4. Hetherington, A.M. (2001). Guard cellsignaling. Cell 107, 711–714.

5. Vavasseur, A., and Raghavendra, A.S.(2005). Guard cell metabolism and CO2sensing. New Phytol. 165, 665–682.

6. Fukuda, M., Hasezawa, S., Nakajima, N.,and Kondo, N. (2000). Changes in tubulinprotein expression in guard cells of Viciafaba L. accompanied with dynamicorganisation of microtubules during thediurnal cycle. Plant Cell Physiol. 41,600–607.

7. Hugouvieux, V., Kwak, J.M., andSchroeder, J.I. (2001). An mRNA capbinding protein, ABH1, modulates earlyabscisic acid signal transduction inArabidopsis. Cell 106, 477–487.

8. Li, J., Kinoshita, T., Pandey, S., Ng,C.K.Y., Gygi, S.P., Shimazaki, K., andAssmann, S.M. (2002). Modulation of anRNA-binding protein by abscisic-acid-activated protein kinase. Nature 418,793–797.

9. Yang, S.H., Berberich, T., Sano, H., andKusano, T. (2001). Specific association oftranscripts of tbzF and tbz17, tobaccogenes encoding basic region leucinezipper-type transcriptional activators,with guard cells of senescing leavesand/or flowers. Plant Physiol. 127, 23–32.

10. Parcy, F., and Giraudat, J. (1997).Interactions between the ABI1 and theectopically expressed ABI3 genes incontrolling abscisic acid responses inArabidopsis vegetative tissues. Plant J.11, 693–702.

11. Taylor, J.E., Renwick, K.F., Webb, A.A.,McAinsh, M.R., Furini, A., Bartels, D.,Quatrano, R.S., Marcotte, W.R., Jr., andHetherington, A.M. (1995). ABA-regulated promoter activity in stomatalguard cells. Plant J. 7, 129–134.

12. Aghoram, K., Outlaw, W.H., Bates, G.W.,Cairney, J., Pineda, A.O., Bacot, C.M.,Epstein, L.M., and Levenson, C.W.(2000). Abg1: a novel gene up-regulatedby abscisic acid in guard cells of Viciafaba L. J. Exp. Bot. 51, 1479–1480.

13. Shen, L.M., Outlaw, W.H., and Epstein,L.M. (1995). Expression of an mRNA withsequence similarity to pea dehydrin(Psdhn-1) in guard cells of Vicia faba inresponse to exogenous ABA. Physiol.Plant. 95, 99–105.

14. Leonhardt, N., Kwak, J.M., Robert, N.,Waner, D., Leonhardt, G., andSchroeder, J.I. (2004). Microarrayexpression analyses of Arabidopsisguard cells and isolation of a recessiveabscisic acid hypersensitive proteinphosphatase 2C mutant. Plant Cell 16,596–615.

15. Cominelli, E., Galbiati, M., Vavasseur, A.,Conti, L., Sala, T., Vuylsteke, M.,Leonhardt, N., Dellaporta, S.L., andTonelli, C. (2005). A guard-cell-specificMYB transcription factor regulatesstomatal movements and plant droughttolerance. Curr. Biol. 15, 1196-1200.

16. Liang, Y.-K., Dubos, C., Dodd, I.C.,Holroyd, G.H., Hetherington, A.M., and

Campbell, M.M. (2005). AtMYB61, anR2R3-MYB transcription factorcontrolling stomatal aperture inArabidopsis thaliana. Curr. Biol. 15,1201-1206.

17. Riechmann, J.L., Heard, J., Martin, G.,Reuber, L., Jiang, C., Keddie, J., Adam,L., Pineda, O., Ratcliffe, O.J., Samaha,R.R., et al. (2000). Arabidopsistranscription factors: genome-widecomparative analysis among eukaryotes.Science 290, 2105–2110.

18. Stracke, R., Werber, M., and Weisshaar,B. (2001). The R2R3-MYB gene family inArabidopsis thaliana. Curr. Opin. PlantBiol. 4, 447–456.

19. Chen, W., Provart, N.J., Glazebrook, J.,Katagiri, F., Chang, H.S., Eulgem, T.,Mauch, F., Luan, S., Zou, G., Whitham,S.A., et al. (2002). Expression profilematrix of Arabidopsis transcription factorgenes suggests their putative functionsin response to environmental stresses.Plant Cell 14, 559–574.

Department of Molecular Biology &Biotechnology, University of SheffieldFirth Court, Western Bank, SheffieldS10 2TN, UK. E-mail: j.e.gray@sheffield.ac.uk

DOI: 10.1016/j.cub.2005.07.039

Dispatch R595

Manuel Mendoza, Caren Nordenand Yves Barral

During cell division the cleavageplane must be positionedcorrectly between the segregatingchromosomes. Animal cells solvethis problem by specifying thedivision plane during mitosis. Themitotic spindle dictates the site offurrowing, ensuring that thecontractile ring cleaves the cellinto two halves, each containing acomplete set of chromosomes [1].At first sight, the fission yeastSchizosaccharomyces pombeappears to use a differentmechanism. Fission yeast cellsare rod-shaped and divide in themiddle. There is a tight correlationbetween the position of theinterphase nucleus and that of thedivision site. The fission yeastnucleus is maintained at the cellmiddle during interphase, and

mutants that exhibit abnormalnuclear positioning often divideoff-center [2].

These and other observationshave strongly suggested that, infission yeast, the position of theinterphase nucleus determines thatof the cleavage plane. Thus, tounderstand how the position of thecleavage plane is set, we first needto determine how the nucleus ismaintained in the center of the cellduring interphase. The answer haslong been thought to lie withmicrotubules. But in the absenceof techniques for manipulating theposition of the nucleus, the exactrole of microtubules has beendifficult to address.

Interphase microtubules in S.pombe are organized in four to sixbundles which span the long axisof the cell. These bundles areanchored by their minus ends atmultiple points on the nuclear

membrane, so that the highlydynamic plus ends are orientedtoward the cell tips [3,4].Microtubules that contact the celltips buckle under tension,generating forces capable ofdeforming the nuclear membrane.It has been suggested that thecombined pushing forces ofmicrotubules at opposite cell tipsmaintain the nucleus in the cellcenter [4]. Two recent papers [5,6]describe elegant physicalapproaches to displacing thenucleus of S. pombe cells. Theresults of these studies confirmthe role of microtubule pushingforces in nuclear positioning, andbring further insight into themechanism of cleavage planespecification in fission yeast.

Tolic-Nørrelykke et al. [5] usedoptical tweezers to trap anaturally occurring lipid granulein the fission yeast cytoplasm. Bypushing the granule against thenucleus, they could displace it byalmost 1 µµm in an interphase cell(fission yeast cells are 7–12 µµm inlength). In most cases, thenucleus returned to the cellcenter after release from theoptical trap, and cells placed thedivision site in the middle.Visualization of GFP-labeledmicrotubules showed that the

Division-Plane Positioning:Microtubules Strike Back

Two groups have recently developed physical techniques tomanipulate the position of the nucleus in fission yeast. Their studiesreveal how microtubules confine the nucleus to the cell center, andindicate how the position of the cleavage plane during cell division iscoordinated with that of the nucleus.

movement of the nucleustowards the center wasassociated with microtubulepushing on the cell end.Moreover, when microtubuleswere depolymerized by treatmentwith thiabendazole, themanipulated nuclei failed toreturn to the cell center.

Remarkably, displacement ofthe interphase nucleus in theabsence of thiabendazole causedsome cells to divideasymmetrically: the cleavageplane was no longer in the cellcenter but correlated with theposition of the displaced nucleus.This demonstrates first, that theposition of the cleavage plane isspecified by the nucleus; andsecond, that this specificationoccurs prior to mitosis. In supportof this last conclusion,displacement of the nucleusduring prometaphase did notdisplace the division site. Incontrast, moving the nucleus at anearlier stage — during prophase— caused some cells to divideasymmetrically (Figure 1). Fromthese results, Tolic-Nørrelykke etal. [5] conclude that the nucleus

specifies the division site beforeprometaphase.

How does the nucleus specifythe site of division? Mid1p hasbeen proposed to be a key proteinin this process. Mid1paccumulates in the nucleus duringinterphase and is exported to abroad band at the cell cortexoverlying the nucleus, starting longbefore mitosis [7]. Duringanaphase, this band condensesinto a sharp ring that correspondsto the site of cleavage. Importantly,cells lacking mid1p often divideasymmetrically, even if their nucleiare maintained at the cell center[8–10]. So mid1p may couple theposition of the cleavage plane tothat of the nucleus. Themicromanipulation experiments ofTolic-Nørrelykke et al. [5] areconsistent with the dynamics ofmid1p localization: they show thatthe position of the cleavage planeis set shortly after the start ofmitosis, which is exactly the timeat which mid1p transfer to themedial cortex is complete.

Independent experiments byDaga and Chang [6] directlyaddress the relationship between

nuclear position and mid1plocalization. Instead of opticalmicromanipulation, these authorsused centrifugation to displace thenucleus of S. pombe cells. Inagreement with Tolic-Nørrelykkeet al. [5], they found that cellstreated with MBC, anothermicrotubule-depolymerizing drug,failed to re-center displacednuclei, whereas they did repositiontheir nuclei in MBC-free medium.Moreover, in contrast to cellscentrifuged during interphase,which went on to divideasymmetrically, cleavage planepositioning was not affected bycentrifugation during mitosis.Daga and Chang [6] found that,during interphase, the position ofthe cortical mid1p domainfollowed that of the displacednucleus. Imaging of mid1p-GFPrevealed that cortical mid1pmoved back to the cell centertogether with the nucleus. Thus,the mid1 cortical domain isregulated by the underlyingnucleus in a dynamic manner.

Now that the role of the nucleusin determining the position of thecleavage plane is demonstrated,

Current Biology Vol 15 No 15R596

Figure 1. Micromanipulation of the S. pombe nucleus during different cell cycle stages, showing the relationship between nuclearposition and cleavage plane establishment. Red boxes mark the time of nuclear displacement.

(A) Cytoplasmic microtubules (green) are associated with the nucleus (grey circle) through their minus ends (‘–’). Microtubule plusends (‘+’) push on the cell tips and maintain the nucleus in the cell center during interphase. Mid1p (blue) accumulates in a broad cor-tical region overlying the nucleus. Upon spindle formation, cortical mid1p is restricted to a sharp ring, which coincides with the posi-tion of actomyosin ring formation (red). Accordingly, cytokinesis and septum formation take place in the cell middle. This leads tosymmetric division and two daughter cells of the same length. (B) When the nucleus is displaced during interphase, microtubules pushit back to the cell center. Nuclear movement is accompanied by re-centering of cortical mid1p. This results in symmetric cell division,like in non-manipulated cells. (C) During prophase, the duplicated spindle pole bodies (depicted in purple) separate and cytoplasmicmicrotubules are disassembled. As a result, microtubules cannot correct the position of the nucleus after manipulation. This causesmispositioning of both the mid1p cortical domain and the actomyosin ring, and cells divide off-center. (D) Displacement of theprometaphase nucleus does not affect cleavage plane positioning. The sharp mid1p ring is no longer coupled to the nucleus: the acto-myosin ring assembles at the cell middle, even if the nucleus is off-center.

Microtubule

Mid1pSpindle pole bodyContractile ring

Nucleus

+

+

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Interphase Prophase Prometaphase Cytokinesis (symmetric)

No manipulation+

+

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Cytokinesis (symmetric)

Nucleus displacedduring interphase

Cytokinesis (asymmetric)

Nucleus displacedduring prophase

Cytokinesis (symmetric)Nucleus displacedduring prometaphase

Current Biology

A

B

C

D

the next question to be solved is:how does the nucleus specify the‘mid1 domain’ at the cell cortex?One possibility is that nuclearexport of mid1p, together withdiffusion, targets mid1p to the cellcortex overlying the nucleus.However mid1p associates withthe cortex, the question is how itscortical localization is maintainedat the cell equator.

Double septin rings assemble atthe division site in both fissionand budding yeast and, at least inthe latter organism, they functionas diffusion barriers to confinecytokinesis factors to the divisionsite [11]. But double septin ringsform only during anaphase in S.pombe, which is too late toinfluence mid1p distribution.Alternatively, a physical linkbetween the nucleus and thecortex may control mid1plocalization. A possible candidateis the endoplasmic reticulum (ER),which is formed by membranesextending from the nuclearenvelope to the entire cell cortex[12]. Interestingly, recent studiesshow that, in budding yeast, thisorganelle is highlycompartmentalized [13],indicating the existence of ERdiffusion barriers in this, andperhaps other, organisms.Whether mid1p distribution isdependent on ER membranes orother, as yet unidentifiedstructures, is an issue thatremains to be addressed.

Beyond advancing ourunderstanding of cleavage planespecification, these studies [5,6]also highlight the role ofmicrotubule networks inmaintaining subcellularorganization. Many processesinvolving directed nuclearmovements depend onmicrotubule pulling, as opposedto pushing. For example,migration of the male pronucleustowards the center ofCaenorhabditis elegans eggsdepends on the combined actionof microtubules and dyneinmotors [14], as does nuclearmigration into the bud in buddingyeast [15]. On the other hand,microtubule-based pushing of anorganelle could be the bestsolution to the problem of findingthe geometrical center in a

confined space. As has beennoted, precise regulation ofmicrotubule dynamics is essentialeven in this simple case, sincemicrotubules that are too stablewould bend around the cell endsresulting in mispositioning of thenucleus [16]. The use ofmicromanipulation techniques incells defective in specific aspectsof microtubule dynamics promisesrapid advances in ourunderstanding of how cellsmonitor their spatial organization.

References1. Glotzer, M. (2001). Animal cell

cytokinesis. Annu. Rev. Cell Dev. Biol.17, 351–386.

2. Chang, F., and Nurse, P. (1996). Howfission yeast fission in the middle. Cell84, 191–194.

3. Drummond, D.R., and Cross, R.A. (2000).Dynamics of interphase microtubules inSchizosaccharomyces pombe. Curr.Biol. 10, 766–775.

4. Tran, P.T., Marsh, L., Doye, V., Inoue, S.,and Chang, F. (2001). A mechanism fornuclear positioning in fission yeastbased on microtubule pushing. J. CellBiol. 153, 397–411.

5. Tolic-Nørrelykke, I.M., Sacconi, L.,Stringari, C., Raabe, I., and Pavone, F.S.(2005). Nuclear and division-planepositioning revealed by opticalmicromanipulation. Curr. Biol. 15, 1212-1216.

6. Daga, R.R., and Chang, F. (2005).Dynamic positioning of the fission yeastcell division plane. Proc. Natl. Acad. Sci.USA 102, 8228–8232.

7. Wu, J.Q., Kuhn, J.R., Kovar, D.R., andPollard, T.D. (2003). Spatial and temporalpathway for assembly and constrictionof the contractile ring in fission yeastcytokinesis. Dev. Cell 5, 723–734.

8. Sohrmann, M., Fankhauser, C.,Brodbeck, C., and Simanis, V. (1996).

The dmf1/mid1 gene is essential forcorrect positioning of the divisionseptum in fission yeast. Genes Dev. 10,2707–2719.

9. Chang, F., Woollard, A., and Nurse, P.(1996). Isolation and characterization offission yeast mutants defective in theassembly and placement of thecontractile actin ring. J. Cell Sci. 109,131–142.

10. Paoletti, A., and Chang, F. (2000).Analysis of mid1p, a protein required forplacement of the cell division site,reveals a link between the nucleus andthe cell surface in fission yeast. Mol.Biol. Cell 11, 2757–2773.

11. Dobbelaere, J., and Barral, Y. (2004).Spatial coordination of cytokineticevents by compartmentalization of thecell cortex. Science 305, 393–396.

12. Pidoux, A.L., and Armstrong, J. (1993).The BiP protein and the endoplasmicreticulum of Schizosaccharomycespombe: fate of the nuclear envelopeduring cell division. J. Cell Sci. 105,1115–1120.

13. Luedeke, C., Frei, S.B., Sbalzarini, I.,Schwarz, H., Spang, A., and Barral, Y.(2005). Septin-dependentcompartmentalization of theendoplasmic reticulum during yeastpolarized growth. J. Cell Biol. 169, 897-908.

14. Kimura, A., and Onami, S. (2005).Computer simulations and imageprocessing reveal length-dependentpulling force as the primary mechanismfor C. elegans male pronuclearmigration. Dev. Cell 8, 765–775.

15. Pearson, C.G., and Bloom, K. (2004).Dynamic microtubules lead the way forspindle positioning. Nat. Rev. Mol. CellBiol. 5, 481–492.

16. Dogterom, M., Kerssemakers, J.W.,Romet-Lemonne, G., and Janson, M.E.(2005). Force generation by dynamicmicrotubules. Curr. Opin. Cell Biol. 17,67–74.

Institute of Biochemistry, Swiss FederalInstitute of Technology, 8093 Zürich,Switzerland.

DOI: 10.1016/j.cub.2005.07.040

Dispatch R597

Anne E. Magurran

Population geneticists andcommunity ecologists often asksimilar sorts of questions. Bothsets of scientists are interested inmeasuring, and understanding,the number and distribution ofbiological variants found in nature.Indeed the conceptual linksbetween the disciplines arerecognized in the most widely

used definition of biologicaldiversity, devised by theConvention on BiologicalDiversity, which refers directly to‘variability among livingorganisms’ and stresses that thisincludes diversity within speciesas well as between species [1].But despite striking parallels inapproach there have been fewattempts to examine these levelsof diversity simultaneously.

Ecology: Linking Species Diversityand Genetic Diversity

Although there is a great deal of interest in the biological diversity ofspecies and of genes, it is only recently that researchers have begun toinvestigate the processes that exert parallel influences on thesedifferent levels of diversity.