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rhythmic respiratory behaviour referred toas the discontinuous gas-exchange cycle(DGC)4. In insects exhibiting DGC, the spir-acles close for long periods (up to severalhours or even days) and open occasionallyfor only a few minutes. This unusual respira-tory pattern has been observed in many adultinsects, as well as in resting butterfly andmoth pupae. Two main hypotheses havebeen proposed to explain why some insectsdisplay DGC: to reduce water loss throughthe spiracles, or to adapt to an undergroundlifestyle. But these ideas were disproved oncloser inspection because DGC could beassociated with neither the humidity5 northe carbon dioxide concentration6 of theenvironment.

Hetz and Bradley1 propose a differenttheory to explain DGC, and their hypothesishas far-reaching implications for how weview animal respiration. They provide com-pelling evidence that insects use DGC not toacquire but to avoid oxygen.Using the pupaeof the moth Attacus atlas as a model system,the authors varied the environmental oxy-gen concentrations from partial pressures of5 to 50 kPa (the normal atmospheric oxygenpartial pressure at sea level is about 21 kPa).Nevertheless, the intra-tracheal oxygen levelsin the resting pupae remained low, close to 4 kPa, across the whole range of partial

An aurora is not just a visual treat forEarth’s latitudinally advantagedresidents. These events also providenatural laboratories for physicistsinvestigating the complex interplay ofelectromagnetic waves and ionizedparticles in plasmas. And we are notmerely passive observers of suchphenomena — we have the capabilityof manipulating these processes fromthe ground, as shown to strikingeffect by Todd R. Pedersen andElizabeth A. Gerken elsewhere in thisissue (Nature 433, 498–500; 2005).

The playground for theseluminous processes is theionosphere — the ionized upperreach of the atmosphere thatstretches from a height of around 100 km to the base of the magnetosphere far above. InEarth’s polar regions, the geometryof the geomagnetic field is such that electrons and ions canoccasionally be driven down from on high: an aurora (pictured) is the visual manifestation of the collision of these energeticparticles with gases in the upper atmosphere and ionosphere.

With this basic understanding of the mechanism in place,researchers have shown previouslythat it is possible to induce suchoptical processes artificially. Bypumping high-power radio wavesinto the ionosphere (the frequency of the waves being tuned to thelocal plasma environment), electronscan be locally energized to collidewith atmospheric gases in a manneranalogous to the natural auroralprocess. But the resulting opticaleffects are small, with emissionintensities falling well below thedetection limit of the human eye.

Now Pedersen and Gerken haveshown that, if ionospheric conditionsare just right, much strongeremissions can be generated by this approach — so strong, in fact,that they are in principle visible tothe naked eye. The ‘trick’ underlyingthis demonstration was to choose a time when a natural aurora wasalready active (previous efforts were directed at quiet regions of the ionosphere). And rather thantargeting the main ionospheric layer,the researchers tuned the radiowaves to excite a much lower layer,which had been transiently ionized

by the inbound charged particles.These observations raise

many questions about the processes involved. For example, isan active aurora really a prerequisitefor generating such brightemissions, or is it instead largely a picturesque bystander? Should the answer turn out to be the latter, we are left with the tantalizing(some would say disconcerting)possibility that such radio-fuelledemissions could form the basis of a technology for urban lighting, celestial advertising, andmore... Karl Ziemelis

Atmospheric physics

Seeing the light

Physiology

A welcome shortage of breathThorsten Burmester

The respiratory systems of animals must guarantee an efficient oxygensupply. But it seems that, in some insects, they have evolved to restrict the flow of oxygen too.

L ike most other animals, insects need to inhale oxygen and get rid of carbondioxide. Oxygen fuels energy produc-

tion in the cells’power plants, the mitochon-dria,and carbon dioxide is released as a wasteproduct. On page 516 of this issue, Hetz andBradley1 show that, contrary to what mightbe expected, the insect respiratory systemmay limit rather than assist the uptake ofoxygen.

In insects, the exchange of gas with the atmosphere is restricted by a mostly impermeable and inflexible outer layer —the cuticle. Therefore, insects have smallopenings called spiracles in their cuticle.These are connected to the inner organs by a system of highly branched, gas-filledtubes called tracheae2 (Fig. 1, overleaf).Oxygen uptake and carbon dioxide releaseby the cells mainly occur at the tips of thesmallest branches. In highly active organs,

such as flight muscle, the tracheal endingscan even enter the cells and reach the mitochondria directly.

A common misconception is that theinsect’s tracheal system is a very inefficienttransport pathway. In fact, oxygen and carbon dioxide are respectively deliveredabout 200,000 times and 10,000 times fasterin tracheal air than in the aqueous environ-ment of the blood2,3. Therefore, simple diffusion through the tracheae would proba-bly be sufficient to supply adequate oxygenand remove carbon dioxide waste even in thelargest insects known historically (for exam-ple, the dragonfly Meganeura monyi, whichlived about 280 million years ago and had awing-span of 70 cm).

The spiracles in the cuticle behave likevalves, opening and closing to allow orrestrict the insect’s gas exchange. Physiolo-gists have long been puzzled by a peculiar

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472 NATURE | VOL 433 | 3 FEBRUARY 2005 | www.nature.com/nature

pressures. Thus, the moth pupa limits theamount of oxygen taken in by keeping thespiracles closed for as long as possible, andopening them only to get rid of the accumu-lated carbon dioxide.

At first glance, the idea that an air-breathing animal should try to limit apparently normal oxygen levels seems per-plexing.But oxygen is a double-edged sword:although required to fuel energy production,it is also a potent source of toxic compoundsknown as reactive oxygen species (ROS),which can damage proteins, DNA andlipids7. In recent years, ROS have been recog-nized as a major threat to cell survival, andtoxic ROS effects are suggested to underlieageing and cell death. Therefore, it is advan-tageous to keep cellular oxygen levels justhigh enough for efficient mitochondrial respiration, and as low as possible to mini-mize oxidative damage. Obviously, the critical oxygen concentration in moth pupaeis far below the normal atmospheric level ofabout 21%. This is probably true for otheranimals too; for instance, quite low oxygenlevels (0.4–5 kPa) are also found in mam-malian tissues8.

But if atmospheric oxygen concentra-tions are toxic to resting pupae, why aren’tthey noxious to the insects that rarely ornever close their spiracles? The answer prob-ably lies in differences in metabolic activity.The insects’ tracheal system is well designedfor efficient oxygen supply during periods ofhigh activity, when oxygen never accumu-lates to critical concentrations in the cellbecause it is rapidly converted into water bythe respiratory chain. However, in periodswhen respiration falls, such as in the restingbutterfly pupa, oxygen consumption is toolow to prevent it building up to harmful levels. It seems that a particular breathingpattern, the DGC, has evolved to ensure oxygen homeostasis. ■

Thorsten Burmester is at the Institute of Zoology,

RNA interference

Methylation mysteryMichael Ronemus and Rob Martienssen

Tiny RNA molecules called microRNAs are important in development,and are thought to function by causing the degradation of matchingmessenger RNAs. That may not be their only mode of action, however.

RNA molecules come in various sizes,ranging from the very long to the veryshort. The smaller RNAs fall into two

major classes: microRNAs (miRNAs), whichguide development in an organism by regu-lating target genes1; and small interferingRNAs (siRNAs), which target viruses,inserted genes and mobile genetic elements— a significant function being defence of thegenome2. One way in which siRNAs work isby guiding the modification (by methyla-tion) of DNA strands, as well as the modifi-cation of the histone proteins around whichDNA is wrapped, thus silencing gene expres-sion3. Writing in Developmental Cell, Bao et al.4 suggest that miRNAs might also —contrary to expectation — contribute toDNA methylation.

At a casual glance, miRNAs and siRNAsdon’t seem all that different. Both are short,single-stranded RNA molecules, generally21–22 nucleotides in size. Both are processedfrom true or transiently double-strandedprecursor RNA molecules, by specializedenzymes called Dicers.And both programmethe activity of Argonaute proteins in RNA-

induced silencing complexes (RISCs)5.But look more closely, and it seems that

any similarity is nothing more than a conse-quence of an ancient common origin. AnmiRNA precursor originates from a non-protein-coding gene and is processed in amulti-step pathway that is coupled to itsexport from the cell nucleus. Once chan-nelled to the cytoplasm,the miRNA interactswith a target messenger RNA (mRNA), viathe RISC, to trigger the destruction of themRNA,or to prevent it from being translatedinto protein1. This, then, effectively silencesthe gene that encodes the mRNA.

In contrast, the precursor of an siRNA isany double-stranded RNA molecule, a fea-ture that allows for multiplication of theoriginal siRNA through reiterative dicingand synthesis6. siRNAs target complemen-tary RNAs for degradation and also influ-ence the modification of repetitive DNA and the proteins that package it. Adding to the differences between the two RNAs,certain miRNA sequences are conservedthroughout the animal kingdom or betweendistantly related plant species,whereas siRNA

University of Mainz, Müllerweg 6, D-55128 Mainz,Germany.e-mail: burmeste@uni-mainz.de1. Hetz, S. K. & Bradley, T. J. Nature 433, 516–519 (2005).

2. Kestler, P. Environmental Physiology and Biochemistry

of Insects (ed. Hoffmann, K. H.) 137–186 (Springer, Berlin,

1985).

3. Krogh, A. Pflügers Arch. Ges. Physiol. 179, 95–120 (1920).

4. Lighton, J. R. B. Annu. Rev. Entomol. 41, 309–324 (1996).5. Quinlan, M. C. & Hadley, N. F. Physiol. Zool. 66, 628–642

(1993).6. Gibbs, A. G. & Johnson, R. A. J. Exp. Biol. 207, 3477–3482

(2004).7. Halliwell, B. & Gutteridge, J. M. C. Free Radicals in Biology and

Medicine (Oxford Univ. Press, 1999).8. Vanderkooi, J. M., Erecinska, M. & Silver, I. A. Am. J. Physiol.

Cell Physiol. 260, C1131–C1150 (1991).

Figure 1 Insect breathingapparatus. Left, the trachealsystem of the beetle Zophobasrugipes; below, a spiracle ofthe same species.

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