plane6,7. But if the S paths were significantlyinclined with respect to the shear plane, theobservations could possibly be matched1.Roughly horizontal shearing of perovskiteassociated with a deforming Tonga slab8 maytherefore explain the S-wave observations ofWookey et al.1 as well as the apparent absenceof large splitting from the lower mantle inSKS and ScS phases that travel almost verti-cally beneath the Tonga subduction zone9,10.

The work of Wookey et al.1 and McNamaraet al.5 suggests that strong anisotropy may bea common feature of regions where subduct-ing slabs enter the lower mantle. Accuratelymodelling the full waveform interaction ofseismic phases with anisotropic structure,rather than just their arrival times, is a formi-dable task. But applying such approaches to waves that sample the mid-mantle couldhelp to explain the dynamics of mantle convection. ■

Karen M. Fischer is in the Department of GeologicalSciences, Brown University, Providence, RhodeIsland 02912, USA. e-mail: karen_fischer@brown.edu1. Wookey, J., Kendall, J.-M. & Barruol, G. Nature 415, 777–780

(2002).2. Gaherty, J. B. & Jordan, T. H. Science 268, 1468–1471 (1995).3. Debayle, E. & Kennett, B. L. N. Earth Planet. Sci. Lett. 184,

339–351 (2000).4. Simons, F. J., van der Hilst, R. D., Montagner, J.-P. & Zielhuis,

A. Geophys. J. Int. (in the press). 5. McNamara, A. K., Karato, S. & van Keken, P. E. Earth Planet.

Sci. Lett. 191, 85–99 (2001).6. Karato, S. Pure Appl. Geophys. 151, 565–587 (1998).7. Stixrude, L. in The Core–Mantle Boundary Region (eds Gurnis,

M., Wysession, M., Knittle, E. & Buffett, B.) 83–96 (AmericanGeophysical Union, Washington, DC, 1998).

8. Van der Hilst, R. Nature 374, 154–157 (1995).9. Fischer, K. M. & Wiens, D. A. Earth Planet. Sci. Lett. 142,

253–260 (1996).10. Iidaka, T. & Niu, F. Geophys. Res. Lett. 28, 863–866 (2001).

First, a localized source of morphogen isestablished in the embryo. Subsequent diffu-sion (or active movement) and degradationof the morphogen produces a concentrationgradient that imparts information to cellsabout their position within a developmentalfield. So, for example, the gradient of Bicoidprotein in fruitflies tells cells where they arealong the head-to-tail (anterior–posterior)axis of the embryo. Bicoid is provided in the form of messenger RNA (mRNA) fromthe mother during egg development andbecomes localized to the anterior pole. As the mRNA is translated after fertilization, theresulting protein begins to diffuse from itssource. It is not hindered by cell boundariesbecause the embryo initially develops as asyncytium, in which thousands of cell nucleishare the same cytoplasm.

The formation of the Bicoid protein gradient is presumably influenced by suchfactors as the quantity of maternal mRNA,the precision of mRNA localization, the rateof translation into protein, and the activityof enzymes that subsequently degrade theprotein. All of these processes may be sus-ceptible to perturbations. So how much does the Bicoid gradient differ from embryoto embryo? Houchmandzadeh et al.1 care-fully measured the profile of Bicoid proteinin a large number of wild-type embryos, andfound that it is indeed quite variable (Fig. 1).After normalizing the profile for embryolength, the position at which the Bicoid concentration crosses a chosen level can beanywhere within a region encompassingabout 30% of the length of the embryo.

One of the functions of Bicoid protein isto control the production of mRNA (tran-scription) from a series of ‘gap genes’ in different spatial domains along the embryo’santerior–posterior axis. One of these, theHunchback gene, is transcribed in an anteriordomain, with a sharp boundary of expres-sion about halfway down the embryo. Previ-ous studies2,3 suggested that the Hunchbackgene is switched on in response to a specificthreshold concentration of Bicoid protein.Indeed, embryos produced by mothers containing two or four extra copies of theBicoid gene (introduced as ‘transgenes’)show a prominent posterior shift in the position of the Hunchback boundary3. So,the posterior edge of Hunchback expressionapparently acts as a direct readout of the concentration of Bicoid protein.

Yet when they looked again at wild-type

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Developmental biology

Precision patterningNipam H. Patel and Sabbi Lall

Many developmental events depend on gradients of key molecules to tellcells where they are. These gradients vary between embryos, but a noise-filtering mechanism ensures that development proceeds normally.

Embryos develop in a precise pattern thatvaries little from individual to individualwithin a given species. Such precision

and reproducibility make this an excellentexample of biological robustness. Forinstance, embryos tend to be patterned nor-mally, regardless (within reason) of how largeor small they are, and even in the face of environmental perturbations such as fluctu-ations in temperature. This seems remark-able given that certain patterning steps relyon molecular gradients to tell cells where theyare in the embryo, and hence how they shoulddevelop. Variations in temperature and sizewould be expected to disrupt such gradientsand to lead to errors later on in development.

On page 798 of this issue, Houch-mandzadeh and colleagues1 investigate thisrobustness by taking a close look at one of the classic gradient systems — that of theBicoid protein — in embryos of the fruitflyDrosophila melanogaster. They find that thegradient can vary greatly from embryo toembryo, but that the positional readout is still quite precise. In other words, the noise in the system is filtered out.

Proteins such as Bicoid are known as morphogens, and work in the following way.

Twice a day, coinciding withtidal motion, climbing crabsSesarma leptosoma leave thecanopy of the mangroves theyinhabit and migrate down thetrunks to the swamp below. Butnature being what it is, they runthe risk of being eaten on theirjourney by another crab,Epixanthus dentatus.

Stefano Cannicci et al.decided to investigate whatvisual cues alert the climbingcrabs to the predator (AnimalBehaviour 63, 77–83; 2002).They wrapped two 70-cmlengths of mangrove trunk with PVC sheeting, and placedraffia on the top to create a test pathway. The authors then recorded their subjects’response to the dummiesshown here.

The first is a preservedspecimen of E. dentatus inambush posture. The second is a wooden rectangle, painted naturalistically, but with real claws attached. Thethird is a wooden trapezium, the same size and coloration as E. dentatus, but otherwisenot like the real thing. Cannicci et al. kept all thedummies in mud for a weekto give them an authentic

swampy tang. In tests, S. leptosoma recognized thefirst two dummies as dangerousfrom about 30 cm away, andtook avoiding behaviour, butthey were unperturbed by the third. So it seems that claws in particular provide the warning. With up to sixpredators per tree, the climbingcrabs probably don’t get asecond chance to mistakeidentity. Tim Lincoln

Animal behaviour

Snap judgements

748 NATURE | VOL 415 | 14 FEBRUARY 2002 | www.nature.com© 2002 Macmillan Magazines Ltd

embryos, Houchmandzadeh et al.1 found thatthe Hunchback boundary — at the level ofboth mRNA and protein — is very precise,showing far less variation than the Bicoid gradient (Fig. 1). Normalizing for egg length,about two-thirds of the embryos showed aprecise Hunchback boundary in a range thatspanned only about 1% of the total egg length,a precision equivalent to the width of a singlenucleus. Furthermore, unlike the Bicoid gradient, the Hunchback boundary positiondirectly correlates with egg length, suggestingthat information about proportion is includedin the Hunchback expression pattern.

This observation, that the precision of thewild-type Hunchback boundary is unaffectedby variations in the Bicoid gradient, seems tobe at odds with the finding3 that increasing thenumber of maternal Bicoid transgenes leadsto posterior shifts in the Hunchback bound-ary. However, when Houchmandzadeh et al.repeated the transgene experiments theyfound that the shift in Hunchback expressionwas clear, but smaller than expected. Whenthey raised embryos at different temperaturesthey found that, although the Bicoid proteinprofile at equivalent developmental stages issignificantly altered by temperature changes,the position of the Hunchback boundary isalmost unaffected. The implication is thatthis boundary is subject to correction mecha-nisms that filter out variability in the Bicoidgradient, as well as a mechanism that impartsscaling information.

Houchmandzadeh et al. conclude that the precision of the Hunchback boundary is independent of Bicoid. So what does regulatethe Hunchback boundary? The most obviouscandidates are other embryonic gap genes. Theauthors show, however, that eliminating anyone of these genes has little or no effect on theabsolute position of the Hunchback boundary

and, more importantly, has no effect on theboundary’s precision. The authors screened80% of the fruitfly genome by eliminatingentire chromosomes, and still found noembryonically expressed gene that disruptsthe precision of the Hunchback boundary.

The most obvious maternally derivedcandidates — the posteriorly focused gradi-ent of Nanos protein, and maternal Hunch-back — affect the position of the embryonicHunchback boundary but not its precision1.But Houchmandzadeh et al. identified spe-cific mutant forms of the maternal Staufengene that did affect this precision. Staufen isknown to affect the localization of maternallyderived mRNAs at both embryonic poles4,but Houchmandzadeh et al. show that theeffect of Staufen on the Hunchback bound-ary appears to be independent of an effect on Bicoid localization.

These results have several implications.First, the obvious embryonic candidates forregulating the Hunchback boundary are notsolely responsible for its precision. So theoriesproposing that interactions between gap genesare responsible for generating precise expres-sion boundaries may be missing a crucial com-ponent. The identification of mutant forms of the Staufen gene that affect precision mightprovide the key to unravelling the mechanism.

Second, it is thought that the Bicoid gene evolved relatively recently, within theDipterans (the large group of insects thatincludes Drosophila). But evolutionarily dis-tant insects such as grasshoppers also showanterior domains of Hunchback expression5

that are presumably wholly independent of Bicoid. So maybe the genetic system thatproduces precise Hunchback boundaries inDrosophila will give us clues to the regulationof Hunchback in these other insects.

Finally, the phenomenon of noise filtra-tion may be a general property of morpho-genetic systems, made necessary by theirinherent susceptibility to perturbations.Indeed, studies6 of the morphogen Dpp in theDrosophila wing disc indicate that the con-centration gradient of Dpp can differ from its ‘activity’ gradient (its output). So mecha-nisms that correct and modulate morphogengradients may be common to, and requiredfor, the precise, robust and elaborate pattern-ing of different developmental fields. ■

Nipam H. Patel and Sabbi Lall are in theDepartment of Organismal Biology and Anatomy,and the Howard Hughes Medical Institute,University of Chicago, 5841 South MarylandAvenue, MC1028, Chicago, Illinois 60637, USA.e-mails: npatel@midway.uchicago.eduslall@midway.uchicago.edu1. Houchmandzadeh, B., Weischaus, E. & Leibler, S. Nature 415,

798–802 (2002).2. Driever, W. & Nusslein-Volhard, C. Nature 337, 138–143

(1989).3. Struhl, G., Struhl, K. & Macdonald, P. M. Cell 57, 1259–1273

(1989).4. Micklem, D. R. Dev. Biol. 172, 377–395 (1995).5. Patel, N. H. et al. Development 128, 3459–3472 (2001).6. Teleman, A. A. & Cohen, S. M. Cell 103, 971–980 (2000).

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Daedalus

Fresh flavoursOur past as hunters and gatherers has leftus with distinctive taste in food. We like itfresh. An animal or vegetable that wasliving and growing only a few minutes agohas quite a different taste to one that hasbeen stored. Many foods, from sprouts tofish, lose their pleasant flavour veryquickly. Even the British diet would bedelightful if it were fresh. But in bulkfarming, a large amount of food isharvested at the same time and is thenstored. This is clearly at variance with ouranimal nature. But this type of farming ishighly efficient, and so has sadly become a fact of life.

While an animal or vegetable is alive, itsimmune system protects its evanescentcompounds or regenerates them. When itdies, all this stops. Bacterial attack,crosslinking and decomposition all start atonce. Freezing, that brutal attempt to stopthe clock, seems to work best with the bulkcomponents. One food-processingcompany claims to freeze its vegetableproduct within 2 hours of picking it,hoping to trap the brief trace compoundsof freshness while they last. Daedalus alsorecalls how the makers of instant coffee puta key flavour volatile in the space at the topof each jar, so the illusion of the real thingsurvives at least for a moment. DREADCObiochemists are now studying the tracecompounds present in fresh foodstuffs.

This delicate and tricky work must bedone quickly, using food picked or killedand transported to the lab with equalrapidity. For each foodstuff, Daedalushopes to identify or synthesize just thoseelusive volatiles that restore the illusion of freshness to the long-stored product.Farming, that dreary but efficientbusiness, will at last be matched to ourinstinctive nature.

Standard condiments, such as salt,pepper or monosodium glutamate, are‘amplifiers’: they exaggerate whatever tastethe food has at the time. By contrast, eachDREADCO ‘elixir of freshness’ will restorethe food’s own character, so it tastes freshagain. Like pepper, it will be added at thetable rather than in the pot. It may take theform of an inert tasteless powder with anadded volatile, or a spray-can of liquid orvapour. Daedalus cannot guess how manywill be needed. In the worst case, everyfoodstuff will need its own elixir. But withluck, only a few elixirs will be needed torevive that elusive sense of freshness — onefor meat, say, and one for vegetables. Evencalorie-counting, vegetarianism and otherdietary extremes will gain new pleasureand respectability. David Jones

Figure 1 Filtering out noise in development. a, Houchmandzadeh et al.1 find that, within agroup of wild-type fruitfly embryos, the profile of the Bicoid gradient (red) can varysignificantly (exaggerated somewhat here). b, However, the profile of Hunchback expression(blue) is relatively precise and constant, eventhough Bicoid regulates Hunchback expression.In fact, the addition of extra copies of the Bicoidgene does lead to shifts in the Hunchbackpattern3 (not shown). But Houchmandzadeh etal. find that these shifts are not as great as mightbe expected if the Hunchback pattern dependedsolely on Bicoid concentration.

Bicoid Hunchbacka b

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