news and views fore chose simply to tabulate the percentage common spice combinations bring together circuits of my salsa- and vindaloo-loving of all bacteria tested towards which a given complementary antimicrobials. family are a legacy of unspicy northern spice or its extract has consistently shown The strong biological slant of Billing and Europe, and they suggest not. at least some inhibitory activity, regardless of Sherman’s hygienic hypothesis will also help Harold McGee is at 14 Chemin Las Crozes, 31450 dose. Four spices are said to be inhibitory sharpen the thinking of anyone who is Pompertuzat, France. He is author of On Food and towards all bacteria tested — garlic, onions, intrigued by the complexities of human spice Cooking: The Science and Lore of the Kitchen, and allspice and oregano — and a total of 15 use and its persistence. The past few decades The Curious Cook (Scribner’s/Allen & Unwin). inhibited 75% of bacteria, or more. have brought Paul Rozin’s rich behavioural e-mail: [email protected] This simple ranking of antimicrobial analysis of flavourings and the challenges of 1. Billing, J. & Sherman, P. W. Q. Rev. Biol. 73, 1–38 (1998). 4 potencies disguises large differences in the omnivory , fascinating studies of the cere- 2. Thimmayamma, B. V. S., Rau, P. & Radhaiah, G. Ind. J. Nutr.8 5 6 Dietet. 20, 153–162 (1983). extent to which different spices have been monial and mythopoetic , social and eco- 3. Davidson, P. M. in Food Microbiology (eds Doyle, M. P., 7 tested. An appendix reveals that in the most nomic aspects of European spice habits — Beuchat, L. R. & Montville, T. J.) 520–556 (Am. Soc. inhibitory group, garlic was tested against 29 and a continuing boom in per capita spice Microbiol., Washington DC, 1997). 4. Rozin, P. in Flavor: Its Chemical, Behavioral, and Commercial bacteria, but allspice against only five, and consumption in the United States. Is the last Aspects (ed. Apt, C. M.) 101–127 (Westview, Boulder, CO, onion against four. Chillies were successful a manifestation of new selection pressures 1978). against just four of five bacteria. A review of exerted by Escherichia coli, salmonella and 5. Detienne, M. The Gardens of Adonis, transl. Lloyd, J. (1972; reprint Princeton Univ. Press, 1994). chemical food preservation concludes that campylobacter? Of an immigrant popula- 6. Schivelbusch, W. Tastes of Paradise (Pantheon, New York, 1992). of the 15 spices that Billing and Sherman tion reverting to type? The chemosensory 7. Laurioux, B. Food Foodways 1, 43–76 (1985). classify as ‘highly inhibitory’ and find signifi- cantly associated with warm climates, seven, Mantle mineralogy including chillies and allspice, have shown “limited or no activity” towards food Olivine emerges from isolation spoilage microbes3. Even for the apparently Craig R. Bina inhibitory spices (cinnamon and cloves, gar- lic, and the Mediterranean herbs), the prob- lem remains that most evaluations have been ear a depth of 410 km, the speeds at magnesium and iron between olivine and done in vitro rather than in victu. The few which seismic waves travel in Earth’s other mantle minerals is an important factor spices to have been tested in foods show far Nmantle rise sharply, an observation in resolving these apparent discrepancies. less activity than they do in microbiological long attributed to high-pressure transfor- Such discrepancies are significant media, probably because food particles mation of the mineral olivine to wadsleyite. because they constrain the extent to which sequester the inhibitory compounds3. The However, seismological studies of this mantle discontinuities arise from phase active sulphur compounds in garlic and ‘410-km discontinuity’ reveal wave-speed transitions in minerals, such as those in onions are formed in disrupted tissue by increases that are smaller and more abrupt olivine, rather than changes in bulk chemi- enzymatic activity, which ordinary cooking than expected from extrapolation of the cal composition. The mineralogy and chem- procedures limit (but which the Indian laboratory behaviour of these minerals to istry of Earth’s mantle, especially that part practice of grating or grinding probably mantle conditions. On page 702 of this known as the ‘transition zone’ between the maximizes). And little is known about the issue1, Irifune and Isshiki show that the seismic discontinuities at depths of 410 and stability of the inhibitory compounds to hitherto largely neglected exchange of 660 km, govern the dynamics of convective cooking temperatures. Volume fraction (%) In short, we don’t yet have enough infor- Figure 1 Equilibrium mineral proportions mation to say whether spices have significant 0 20 40 60 80 100 0 100 and compositions as functions of mantle antimicrobial effects in ordinary cooking. depth, for (Mg0.89Fe0.11)2SiO4 olivine in On balance, it looks as though a simpler and isolation (Fo89) and for a model opx

Fo89 (pyrolite) mantle composition. Mineral more reliable defence against foodborne dis- Pyrolite ease is to heat food well and often, as Indians proportions are as volume fraction, have long done with highly perishable and 200 shown by bold white lines; compositions unspiced liquid milk. are in terms of Mg/[Mg+Fe], and are If Billing and Sherman are unable to shown by colours and by dotted contours make a convincing case for the hygienic at 1% intervals. With increasing depth, hypothesis, their labours are to be valued for Ȋ Ȋ olivine (Ȋ) transforms to wadsleyite (ȋ) reviving an excellent question and demon- near 410 km, and orthopyroxene (opx) strating the need for in victu research initia- 300 cpx gradually dissolves into clinopyroxene (cpx) which in turn dissolves into garnet tives. After testing culinarily appropriate Depth (km) spice doses in a model food, one would want (gt). In the pyrolite mantle composition, to contaminate genuine, pre-refrigeration all phases grow more Mg-rich with versions of, say, chicken in a sauce — Chinese increasing depth. (Figure based on data red-cooked, Indian curried, Italian caccia- from Irifune and Isshiki1.) tore, French coq au vin, American pot pie, 400 Mexican mole — and follow pathogen growth in each, which will surely be affected by ingredients such as soy sauce, vinegar and ȋ ȋ wine. Given the worldwide popularity and gt yet limited antibacterial activity of black pep- per, Billing and Sherman’s speculative ratio- nale — that pepper acts as a ‘bioavailability enhancer’ to sensitize bacteria to other spices 75Mg/[Mg+Fe] (%) 95 — needs testing, as does the possibility that

650 NATURE | VOL 392 | 16 APRIL 1998 Nature © Macmillan Publishers Ltd 1998 news and views flow of heat and matter in the interior2. This Irifune and Isshiki1 have shown such transition to higher pressures (as can be seen flow, in turn, drives the tectonic evolution artificial separation to be misleading by in Irifune and Isshiki’s Fig. 3 on page 704), so of the surface, including the occurrence of experimentally demonstrating Mg–Fe ex- the onset of the transition in pyrolite is post- volcanism and seismicity. change between Ȋ, ȋ, garnet and clino- poned to greater depths relative to that in Olivine, a solid solution of forsterite pyroxene. We have known5 for some time Fo89. On the other side of the phase change,

(Mg2SiO4) and fayalite (Fe2SiO4), has the that the proportions of these minerals vary ȋ does not partition Fe into garnet as strong- approximate composition (Mg0.89 Fe0.11)2SiO4 with depth, as shown by the bold white lines ly as does Ȋ, so ȋ is not initially Mg-enriched, (called forsterite-89 or Fo89) in samples from in Fig. 1, but Irifune and Isshiki have now and the completion of the Ȋ–ȋ transition the shallow mantle. It transforms from the measured the accompanying variations in is not postponed relative to Fo89. The net olivine (Ȋ) phase to wadsleyite (ȋ), changing the compositions of coexisting minerals, effect of postponing onset but not co8m- again to ringwoodite (ȍ) at greater depths shown by the colours. In isolated Fo89 pletion is to concentrate the transition into and eventually breaking down to a mixture of olivine, mineral compositions must remain a narrower range of depths, decreasing the silicate perovskite and magnesiowüstite. In constant, except in the narrow zone of Ȋ–ȋ depth-extent of the wave-speed increase by standard compositional models of the upper coexistence. In pyrolite, however, all phases some 6 km relative to that expected for Fo89 mantle such as the pyrolite3 model, in which a grow increasingly Mg-rich with depth. (>10 km), in better agreement with seismic6 mix of different minerals is taken to account Given that bulk composition is fixed, this estimates (<5 km). for the rocks and melts erupted at the surface entails an increasing abundance of the most Furthermore, mineral wave-speeds from the interior, olivine accounts for only Fe-rich phase, garnet. Indeed, gradual dis- themselves also vary with composition. about 60% of the mantle by volume. Other solution of the less Fe-rich pyroxenes into Wave-speed increases across the Ȋ–ȋ transi- minerals concerned include orthopyroxene, the garnet both raises the proportion of tion in isolated Fo89 can be measured in clinopyroxene and garnet, which gradually garnet and dilutes the garnet to lower Fe the laboratory, extrapolated to mantle dissolve each into the next with increasing levels. However, given constant Mg–Fe conditions, and scaled for a mantle model depth. Despite a pioneering study nearly 20 partitioning ratios (incidentally confirmed which is about 60% olivine by volume. years ago, in which Akaogi and Akimoto4 by Irifune and Isshiki), such dilution drives Although subject to remaining uncertain- pointed out that knowledge of element parti- Fe-depletion (that is, Mg-enrichment) of ties7 in the pressure- and temperature- tioning amongst these minerals is ‘indispen- Ȋ to maintain equilibrium levels of Fe in dependence of mineral wave-speeds, some sable’ for understanding the mantle, to this garnet. As a result, olivine above the Ȋ–ȋ such analyses8 yield increases whose magni- day the olivine polymorphs are treated in transition in pyrolite becomes enriched in tudes are rather larger than those inferred isolation from pyroxenes and garnets in most Mg relative to that in Fo89. for the 410-km seismic discontinuity. In models of the mantle. Such Mg-enrichment of Ȋ shifts the Ȋ–ȋ pyrolite, on the other hand, the Mg-

Palaeontology The eyes have it

Life on land is so challenging that many land vertebrates have taken the plunge and gone back to the sea. From porpoises to placodonts, pliosaurs to pinnipeds, the examples are legion. But life in the sea poses a unique problem for a returning land vertebrate — salt. A marine vertebrate with a terrestrial ancestry has body fluids that are much less salty than sea water, so an unprotected vertebrate immersed in the sea runs the risk of dehydration. Water can be replaced by drinking sea water. This raises the saltiness of the body with respect to the sea, so there can be no net gain of water unless the salts are excreted in a solution at least as concentrated as that of sea water. The kidneys of marine reptiles, such as turtles, do not have this concentrating power: without other methods of voiding excess salt, a marine reptile cannot the Lower Cretaceous Santana Formation contrast, far less haphazard. Marine turtles eat salty food or drink sea water without of north-east Brazil, extends the fossil have lachrymal glands (each one larger, in becoming dehydrated in the midst of record of marine turtles back ten million some cases, than the brain) modified to plenty. years. The turtle is primitive in the sense excrete a concentrated salt solution. The Marine life is simply unliveable unless that the bones in its wrists, ankles and skull of the Santana turtle shows evidence an animal solves the salt problem. This digits have not become consolidated into that it had enormous salt glands around 200-millimetre-long, 110-million-year-old rigid paddles. In other words, its feet the eyes. In which case, the evolution of fossil marine turtle, described by Ren would have looked more like those of salt-excreting glands preceded that of rigid Hirayama on page 705 of this issue (Nature freshwater terrapins than fully seagoing paddles. These salt glands were the turtles’ 392, 705–708; 1998), has taken this lesson turtles. passport back to the sea. on board. The creature, which comes from Its salt-excreting arrangements were, in Henry Gee

NATURE | VOL 392 | 16 APRIL 1998 651 Nature © Macmillan Publishers Ltd 1998 news and views enriched Ȋ is seismically faster than the Ȋ part determine the properties of the 660- petunia. The gene was modified with border in Fo89, but its non-enriched ȋ exhibits the km seismic discontinuity at the bottom of sequences that allow integration into the same speeds as ȋ in Fo89. Thus the Mg–Fe the transition zone. Interestingly enough, chloroplast genome by homologous recom- partitioning which leads to a narrower Akaogi and Akimoto’s early study also bination and, using the polymerase chain transition yields one which is also smaller suggested that Mg–Fe partitioning between reaction and Southern blot analysis, the in magnitude (by about 10% according to garnet and ȍ should be reversed relative to ȋ authors confirmed that the gene had inserted Irifune and Isshiki’s estimate), in better in the middle of the transition zone. Future into chloroplast DNA of regenerated plants. agreement with seismic estimates. work may or may not show that this behav- All of the chloroplasts were transgenic, and So Irifune and Isshiki have demonstrated iour does indeed occur. If it does, it could the very high copy number of the gene (up to that the olivine polymorphs exchange Fe and affect the properties of the broader ȋ–ȍ 10,000) resulted in a tenfold increase 8in Mg with their pyroxene and garnet compan- transition, too, with implications for our glyphosate tolerance compared with trans- ions under mantle conditions rather than understanding of the enigmatic seismic genic plants that carry the same gene in the residing in isolation, confirming Akaogi and reflectors10 that occur at around 520 km nuclear genome2. Akimoto’s early indications that Mg–Fe depth. Genes that confer tolerance to herbicides partitioning relative to garnet differs sig- Craig R. Bina is in the Department of Geological have been the target of genetic engineers for nificantly among the olivine polymorphs. Sciences, Northwestern University, Evanston, Illinois more than a decade. Up to January this year, Although other factors, such as mineral 60208-2150, USA. there were nearly 2,300 experimental releas- hydration or nonlinear variations in wave e-mail: [email protected] es of herbicide-tolerant crops in OECD speed, may also affect the seismic signature 1. Irifune, T. & Isshiki, M. Nature 392, 702–705 (1998). countries (about 35% of all genetically mod- of phase changes, Irifune and Isshiki have 2. Christensen, U. Annu. Rev. Earth Planet. Sci. 23, 65–87 (1995). ified crop releases up to then)3. As well as its 3. Ringwood, A. E. Composition and Petrology of the Earth’s Mantle helped to reconcile the properties of the (McGraw-Hill, New York, 1975). agronomic objective, herbicide tolerance has Ȋ–ȋ transition in a pyrolite model mantle 4. Akaogi, M. & Akimoto, S. Phys. Earth Planet. Inter. 19, 31–51 been widely used as a selectable marker, with observations of the 410-km seismic (1979). enabling the easy detection of successful discontinuity. 5. Ringwood, A. E. Spec. Publ. Geol. Soc. Australia 14, 457–485 (1989). transformations. 6. Neele, F. Geophys. Res. Lett. 23, 419–422 (1996). Although their study pertains largely to 7. Li, B., Gwanmesia, G. D. & Liebermann, R. C. Geophys. Res. Herbicide-tolerant crops such as the the top of the transition zone, other work9 Lett. 23, 2259–2262 (1996). glyphosate-tolerant soybean Glycine max suggests that similar phenomena — Al–Fe 8. Duffy, T. S., Zha, C. S., Downs, R. T., Mao, H. K. & Hemley, were among the first genetically modified R. J. Nature 378, 170–173 (1995). exchange involving ȍ, garnet, silicate 9. Wood, B. J. & Rubie, D. C. Science 273, 1522–1524 (1996). species to be grown commercially in North perovskite and magnesiowüstite — may in 10.Shearer, P. M. J. Geophys. Res. 101, 3053–3066 (1996). America. Such crops are also near to market in Europe — for example, both glyphosate- Crop genetics and glufosinate ammonium-tolerant oilseed rape, Brassica napus, are undergoing variety Reducing transgene escape routes trials in the United Kingdom. Tolerance to glyphosate has featured in almost one-third Alan J. Gray and Alan F. Raybould of all herbicide-tolerance field trials and in 20 different crops, ranging from lettuce and enetically modified crops and the is correctly targeted to the chloroplasts), or a tomato to eucalyptus, chestnut and poplar, food derived from them have had a gene that inactivates the herbicide. although the major crop targets have been bad press, but a rare piece of good But Daniell et al. have now achieved maize, soybean, oilseed rape, sugar beet and G 1 news is provided by Daniell et al. in this maternally inherited glyphosate tolerance in cotton. month’s Nature Biotechnology. They report tobacco by particle bombardment of leaves The environmental and agricultural a development that has profound implica- with a vector containing the EPSPS gene of implications of the large-scale cultivation Y tions for the risk assessment of genetically A modified crops. Most crops are modified by . GR inserting genes into the nucleus, and the A. J genes can therefore spread to other crops or wild relatives by movement of pollen. By engineering tolerance to the herbicide glyphosate into the tobacco chloroplast genome, however, the authors have not only obtained high levels of transgene expression, but, because chloroplasts are inherited maternally in many species, they have also prevented transmission of the gene by pollen — closing a potential escape route for transgenes into the environment. Glyphosate is the most widely used herbi- cide in the world. It interferes with 5-enol- pyruvyl shikimate-3-phosphate synthase (EPSPS), an enzyme that is encoded by a nuclear gene and catalyses a step in the biosynthesis of aromatic amino acids in the chloroplasts. Conventional strategies for producing glyphosate-tolerant plants are to Figure 1 Off the beeten track — sea beet on a sea wall in Somerset (UK), flowering at the same time as insert, into the nucleus, an EPSPS gene from a nearby sugar beet. Genes from sugar beet are known to transfer to wild, weedy beets in areas where plant or a glyphosate-tolerant bacterium (the sugar beet seed is produced12. The same process can occur when sugar beet plants flower before bacterial gene is modified so that the enzyme harvest in coastal regions where sea beet occurs.

NATURE | VOL 392 | 16 APRIL 1998 653 Nature © Macmillan Publishers Ltd 1998