of oxygen as the controlling factor for the adult flash inspired suggestions (reviewed in Ghiradella, 1998
, 2004, and Greenfield, 2001) as to possible mechanisms. All had features in common: they posited that the peroxisomes are charged with active intermediate that needs only a puff of oxygen to complete the flash reaction. The question then became what keeps the oxygen from entering the reaction prematurely, i.e., how does the interior of the photocyte stay anaerobic enough to avoid setting off the light reaction? On the basis of photocyte morphology, Ghiradella (1998, and in papers referenced therein) and before her, Dahlgren (1917, in a remarkably prescient paper) suggested that because of their deployment at the tracheal system/photocyte interface, the photocyte mitochondria might serve as "gatekeepers," absorbing and using any incoming oxygen to keep it from the peroxisomes until the nerve fires and the octopamine sets in motion a cascade of events that drops the barrier and allows the oxygen to reach the peroxisomes and complete the photochemical reaction.
Trimmer et al. (2001) provided a possible mechanism for this action when they realized that the gaseous transmitter nitric oxide (NO) might be the needed link between events at the synapse and the rise of the flash. In Figure 4 (center) the shaded areas (the TEC, the tracheolar cell and its extensions, and differentiated zones of the photocytes) represent sites of nitric oxide synthetase (NOS). In their model, Trimmer et al. suggested that the NOS is activated and produces NO in response to calcium transients raised by the action of octopamine at the cell surface. NO reversibly inactivates mitochondria by complexing mitochondrial cytochrome c oxidase, without which the mitochondria cannot bind oxygen. With the mitochondria "off," oxygen can diffuse through the photocyte differentiated zones to the peroxisomes. The authors extend the "gatekeeper" hypothesis to the TEC and suggest that its prominent mitochondria might serve as a first (outlying) line of defense against oxygen invasion until the NO in the TEC shuts down its mitochondria and allows additional oxygen access to the photocyte.
Built into this model are welcome additions to previously suggested mechanisms for flash termination. In vitro studies by McElroy and Hastings (1956) suggested that the flash might be self-limiting because the formation of active intermediate is much slower than the flash reaction itself. Trimmer et al. point out that NO degrades and diffuses away quickly, it is inhibited by oxygen (which will build up when the mitochondria cease absorbing it), and it is inhibited by light itself (Aprille et al., 2002, 2004). All these factors can be expected to contribute to flash termination.
The NO model does not, however, address the specializations of the tracheal system, notably the ion pump morphology of the TEC and tracheolar cell, the reinforcement of the tracheoles and the existence of the flimsy tracheal twig. A different group of investigators (Timmins et al., 2001) has focussed on these elements and revived an old osmotic model of oxygen control (Maloeuf, 1938) that is based on presumed shifts by the ion pump of tissue fluid in and out of the tracheoles, a process that mimics one that occurs generally in insect tissues as a result of metabolic activity (Wigglesworth, 1983).
In resting insect tissues, tracheoles are typically filled with liquid drawn by capillary action from the surrounding tissue. Metabolic activity raises tissue osmotic pressure, which in turn draws the liquid back from the tracheoles, filling them with air. Since oxygen diffuses more rapidly by a factor of 104 in air than in water, this allows much faster oxygen delivery to the needy active tissues. Timmins et al. studied the kinetics of oxygen diffusion in tracheoles under different conditions and concluded that the singular morphologies of the TEC and tracheolar cell suggest that they might respond to the octopamine by actively pumping tissue fluid out of the tracheoles, thereby clearing them for fast oxygen passage. The burst of incoming oxygen would overwhelm mitochondrial ability to absorb it and would reach the peroxisomes, initiating the flash. Contrary to the conventional wisdom that pumping/ osmotic mechanisms are slow, these authors assert that for such short distances the process could be fast enough to work within the required time frame. They also echo an older idea (Ghiradella, 1977) that reinforcement of tracheoles and intercellular gaps would keep the tubes and their attendant cells stable under shifting osmotic pressures.
The osmotic control model of Timmins et al. does not contradict or preclude an NO effect on the photocyte mitochondria. In a combined model, oxygen diffusing rapidly through air-filled tracheoles would arrive at a photocyte "opened" by NO shut-down of the mitochondrial gatekeepers and would quickly and effectively reach the peroxisomes. More troubling is the lack of mechanism connecting the octopamine with the osmotic activity at the TEC. If NO shuts down photocyte mitochondria, it should also shut down TEC mitochondria, presumably turning off the pump. There may be enough stored ATP to run the pump temporarily (J. R. Aprille, personal communication), but we are then left with the question of what difference the octopamine makes in TEC function, i.e., why this cell has NOS if the pump is on regardless of the presence or absence of NO. It is more likely that TEC pumping is important when the lantern is off or that it may serve a function not directly related to control of individual flashes.
The crypts of the TEC are oriented towards the tracheal twig. On the basis of this morphology Ghiradella (2003) revived an old suggestion that in the tracheal twig the system might have a mechanical valve which could open or close in response to changes in the TEC. (The reinforcement of the intercellular gap and the tracheoles would then be against mechanical as well as osmotic stress.) In whole mounts (Fig. 4) the twig is always collapsed shut, but in fixed and sectioned material, it always appears open. This latter fact is not necessarily informative in itself, since fixed material is subject to shrinkage, but the twig is typically surrounded by a calyx of fibrils which appear to come from the crypts of the luminal surface of the TEC and which may bind TEC and twig together. Osmotic changes in the region should swell or shrink the TEC, and this in turn might be a mechanism for opening or shutting the tracheal twig (and facilitating or delaying the passage of oxygen). But we still need to know why and under what circumstances.
Some additional special features of the lantern deserve mention. It has an extremely potent octopamine-sensitive adenyl cyclase (Nathanson, 1979), which appears to be localized in the tracheal system and which produces the highest cAMP levels recorded for any insect (Zeng et al., 1996). Case and Strause (1978) point out that although the adult lantern has relatively sparse innervation, action potentials are easily recorded from the surface, suggesting that another component—perhaps the tracheolar cells—may be capable of generating spikes. Certainly these cells, with their long branches, their close associations with the photocyte differentiated zones and their ability to make NO, are well situated to carry the message, whatever it may be, deep into the photocyte interiors. But all these specializations must be incorporated into any model of flash control.
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PROBLEMS WITH THE OXYGEN CONTROL THEORY |
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If external oxygen is a simple trigger, a cut or break in the lantern cuticle or surface should light up the whole lantern as oxygen rushes in through the break. This does not happen: unexposed parts of a cut or crushed lantern do not glow (Dahlgren, 1917
), and a cut part glows only locally and quickly extinguishes (H. Gh., unpublished data). Other studies (Hastings and Buck, 1956; Carlson, 1965) suggest strongly that the oxygen supply within the lantern does not limit or control the glow. Furthermore, the lantern is not in any way isolated from the rest of the body. It floats in circulating hemolymph that is presumably oxygenated from non-lantern tracheal sources. It seems clear that the photocyte has substantial local control of its luminescence and is not dependent on external oxygen in any simple way. Let us take a closer look at this cell, and in particular at the peroxisomes that fill it so robustly.
Implicit in the whole oxygen control model has been our assumption that the photocyte must work hard to keep its interior anaerobic between flashes. This has seemed to be a unique and exotic qualification, but as Lane (2002) points out, all eukaryotic cells are functionally anaerobic. Indeed, mitochondria may have originally evolved to "compartmentalize" respiration, keeping the oxygen levels in the cytosol just high enough to support oxygen demands while minimizing the exposure to potentially dangerous reactive oxygen species (ROS) (Abele, 2002). In other words, any healthy cell can presumably keep itself safe from oxygen overload and the cytotoxic species (superoxide, peroxynitrite, hydrogen peroxide and [more recently described] ozone, dihydrogen trioxide, and the hydrotrioxy radical) that are likely to result from it. Given the great ability of biological systems to build on and modify common systems for exotic purposes, if oxygen levels needed to be especially manipulated, one would expect some sort of mitochondrial elaboration to provide the needed control.
But it is the peroxisomes in the lantern that light up, and while they come in a variety of forms, peroxisomes have a specific talent—they can manipulate peroxides. They typically form hydrogen peroxide and superoxides, which they generate through various peroxidative reactions, and they then use catalase, the one enzyme common to them all (de Duve, 1969) to break the hydrogen peroxide down to oxygen and water. Furthermore, they do this safely, i.e., without production of free radicals (Lane, 2002). Theoretically, photocyte peroxisomes could store oxygen in the form of hydrogen peroxide, liberating it as needed through the action of catalase. But NO inhibits catalase (Brown, 1995) and therefore precludes this release of oxygen, which inhibition might lower oxygen levels somewhat, even as oxygen coming in from the tracheoles would raise them. In other words, in the active lantern we would have forces working in opposing directions, which hardly seems an effective way of initiating an explosively fast chemical reaction (the rise of the flash in the insect can be even more rapid than that achieved with a step pulse [something a biological system cannot produce] of oxygen in a stop-flow apparatus).
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PEROXIDE AS THE AGENT CONTROLLING THE FLASH |
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If we assume that in the intact lantern the activating species is not oxygen but hydrogen peroxide, things look very different. The various peroxisome oxidases (urate-, D,L amino acid-, glycolate-, and L-hydroxy acid oxidases) would use the burst of external oxygen arriving through the "open" mitochondria to make hydrogen peroxide, and the shutdown of catalase would allow this compound to build up explosively, completing the photochemical reaction and raising the flash. We need now to revisit the chemistry of the lantern light reaction and ask if hydrogen peroxide in fact can serve as this trigger.
We propose the following speculative scenario (Fig. 5): The release of octopamine at the synapse both depolararizes the tracheolar cell to fire an action potential and activates the octopamine-sensitive adenyl cyclase in the end organ complex to form cAMP. The action potential depolarizes the tracheolar cell and all its branches, deep between the photocytes, opening voltage dependent calcium channels (VDCC) in the cell membrane. The cAMP acts on protein kinase A to phosphorylate calcium channels and augment the calcium influx (Nishiyama et al., 2003). The resulting rise in intracellular calcium activates the NOS to produce NO throughout the whole tracheolar cell. The NO diffuses rapidly into the photocyte, where it shuts down the mitochondria, opening the photocyte interior to incoming oxygen. But it also reaches the peroxisomes where it shuts down the catalase. This is possible because the Ki for catalase inhibition (180 nM) is even lower than that for cytochrome c oxidase inhibition (270 nM: Brown, 1995). Hydrogen peroxide levels rise sharply, abetted both by peroxide formation from the incoming oxygen and by the inhibition of catalase. We propose that it is the rise in the highly reactive peroxide that completes the photochemical reaction. Indeed, one might expect such a fast burst of hydrogen peroxide production from the action of another common peroxisomic enzyme, urate oxidase (assuming that the lantern is rich, as it may be, in urate inherited from its ancestral fat body).
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In this model, the end organ and tracheolar cell branches are in fact serving at least three purposes: they are delivering oxygen, they are carrying excitation rapidly along their lengths, and they are making and broadcasting NO throughout the lantern. These functions can be related to the physiological differences between species. Carlson (2003)
has demonstrated effectively that two species, Photuris versicolor, which has a compound flash, and P. lucicrescens, whose flash is a slowly rising crescendo, respond quite differently to similar nervous stimuli and to various high K+ and Ca2+ solutions. He concludes that there must be a fundamental difference at the photocyte, possibly involving voltage-dependent calcium channels (VDCC). He also points out that Ca2+ depolarizes mitochondria and could presumably function directly in liberating oxygen from them as a result of octopamine action. We suggest that the tracheolar cell may be another candidate for this level of control: if it is in fact capable of spiking, its response to synaptic activity may differ in the different species, with concommitant differences in the flash pattern.
We can also further provide for flash shut-down. As Trimmer et al. (2001) note, nitric oxide will diffuse out quickly, its binding to cytochrome c oxidase is inhibited by rising oxygen levels (caused by its inactivation of mitochondrial cytochrome c oxidase), and it is inhibited by light, all of which will render it inactive. In addition, catalase can, in the presence of hydrogen peroxide, degrade NO (Brown, 1995). As NO drifts off and/or is degraded, oxygen levels in the peroxisomes will fall, and catalase activity in the peroxisomes will rise, with the result that peroxide levels will plummet, allowing the system to revert back to its quiet state where it will presumably make more ATP and break down any reaction products (oxyluciferin, AMP, CO2) that have accumulated.
All this depends, of course, on the effectiveness of hydrogen peroxide as the trigger for the photochemical reaction. The proposed chemistry of the peroxide-stimulated light reaction is shown in Figure 6. The luciferin-AMP intermediate (LH2-AMP) that was shown to be present before the flash is activated by peroxide/ hydroxyl radical attack on two hydrogens, first at the usual C4 carbon (site of postulated proton extraction in the oxygen model; H2O2 removing H· to make H2O and ·OH) and at the important 6'OH (luciferin with a methyl group substituted here does not emit light: McCapra, 1970). Although this would nominally leave behind radicals at both points, the formation of these is averted by a concerted rearrangement of the double bonds to give a neutral, conjugated L-AMP (dehydro form 1 of Fig. 6). Note that even the carboxyl double bond is now included in the conjugated system. Occasional formation of the classical (unrearranged) dehydro form 2 is unproductive for light formation (McElroy and DeLuca, 1978). The L-AMP intermediate and a form without AMP apparently can dissociate from luciferase before they emit light (Min and Steghens, 1999; Brau et al., 2000). We envision further steps of peroxide attack at the double bond and formation of a peroxy-bridge before driving off AMP, and later decomposition to the ketone, producing CO2 (as in the oxygen model). The difference is that this leaves the new ketone double bond unconjugated with the rest of the system. Simultaneous protonation at the 6'O and deprotonation of the nitrogen give rise to electron shifts associated with a rearrangement back into the original configuration of conjugated double bonds, but with the favorable addition of the ketone double bond to the conjugated system. (Alternatively this can be thought of as going to the enol structure [N=C-OH], which would rearrange after deprotonation of the hydroxy group.) This model does not preclude the reaction of molecular oxygen facilitated by the luciferase, as obviously happens in the lab. But peroxide activation, rather than oxygen activation, could explain the extremely rapid rise of the flash in the insect.
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In presenting this admittedly speculative model we have wished to launch a new direction in thinking about flash control in the firefly. We believe that we have made a strong case for hydrogen peroxide as the key agent in flash control, but we acknowledge that the system might not be that simple. Current research is showing that oxygen can exist in more forms than have previously been suspected (Hoffman, 2004
) and that biological systems can handle and control many of them (Lerner and Eschenmoser, 2003; Wentworth et al., 2003). Besides singlet oxygen and hydrogen peroxide, already implicated in the firefly flash reaction, these "manageable species" include ozone, dihydrogen trioxide (H2O3), and the hydrotrioxy radical (HO3·); in antibody action, this last may produce or may mimic the action of the notoriously toxic HO· radical. The field is advancing quickly, and as new knowledge accumulates, we shall almost certainly have to revisit the question of reactive oxygen species in firefly flash control. Several additional questions remain. What of the TEC and its complicated morphology? As mentioned above, it could serve as a mechanical valve, helping to hold oxygen out of the lantern between flashes. Alternatively, it may be doing something similar on a longer time scale. A "daytime" firefly cannot flash when it is first awakened: there are seconds or even minutes of disorganized lantern activity before the system can begin to produce coordinated flashes. This has been generally interpreted as a need for some sort of neural priming, but it may be that a physiological adjustment needs to be made, such as (for example) the TEC turning on the pump and clearing the tracheoles of tissue fluid in preparation for the coming lantern activity. The presence of NOS in the TEC may also be related in some as yet unknown fashion to the "off" process in the flash.
We need to know more about the other enzymes in the lantern peroxisomes. As a class peroxisomes have been found to have a huge variety of oxidases, many of which have been selectively shed in various lineages so that today's organelles are more or less tailored for their specific environments and functions. As mentioned above, urate oxidase may be of particular interest here: we need to determine if urate is actually present in the lantern and being used to form the hydrogen peroxide that we are proposing as the control for the light reaction.
We need more studies such as that of Carlson (2003) into control mechanisms for extended, compound and multiple flashes (Fig. 1). We believe that we can safely assume that the underlying biochemistry of the flash reaction is conserved among firefly species, but the details of flash patterning (especially in those species with complex flash repertoires) add a level of complexity that must ultimately be explained by any model of flash control.
Finally, drawing on this model, can we say anything about the function of the larval lantern? We do not know if there is NOS somewhere in the larval system. There certainly is no end organ with its specialized morphology. The photocyte membrane is highly convoluted and receives many nerve terminals whose liberation of octopamine leads to a slow depolarization that may involve cAMP; repeated stimuli facilitate the light response (Oertel and Case, 1976). Unlike the adult, which can glow for several minutes in a nitrogen atmosphere, the larva can do so for only a few seconds (Carlson, 1965), which has been interpreted to mean that it has limited oxygen stores in the lantern. However, if we think peroxide rather than oxygen, we can suggest that lacking extensive tracheation to bring in oxygen for mitochondrial function at rest and the generation of peroxide during the flash, the larval peroxisomes may have relatively low peroxidatic activity and so be unable to accumulate enough peroxide rapidly enough to mount a flash.
We hope that our model will provoke further thought, especially in light of current research into the complexity and versatility of biological control of oxygen in its many forms and compounds. We can think of no more exciting or rewarding quest than that to understand how a biological system can evolve such ability to control the release of stored light.
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ACKNOWLEDGMENTS |
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First and foremost, we are most grateful to John Buck, who first introduced one of us (H. Gh.) to the complexities of the firefly lantern and who, together with his wife Elizabeth, has been a constant source of help and support. We thank James Case, Al Carlson, Jon Copeland, J. W. Hastings, and the rest of the firefly community for decades of sharing data, interpretations and general insights. Helmut Hirsch has provided help, insight and critical evaluation of ideas. And finally, we thank a host of colleagues—John Buck, Thomas Eisner, Helmut Hirsch, J. W. Hastings, Su Tieman, and our anonymous "Reviewer No. 1"—for their careful review of the manuscript and for their thoughtful and constructive suggestions. Supported in part by NSF grant PCM 78-22924 and SUNY Research Foundation grant 320–7328A, both to H. Gh.
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FOOTNOTES |
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1 From the Symposium Flash Communication: Fireflies at Fifty presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 4–8 January 2003, at Toronto, Canada.
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References |
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