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surprising. Although the diatoms werereproducing and taking up nitrate morequickly, the effect of this extra growth on silica uptake was minimal. In consequence,Si:N (and by inference, presumably also Si:Cand Si:P) ratios were two to three times ashigh in the unfertilized controls as in theiron-enhanced cultures.

Takeda3 has extended silica monitoring to the three previously established HNLCprovinces, and has also isolated two speciesof Antarctic diatom in culture to examine thespecific response of these species in additionto that of the complex natural community. In all cases, he found the same result asHutchins and Bruland — extra iron resultedin enhanced diatom reproduction rates andnitrate uptake, but only slightly increaseduptake of silica. He also measured Si:P ratios,and verified that Si:P ratios were lower in theiron-addition experiments.

The two papers2,3 show that iron-limiteddiatoms grow thicker silica shells. When theydie, these diatoms can be expected to sinkfaster, lose less silica on their descent to thesea floor, and so be better preserved on thesea floor in the form of opal deposits (whichconsist of hydrated SiO2). Opal accumula-tion rates are used to trace past levels of pro-ductivity, and the new results suggest that theapproach may be biased. This is because adusty glacial atmosphere may have suppliedmore iron to the oceans and increaseddiatom carbon export to the sea floor with-out greatly enhancing silica export. If thiswas so, higher productivity would not bereflected by higher accumulation rates ofsedimentary opal and would eliminate anapparent contradiction — the lack ofobserved higher accumulation rates of opalhad been a sticking point for the hypothesisthat enhanced Southern Ocean productivitywas a factor driving reduced levels of atmos-pheric CO2 during glacial periods7,8.

A further consequence of these findings isthe implication that the distribution of silica,nitrate and phosphate within the ocean maydepend on iron. In today’s ocean, silica ismore rapidly removed into the deeper partsof the ocean than the more labile nutrientsnitrate and phosphorus. This situation maynot be normal, and may occur only duringrelatively infrequent, dust-starved, warmperiods such as the Holocene (the climateperiod of the past 10,000 years). Over thePleistocene, the past two million years, periods of higher dust-flux have been moretypical.

So, in the past, diatoms may have beenless silicified, resulting in less efficient trans-port of silica to the bottom of the ocean, withmore rapid dissolution and recycling of thesilica skeletons that survived transit. The vertical and inter-ocean contrast of silicacompared to nitrate and phosphorus mayhave been less during glacial periods than it has been in warm intervals such as the

past 10,000 years. Furthermore, in glacialperiods, higher uptake of nitrogen and phos-phorus in areas that are currently HNLCwould have affected the distribution of theseelements in the surface and upper ocean.

With these new findings, the power of anelement — iron — that occurs at low con-centrations in the ocean (10–9 mol kg–1 andless) is becoming more evident. It appearsthat iron influences the oceanic distributionsof nutrients at the level of micromolar con-centrations, and also influences the preser-vation of opaline fossils on the sea floor.Because the distribution of other chemicalspecies (some — such as zinc — with biolog-ical importance of their own) are closely cou-

pled to the oceanic cycles of these nutrients,we can expect more surprises as the bio-geochemistry of trace metals is investigatedfurther. Ed Boyle is in the Department of Earth andPlanetary Sciences, Massachusetts Institute ofTechnology, E34-258, 77 Massachusetts Avenue,Cambridge, Massachusetts 02139, USA. e-mail: eaboyle@MIT.EDU1. Martin, J. H. & Fitzwater, S. E. Nature 331, 341–343 (1988).

2. Hutchins, D. A. & Bruland, K. W. Nature 393, 561–564 (1998).

3. Takeda, S. Nature 393, 774–777 (1998).

4. Harvey, H. W. J. Mar. Biol. Assoc. UK 22, 205–219 (1937).

5. Coale, K. H. et al. Nature 383, 495–501 (1996).

6. Falkowski, P. G. Nature 387, 272–275 (1997).

7. Martin, J. H. Paleoceanography 5, 1–13 (1990).

8. Charles, C. D., Froelich, P. N., Zibello, M., Mortlock, R. A. &

Morley, J. J. Paleoceanography 6, 697–728 (1991).

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734 NATURE | VOL 393 | 25 JUNE 1998

The genitals of male insects bear a hugenumber of intricate bits and pieces.The genitalic terms for orthopterans

alone (crickets and allies) include phalli, epi-procts, paraprocts, cerci, gonotremes andeven titillators (from the Latin titallo,‘tickle’).

But why have the male’s genitals evolvedto be so complex? Two answers have beenproposed. First, the species-diagnosticnature of these male characteristics suggeststhat species differences in genitals may haveevolved as barriers to insemination, prevent-ing the production of low-quality hybrid offspring. So, in zones of species overlap, the female genital ‘locks’ of each speciesdiverged, imposing Darwinian selectionamong conspecific males for a proper fit ofeach genital ‘key’. The second answer is thatgenital structures diverged through the sexual selection that occurs after insemina-tion for any device that increases fertilizationsuccess. Possible examples are claspers,which hold the struggling female untilinsemination occurs1, or titillating devicesthat deliver ‘internal courtship signals’ to thefemale. Sperm from males with the best

signals are used in preference to stored spermfrom the females’ previous mates, and suchcopulatory discrimination is favoured infemales because they pass on the advanta-geous genital trait to their sons2.

On page 784 of this issue, Göran Arn-qvist3 provides a rigorous test of the lock-and-key and sexual-selection hypotheses.The rigour is in his methods. First, he mea-sures structural divergence geometrically,thereby overcoming the inevitable subjec-tivity problems that occur when researchersassess morphological complexity (see, forexample, ref. 4). Then he compares thesedivergence measures for matched pairs ofinsect groups that share a common ancestorbut differ in the pattern of female mating.One group contains species reported tohave females that mate only once (mon-androus species), whereas the other hasmultiply-mating females (polyandrousspecies). If post-insemination selection isimportant, the genitals should be moredivergent in the polyandrous groupsbecause competition between the ejaculatesof rivals can occur only when females take

Evolutionary biology

Genitally does itDarryl T. Gwynne

Figure 1 Dorsal views of the ‘Swiss army knife’ gadgetry in the genitals of a male oriental cockroach,Blatta orientalis. Arnqvist3 has studied why such complex male genitals have evolved in insects, andhe concludes that it is a result of sexual selection for genital devices that favour a fertilization bias forthe mating male’s sperm over the stored sperm of the female’s previous mates. a, Whole cockroach,from ref. 12; b, c, enlarged genital area from ref. 13.

Nature © Macmillan Publishers Ltd 1998

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on additional partners. Conversely, thelock-and-key hypothesis predicts greatergenital divergence in monandrous species,because of the potentially large cost to thefemale if her single copulation is with thewrong species.

Arnqvist’s results overwhelmingly sup-ported post-insemination selection. In 18 of19 pairs sampled from mayflies, flies, beetlesand lepidopterans, the shape of male genitalsfrom species in the polyandrous groups hasdiverged more than in single-mating species.Moreover, this divergence was found only ingenitals, and not in other body parts such aslegs.

Could any other selective forces explainthe differences in genital divergence betweenthe monandrous and polyandrous groups?After all, the two groups do differ withrespect to other aspects of their life history —variables that are not controlled for in a com-parative study such as Arnqvist’s. For exam-ple, compared with polyandrous species,monandrous females tend to have a lowerfecundity5 and they copulate immediatelyafter emerging as adults6. Perhaps complexgadgetry is not needed to engage the pliantreproductive parts of a newly emergedfemale whose integument has not yet hard-ened. However, hypotheses such as this areunlikely to provide general explanations forthe patterns observed by Arnqvist.

One hypothesis that may contribute tothese patterns involves a combination of the two models considered by Arnqvist.Hybridization costs do exist in species withpolyandrous females, and some post-insemination selection may be a response tothese costs. In a beetle, lacewing, fruitfly,grasshopper and cricket (refs 7–9 and refstherein) there is clearly no lock-and-key iso-lating mechanism — sperm from the wrongspecies (or, in one case, subspecies) can findits way into the sperm storage organs offemales, and even successfully fertilize eggs.But this occurs only when it is the sole ejacu-late present, and such heterospecific spermusually lose the fertilization battle when incompetition with sperm from the samespecies as the female. There is some evidencethat this outcome is partly due to the abilityof the female’s reproductive tract to recog-nize and favour sperm from her own species. Such cryptic communication ofspecies identity is not well understood9.

So here’s the alternative hypothesis.Could male genital modifications in somepolyandrous species include devices that sig-nal to the female that conspecific sperm isbeing delivered (similar to the way in whichgenital structures are thought to signal thequality of a conspecific male2)? This idea isworth investigating, even though it is not ageneral explanation. For example, it does notexplain why divergent genitals exist whenthere is no threat of mis-mating — such as inisolated island species or in parasites that do

not share hosts with related species2. To workout the importance of genital signalling (ofany sort) on the evolution of genitalic com-plexity, we need many more experimentaland observational studies on individualspecies10.

Finally, it is worth noting that Darwin,who was a very competent entomologist, waskeenly aware of “the complex appendages atthe apex of the abdomen in male insects”,structures that he thought could function asmate-holding devices1. He recognized thatsexual selection could produce obscure genitalic traits as well as the more widelyacclaimed sexual structures such as the pea-cock’s tail. As Arnqvist concludes, the factthat male genitals are under sexual selectioncertainly blurs the traditional dichotomy11

between primary reproductive organs —those that deliver ejaculates — and the sec-ondary sexual structures of males such asfancy tails. In considering the intricate genital appendages of male insects, Darwinhad a similar take on this issue when he

concluded that “it is scarcely possible todecide which ought to be called primary andwhich secondary”.Darryl T. Gwynne is in the Department of Zoology,University of Toronto in Mississauga, Mississauga,Ontario, Canada L5L 1C6.e-mail: dgwynne@credit.erin.utoronto.ca 1. Darwin, C. The Descent of Man and Selection in Relation to Sex

2nd edn (John Murray, London, 1874).

2. Eberhard, W. G. Sexual Selection and Animal Genitalia (Harvard

Univ. Press, Cambridge, MA, 1985).

3. Arnqvist, G. Nature 393, 784–786 (1998).

4. Proctor, H. C., Baker, R. L. & Gwynne, D. T. Can. J. Zool. 73,

2010–2020 (1995).

5. Ridley, M. Biol. Rev. 63, 509–549 (1988).

6. Ridley, M. Anim. Behav. 37, 535–545 (1989).

7. Albuquerque, G. S., Tauber, C. A. & Tauber, M. J. Evolution 50,

1598–1606 (1996).

8. Price, C. S. C. Nature 388, 663–666 (1997).

9. Howard, D. J. & Gregory, P. G. Phil. Trans. R. Soc. Lond. B 340,

231–236 (1993).

10.Eberhard, W. G. Am. Nat. 142, 564–571 (1993).

11.Hunter, J. Observations on Certain Parts of the Animal Oeconomy

(London, 1786).

12. Helfer, J. R. How to Know the Grasshoppers, Crickets,

Cockroaches and Their Allies (Dover, New York, 1987).

13. Snodgrass, R. E. The Male Genitalia of Orthopteroid Insects

Smithsonian Misc. Collections 96, 1–107 (1937).

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Chemical kinetics is the study of mechanisms and rates of chemicalprocesses, in anything from biologi-

cal systems to man-made materials. Tomeasure the rate of reaction between twochemical species, they must be mixed muchfaster than the reaction rate — otherwiseonly the mixing rate is observed. Rapidenough mixing is difficult, especially for themany important liquid-phase and biologicalreactions with millisecond or shorter-time-scale dynamics. But the device built byKnight et al.1 (reported in Physical ReviewLetters) reduces the mixing timescale to lessthan 10 ms, by vastly reducing the size of thereaction vessel.

Two reactants in a quiescent liquid mix bydiffusion. The timescale is t = L2/D, where Lis the length scale over which diffusion must

take place and D is the diffusion coefficient.With typical values of D being 10−9 m2 s−1 orless for large molecules, mixing by diffusionis slow. Consequently, investigations of fastkinetics have traditionally speeded mixingby encouraging turbulence, which increasesthe contact area between two fluids and soreduces the diffusion distance. Turbulencecan be created by mechanical stirring, forc-ing fluids through nozzles, or using the wakebehind a ball. But these methods typicallyrequire high flow rates, and correspondinglylarge samples, so they are unattractive whendealing with expensive biological samples orchemicals that could damage the environ-ment. Moreover, the mixed reactants mustusually be pumped from such mixing systems to the diagnostic equipment, whichintroduces dead time, further limiting the

Chemical kinetics

Smaller, faster chemistryKlavs Jensen

Figure 1 Microfluidicapproaches to achievingfast mixing in laminarflows by diffusion. a, Hydrodynamicfocusing1, which workson the smallest scales,and thus the shortesttimescales, yet achieved.b, Repeated layering offluid elements in staticmixer2 (we see successiveslices through channelspointing towards us).

In flow(fluorescein)

Outlet

Side flow(iodine ions)

etc...

a

b