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39 possible species. One wonders whethersome of the insect-related species weredescribed in the past, but traditional meansof classification erroneously interpretedthem as being identical to other knownspecies. This may be true for some of thespecies in, for example, the P. guilliermondiiclade, and it will be necessary to sequence the ‘type’ (reference) strains of all speciesconsidered to be synonyms.

Given the continuing development ofmolecular tools for the rapid identificationof microbes, studies of yeast diversity inhabitats and regions previously unexplored,or only scantily so, have become more feasi-ble and will pay rich dividends.Large parts ofAfrica, Antarctica, Asia, Australia and LatinAmerica are mainly virgin territory in thisrespect8 — it could be that only 1% of yeastspecies have been described9. This endeav-our will not just be for the education of biol-ogists concentrating on biodiversity, but willalso enhance our knowledge of ecologicalinteractions — as, for instance, is the case in research10 on xylose-fermenting yeasts inbeetle guts, which may be involved in thebreakdown of hemicellulose, a componentof plants, and thus contribute to the decom-position of plant biomass. Moreover, someas-yet-undiscovered species are likely to besources of useful compounds or as agents forbiological control.

As far as the work of Suh et al.1 is con-cerned, extra relevance stems from the factthat insects are also major vectors for the dis-persal of yeasts, among them those involved

There is no doubt that many species offungi have yet to be described, yeastspecies included. Many of them must

lurk in habitats and hosts that have still to beinvestigated. But who would have thought to look in the guts of beetles? Suh and col-leagues did so, as they report in MycologicalResearch1, and have found a hotspot of yeastdiversity. About 1,000 yeast species areknown worldwide, to which Suh et al. add atleast 200 more, identified from a variety ofbeetles sampled in the southeastern UnitedStates and Panama. Many of the yeastsbelong to such familiar genera as Candidaand Pichia, some species of which are respec-tively human pathogens and agents used forthe production of drugs.

Yeasts are among the best-studied groupsof microbes, but this research provides a dramatic indication of how limited ourknowledge is. Apart from their traditionalimportance in wine, bread and beer making,they are also used in many (agro)industrialand medicinal applications, and as work-horses in biological research. Suh and col-leagues’studies on beetles show that efficientsampling of poorly explored habitats or geo-graphical areas can yield an extraordinaryextension to our knowledge of microbialbiodiversity.

Since 1820,about 3,000 yeast species havebeen described (Fig. 1), many of which werelater considered to be identical to previ-ously described species, and consequentlyregarded as synonyms. Two recent mono-graphs on yeasts accept only approximately700 species2,3, but this figure has since risencloser to 1,000. Our present knowledge isheavily biased, however. The great majorityof known species come from WesternEurope, Japan and North America. Of thestrains maintained in the collection ofthe Centraalbureau voor Schimmelcultures(CBS), which is the result of a century ofwork, 45% originated from clinical sourcesor from products such as wine, fermentedfoods, bread, fruit juices and dairy products,with 55% from natural substrates — plants,soil, fruits,animals and water4.Insect sourcesaccounted for only 6% of the isolates in theCBS yeast collection. Clearly, this is only thetip of a very large iceberg: Suh et al.1 considerthat further sampling of beetles alone could result in the identification of another350–500 yeast species.

In recent times, most researchers inter-ested in yeast biodiversity have made effective

use of DNA-based taxonomic concepts5.Since the 1990s, a huge effort has been madewith all accepted yeast species to sequencethe specific regions of DNA that encode a cell component known as ribosomal RNA —these are standard sequences in systematics.The result is that comprehensive rDNA data-bases now exist for yeasts6,7. These molecularstudies have produced an unparallelledincrease in the number of yeast speciesdescribed in the years 2000–05 (Fig. 1), evenbefore we had news of the high biodiversityobserved in beetle guts.

That news adds an extra dimension to our appreciation of yeast ecology. Suh et al.found that some clades of yeasts containmany insect-related species (a clade being agrouping consisting of an ancestor and all itsdescendants). Examples are the Pichia guil-liermondii, P. stipitis, Candida tanzawaensisand Stephanoascus–Arxula–Zygoascus cladesin the fungal phylum Ascomycota, and the Trichosporon clade within the phylumBasidiomycota. The C. tanzawaensis cladehas expanded from a single strain of just oneknown species to 164 isolates representing

Biodiversity

Gut feeling for yeastsTeun Boekhout

The startling news that has emerged from studies of the intestines ofbeetles is a reminder of how little is known about the diversity of evensuch comparatively well-characterized groups as the yeasts.

Figure 1 Yeast on the rise — the cumulative increase in yeast species described since 1820 (the totalincludes synonyms). Since 2000, application of DNA-based methods has produced a rapid increase in the yeast species recognized; the last time span covers only five years rather than ten, and includesthe new identifications from the hyperdiverse guts of beetles. Blips in the increase in descriptionsoccurred during the Second World War and at the end of the twentieth century. Inset, beetles of the genus Mycotretus, family Erotylidae, one of the taxonomic groups of beetle sampled by Suh and colleagues1.

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in fermentation and food spoilage. So a better understanding of yeast biodiversity ininsects should help in controlling theseprocesses.All in all, the new observations area clear signpost to a highly promising routein the quest to identify hidden microbial biodiversity. ■

Teun Boekhout is in the Centraalbureau voorSchimmelcultures, Uppsalalaan 8, 3584 CT Utrecht,and the Division of Acute Medicine and InfectiousDiseases, Utrecht University, The Netherlands.e-mail: boekhout@cbs.knaw.nl

1. Suh, S.-O., McHugh, J. V., Pollock, D. D. & Blackwell, M.

Mycol. Res. 109, 261–265 (2005).

The behaviour of electrons is difficult topredict.Undergraduates routinely cal-culate the quantum states of a hydro-

gen atom, but even a helium atom with twoelectrons displays complex dynamics. Howmuch more complex are solids, with elec-trons in the billions. Nanotechnology nowenables researchers to create controlled, or

tunable, models of electronic behaviour insolids. On page 484 of this issue, Jarillo-Herrero et al.1 present perhaps the mostsophisticated example of this approach todate, studying the flow of electrons through a carbon nanotube that is made to act like anexotic magnetic atom.

In attempting to explain the electronicproperties of materials, theorists often construct ‘microscopic’ models, comprisingelectrons that can move freely (become delocalized), sit on localized sites and maketransitions between these states.Such modelsoffer powerful insights but also have draw-backs when applied to conventional bulkmaterials. In particular, electrons on manysites can interact with each other, makingcomplete calculations impractical; and para-meters such as an electron’s tunnelling rateand interaction strength are not tunable,andcan be difficult to measure precisely. Nano-technology can remedy these limitations:by building submicrometre-sized structuresin which one or a few electrons are isola-ted, researchers can know everything about the relevant electronic states, and can studyelectrons at a single site. Perhaps most im-portantly, nearly all of the significant para-meters can be designed, measured and oftentuned in situ.

The interaction of mobile electrons withmagnetic impurities — atoms or ions with anon-zero magnetic moment — in a hostmetal has been a theme in solid-state physicsfor decades. In 1961, Anderson proposedthat a magnetic impurity could be modelledas an electronic quantum state localized at asingle site within a crystal. According to thePauli exclusion principle, the site can beoccupied by up to two electrons (with oppo-site spins),but Coulomb repulsion can make

Nanotechnology

New spin on correlated electrons Ronald M. Potok and David Goldhaber-Gordon

In the Kondo effect, the flow of electrons in a solid is modulated bymagnetic impurities. Nanostructures such as carbon nanotubes can bedesigned to obtain even more complex versions of this intriguing effect.

or

or

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e–

Figure 1 Degeneracy and the Kondo effect. Thebasis of the Kondo effect is the screening of alocalized twofold degeneracy by surroundingdelocalized electrons. The degeneracy normallycomes from spin, but with the advent ofnanotechnology, researchers can create and studyother types of degeneracy; a–c give examples.a, An electron can reside on either of two sites,but not both simultaneously because of Coulombrepulsion8. b, c, A spatial symmetry producesdegenerate orbital levels11. b, The symmetry is the rotational symmetry of a circular quantumdot9. This is analogous to the rotationalsymmetry of a hydrogen atom that producesdegenerate p orbitals. c, The lattice symmetry of a nanotube gives rise to a twofold orbitaldegeneracy1. Non-spin degeneracy may also give rise to the Kondo effect in bulk systems14.

2. Kurtzman, C. P. & Fell, J. W. The Yeasts: A Taxonomic Study

4th edn (Elsevier, Amsterdam, 1998).

3. Barnett, J. A., Payne, R. W. & Yarrow, D. Yeasts: Characteristics

and Identification (Cambridge Univ. Press, 2000).

4. CBS databases www.cbs.knaw.nl

5. Blazer, M. L. Phil. Trans. R. Soc. Lond. B 359, 669–679 (2004).

6. Kurtzman, C. P. & Robnett, C. J. Antonie van Leeuwenhoek 73,

331–371 (1998).

7. Scorzetti, G., Fell, J. W., Fonseca, A. & Statzell-Tallman, A. FEMS

Yeast Res. 2, 495–517 (2000).

8. Hawksworth, D. L. Stud. Mycol. 50, 9–18 (2004).

9. Fell, J. W., Boekhout, T., Fonseca, A. & Sampaio, J. P. in

The Mycota VII, Part B, 2nd edn (eds McLaughlin, D. J.,

McLaughlin, E. G. & Lemke, P. A.) 3–35 (Systematics and

Evolution, Part B) (Springer, Berlin, 2001).

10.Suh, S.-O., Marshall, C. J., McHugh, J. V. & Blackwell, M.

Mol. Ecol. 12, 3137–3146 (2003).

100 YEARS AGODie Kalahari. Now if we could imagine thatMr. Shandy the elder were alive, this is abook that, like many another of its class,would have delighted him. Hereby he couldhave proved triumphantly to Yorick thepotency of that great scheme of education— that “north-west passage to theintellectual world” — which he propoundedso enthusiastically upon a memorableoccasion. His scheme, it will be remembered,was that upon every substantive in thedictionary (so gravely misunderstood by the Corporal and Uncle Toby) should bebrought to bear exhaustively:— “Everyword, Yorick, by this means, you see, isconverted into a thesis or an hypothesis:—every thesis and hypothesis have anoffspring of propositions;— and eachproposition has its own consequences andconclusions; every one of which leads themind on again, into fresh tracts of enquiriesand doubtings.— ‘The force of this engine,’added my father, ‘is incredible…’ ”From Nature 23 March 1905.

50 YEARS AGODr. J. R. Oppenheimer’s address wasostensibly on… “Man’s Right to Knowledgeand the Free Use Thereof”; but he dealt morespecifically with the prospect in the arts andsciences, and above all he emphasized theisolation of the specialist from the broadpath of human knowledge and intercourse.With more means of communication thanever before, we can speak to each other less easily and less effectively. While morepeople are engaged in scientific research to-day... the frontiers of science are nowseparated by long years of study, byspecialized vocabularies, arts, techniquesand knowledge from the common heritageeven of the most civilized society… Thedifficulties which face us are thus derivedfrom the growth of skill, of power, and of our understanding of that world, if not of the conditions which govern their exercise.To assail such changes is futile: we need torecognize them and to take account of ourresources; nor does Dr. Oppenheimer appearto put high among such resources the massmedia of radio and television, the cinema,best-seller or the world tours of successfultheatrical productions. Although art andscience may be purveyed by such means,they have also an inhumanizing influenceand can easily destroy the individual senseof beauty.From Nature 26 March 1955.

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