The acquisition of cell polarity is among the earlieststeps during the development of multicellular eukary-otic organisms. As embryonic cells are incorporatedinto the blastoderm epithelia they begin to display thehallmark characteristics of apical–basal polarity, andduring successive stages of development, cell polaritycontinues to play a significant role, influencing patternsof cell division, shape change and movement.Therefore, developmental biologists have activelysought to elucidate the genetic programs that regulatethe specification and maintenance of cell polarity.Whereas much work has addressed the determinationof apical–basal polarity in epithelia, relatively little isknown about the specification of polarity orthogonal tothe apical–basal axis, known as planar polarity, or tis-sue polarity. Nevertheless, planar polarity is integral tothe function of many tissue-systems, ranging from thespecialized hair cells of the vertebrate ear to the dynamiccilia of the tracheal and reproductive tract epithelia1.

Here, we review recent progress in elucidatingmechanisms of planar polarity specification, with spe-cial attention to the role played by the Frizzled (FZ)family of serpentine membrane receptors and its down-stream signal transducers. Because the role of FZ signal-ing in regulating cell polarity has been studied first andmost thoroughly in Drosophila, we have concentratedon this system. In light of previous excellent reviews1–4,our discussion is focused on the most recent develop-ments and emphasizes the remaining conceptual hur-dles for a coherent model of polarity signaling.

Drosophila FZ was first identified many years agofor its role in regulating planar polarity5. Subsequently,a large number of FZ homologs, including DrosophilaFZ2, have been found in numerous organisms, wherethey have been proposed to function as cognate receptorsfor WNT signaling molecules6,7. WNTs, of which murine

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50 Jeffreys, A.J. et al. (1994) Nat. Genet. 6, 136–14551 Jeffreys, A.J. et al. (1995) Electrophoresis 16, 1577–158552 Baumer, A. et al. (1998) Hum. Mol. Genet. 7, 887–89453 Lopes, J. et al. (1998) Hum. Mol. Genet. 7, 141–14854 Antonarakis, S.E. et al. (1995) Blood 86, 2206–221255 Lazaro, C. et al. (1996) Hum. Genet. 98, 696–69956 Dallapiccola, B. et al. (1993) Am. J. Med. Genet. 47, 921–92457 Overhauser, J. et al. (1990) Am. J. Med. Genet. 37, 83–8658 Lopes, J. et al. (1997) Nat. Genet. 17, 136–13759 Wirth, B. et al. (1997) Am. J. Hum. Genet. 61, 1102–111160 Olson, S.B. and Magenis, R.E. (1988) in The Cytogenetics

of Mammalian Autosomal Rearrangements (Daniel, A.,ed.), pp. 583–599, Liss

61 Short, R.V. (1997) Acta Paediatr. 86, 3–762 Wolfe, K.H., Sharp, P.M. and Li, W.H. (1989) Nature 337,

283–28563 Hughes, A.L. and Yeager, M. (1997) J. Mol. Evol. 45, 125–13064 Smith, N.G.C. and Hurst, L.D. J. Mol. Evol. (in press)65 Eyre-Walker, A. (1991) J. Mol. Evol. 33, 442–44966 Debry, R.W. and Marzluff, W.F. (1994) Genetics 138, 191–20267 Sharp, P.M. et al. (1995) Philos. Trans. R. Soc. London Ser.

B 349, 241–24768 Ohta, T. (1993) Proc. Natl. Acad. Sci. U. S. A. 90,

10676–10680

69 Kisakibaru, Y. and Matsuda, H. (1995) Jpn. J. Genet. 70,373–386

70 Clermont, Y. (1966) Am. J. Anat. 118, 509–52471 Vogel, F. and Motulsky, A.G. (1997) Human Genetics:

Problems and Approaches (3rd edn), Springer-Verlag72 Dryja, T.P., Morrow, J.F. and Rapaport, J.M. (1997) Hum.

Genet. 100, 446–44973 Kitamura, Y. et al. (1995) Hum. Mol. Genet. 4, 1987–198874 Bellus, G.A. et al. (1995) Am. J. Hum. Genet. 56, 368–37375 Jadayel, D. et al. (1990) Nature 343, 558–55976 Szijan, I. et al. (1995) J. Med. Genet. 32, 475–479

Reference added in proof77 Kuick, R.D. et al. (1992) Proc. Natl. Acad. Sci. U. S. A. 89,

7036–7040

L.D. Hurst is in the Department of Biology andBiochemistry, University of Bath, Claverton Down, Bath,UK BA2 7AY. H. Ellegren is in the Department of Animal Breeding andGenetics, Swedish University of Agricultural Sciences, Box597, S-751 24 Uppsala, Sweden.

0168-9525/98/$ – see front matter © 1998 Elsevier Science All rights reserved. PII: S0168-9525(98)01584-4

INT1 and Drosophila Wingless (WG) are the foundingfamily members, are secreted glycoproteins that regu-late cell proliferation and differentiation in a wide rangeof developmental contexts. While the ligand for FZ during

Frizzled signaling and thedevelopmental control ofcell polarityJOSHUA M. SHULMAN (jms78@hermes.cam.ac.uk)

NORBERT PERRIMON (perrimon@rascal.med.harvard.edu)

JEFFREY D. AXELROD (jaxelrod@cmgm.stanford.edu)

Within the last three years, Frizzled receptors have risenfrom obscurity to celebrity status owing to their functionalidentification as receptors for the ubiquitous family ofsecreted WNT signaling factors. However, the foundingmember of the Frizzled family, Drosophila Frizzled (FZ), wascloned almost a decade ago because of its role in regulatingcell polarity within the plane of an epithelium. In this review,we consider the role of FZ in this intriguing context. Wediscuss recent progress towards elucidating mechanisms forthe intracellular specification of planar polarity, and furtherreview evidence for models of global polarity regulation atthe tissue level. The data suggest that a genetic ‘cassette’,encoding a set of core signaling components, could patternhair, bristle and ommatidial planar polarity in Drosophila,and that additional tissue-specific factors might explain thediversity of signal responses. Recently described examplesfrom the nematode and frog suggest that the developmentalcontrol of cell polarity by FZ receptors might represent afunctionally conserved signaling mechanism.

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planar polarity specification remains unknown, itsstrong homology to other FZ receptors suggests thatanother Drosophila WNT could fulfill this role. Parallelstudies of WNT signaling and the FZ polarity pathwayhave indicated that divergent signaling mechanisms areinvolved (Fig. 1). Whereas the WG and FZ signalingpathways both require the function of Dishevelled (DSH),they demonstrate differential requirements for moredownstream factors8,9. In WG signaling, DSH antago-nizes the serine-threonine kinase Zeste-white3/ GSK3b,leading to the intracellular accumulation of Armadillo/b-catenin and the modulation of target gene expres-sion via the Pangolin (PAN)/LEF1 transcription factor10.By contrast, during FZ signaling, DSH might regulatesmall GTPases such as RHOA, as well as a number ofadditional pathway- and tissue-specific factors8,9,11.

The genetic programming of planar polarity inDrosophila

Drosophila is ideally suited for the genetic analysisof planar polarity specification because the adultexoskeleton is covered with easily scored, parallelarrays of cuticular structures that are polarized withrespect to the body or limb axes (Fig. 2). On the thoraxand abdomen, hairs and bristles project posteriorly, andon the wings and legs, these structures point distally.Planar polarity is also apparent in the eye, where eachommatidium possesses an intrinsic polarity owing tothe invariant arrangement and rotation of the con-stituent photoreceptor cluster relative to the dorsal-ventral midline. Using a number of approaches, investi-gators have identified several ‘tissue polarity genes’required for the specification of planar polarity inDrosophila. Table 1 lists these genes, the tissues inwhich they function and the identity of the encodedprotein, if known. In only a few instances has the mol-ecular characterization of these genes yielded clues totheir function. Despite the dramatic differences in themorphogenesis of hairs, bristles and ommatidia, mu-tations in a subset of these genes, including fz, dsh,prickle (pk), RhoA, strabismus (stbm) and Van Gogh(Vang), disrupt the polarity of all three structures(Fig. 2). Throughout this review, we will refer to thesegenes as the ‘core’ group, since they might function as aconserved signaling cassette for the specification of pla-nar polarity (Fig. 1); however, at present, there is still nodirect evidence that pk, stbm or Vang are componentsof the FZ signaling pathway. In the remainder of thissection, we review the development of hair, bristle andommatidial planar polarity, and describe the phenotypicconsequences of mutations in the tissue polarity genes.

Each cell of the developing wing epithelium nor-mally produces a single, distally directed hair (tri-chome) (Figs 2, 3). This planar polarity arises from thepolarized assembly of a single actin bundle ‘pre-hair’ tothe distal vertex of each hexagonal wing cell12.Mutations in the core polarity genes (at least those thathave been examined) abolish this aspect of subcellularasymmetry, resulting in prehair formation at the centerof each cell, and deflecting hairs from their wild-typedistal polarity. Mutations in the tissue-specific genes,inturned (in), fuzzy (fy) and multiple wing hairs (mwh),cause polarity disruptions and the production of extrahairs, suggesting they might normally function to inhibit

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hair initiation and/or outgrowth. The similar phenotypesproduced by a dominant-negative form of Rac1, as wellas a RhoA mutant, implicate these small GTPases in thepathway as well11,13,14. Genetic epistasis studies suggestthat the core polarity genes fz and dsh function sequen-tially and upstream of in, fy and mwh, and that this path-way coordinately functions to derepress hair outgrowthat the distal vertex of each wing cell12,15.

By contrast with wing hairs, very little is knownabout the determinants of bristle planar polarity.Polarized arrays of bristles are found at the wing mar-gin, along the length of each leg and over the thoracicsurface, and mutations in the tissue-polarity genes candisrupt bristle polarity at all of these loci (Fig. 2). Eachbristle is the ultimate product of a single sensory organprecursor cell (pI), which gives rise to a four-cell clusterincluding a shaft, socket, sheath and neuron (Fig. 3)16.A recent report suggests that FZ signaling might regulatethe pattern of asymmetric cell division that generatesthis cluster17. Whereas, on the dorsal thorax of wild-typeflies, the division of pI is invariably parallel to the ante-rior–posterior axis, mutations in fz and dsh randomizethe orientation of the pI mitotic spindle and cleavageplane. Interestingly, however, the asymmetric partition-ing of Numb protein and the orientations of subsequentdivisions are unaffected in these mutants, suggestingthat additional, FZ-independent, pathways contribute tobristle polarity. In addition, it remains unclear to whatextent the oriented pI mitotic division is linked to theultimate polarity of the sensory bristle produced.

Each Drosophila eye is an exquisitely orderedhexagonal array comprised of hundreds of photorecep-tor clusters, termed ommatidia. During the develop-ment of the eye imaginal disc, a wave of differentiationcalled the morphogenetic furrow sweeps from posterior

WG

FZ2

DSH

ZW3/APC/Axin

Gene expression

ARM

PAN

WNT (?)

FZ? pk

stbmVang

DSH

RHOA

Gene expression Cytoskeletal polarization

Tissue-specific factors

FIGURE 1. The WG and FZ signal transduction pathways.Although DSH is required for signal transduction in bothcontexts, distinct downstream pathways are activated. pk, stbmand Vang are required for hair, bristle and ommatidial planarpolarity specification, but have not yet been shown to becomponents of the FZ pathway and might, therefore, have adirect or indirect role in FZ signal transduction. We speculate thatthe small GTPase RHOA could initiate a branched signal withnuclear and cytoskeletal targets.

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to anterior, recruiting the eight photoreceptors into indi-vidual ommatidial units18. The topography of furrow initiation and progression, which is coordinately con-trolled by WG, Hedgehog and Decapentaplegic/TGF-bsignaling factors, ensures the precise stacking of matur-ing ommatidia and might define the position of the eye‘equator’ at the dorsal–ventral midline4. Posterior to thefurrow, each ommatidium rotates 90° towards the equa-tor and acquires chirality, such that the dorsal and ven-tral eye hemispheres ultimately contain ommatidialforms that are reflected across an equatorial axis of mir-ror symmetry (Figs 2, 3). Mutations in the core group oftissue-polarity genes, fz, dsh, pk, stbm and RhoA,induce ommatidial chirality reversals, misrotated clus-ters, and defects in photoreceptor fate specification andpositioning11,19,20. Such phenotypes suggest that thesegenes function in a pathway that specifies ommatidialpolarity. In direct analogy to the wing, it has been pro-posed that this core polarity pathway regulates more

downstream, tissue-specific factors, such as Nemo andRoulette, which have been implicated in the control ofommatidial cluster rotation21.

FZ signaling and global polarity controlWhile phenotypic and epistasis analysis have

helped define a genetic pathway for polarity specifi-cation, they have not addressed the intriguing problemof global regulation at the whole-tissue level. In whichcells is tissue-polarity gene function absolutely required?From where does the cue for polarity originate? How isthe signal propagated from one cell to the next? Muchof the most exciting recent work aims to answer thesequestions through the use of genetic mosaic experiments.

Eleven years ago, the startling discovery was madethat mitotic clones of fz mutant tissue in the wing showa non-autonomous effect, such that hair polarity is dis-rupted distal to, but not proximal to the clone22.Whereas most fz alleles, including nulls, demonstratethis non-autonomy, four independently isolated mis-sense alleles at the identical residue behave cell-autonomously23. More recently, a distal to proximal gra-dient of FZ overexpression was induced via aheat-shock responsive transgene by dripping hot waxover the distal tip of pupal wing discs24. This ‘waxing’protocol induced dramatic reversals in wing hair polar-ity, suggesting that planar polarity points from high tolow levels of FZ signaling (presumably proximal to distalin the wild-type case). By contrast with fz, genes pro-posed to function downstream, such as dsh, in and fy,show solely cell-autonomous effects in mutant clones,consistent with their proposed role in intracellular signaltransduction25–29. Significantly, mutant clones of zw3and arm do not disrupt planar polarity in the wing, sug-gesting that FZ uses a signal transduction mechanismthat is distinct from the WG pathway (Fig. 1)8.

Cumulatively, these studies support at least twopotential models for the global control of FZ signalingin the wing24. One possibility is that the signal is seriallyregenerated in a wave that propagates along the prox-imal–distal axis. In such a scenario, the existence of cell-autonomous and non-autonomous fz allelesmight suggest that distinct pathways downstream of FZmediate cell polarization and intercellular signal relay,respectively15. In an alternative model, the FZ ligandmight diffuse out from a localized source and act over long distances as part of a morphogen gradient.The picture has become still further confused with theidentification of a class of genes, including pk, Vangand dachsous (ds), that might influence the directional-ity of FZ signal propagation or interpretation. Whereasa ds mutant background can enhance the distal non-autonomy of fz clones30, a dominant allele of pkconverts the direction of fz non-autonomy to proximal(P.N. Adler, pers. commun.). It is also remarkable that mutant clones of Vang have an effect completelyopposite to that of fz, disrupting hair polarity proximal,but not distal to the clone56. If these genes regulate sig-nal propagation, they might participate in either the ser-ial regeneration of FZ ligand or the control of ligand dif-fusion. It is also possible, however, that these genesmight have a more downstream role in signal interpre-tation, perhaps by specifying the orientation of cellpolarization.

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FIGURE 2. Wild-type and mutant planar polarity phenotypes. Hairpolarity is shown in wild-type and dsh1 wings; anterior is up anddistal is to the right. Bristle polarity is shown for a wild-type anddsh1 thorax; posterior is down. Examples of ommatidial polarity

are taken from wild-type and pk flies (images kindly provided byD. Gubb). The eye equator is highlighted in wild type, but is

obscured in the mutant. Ommatidial orientations are drawn in forclarity in both panels.

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In a manner analagous to that seen in the wing, fzmutant clones in the eye cause non-autonomousommatidial polarity disruptions that are biased towardsthe polar side of the clone (opposite to the equator)19.These studies also demonstrate a preferential requirementfor fz in the R3 and R4 photoreceptors, such that thecell with greater FZ activity will adopt the R3 cell fate andasymmetric positioning anterior to R4. Cumulatively, thesedata are consistent with a model whereby an equatorialsignal activates FZ leading to: (1) the correct specificationof R3 versus R4 cell fate; (2) the correct choice of ro-tational direction; and (3) the execution of a 90° turn. As inthe wing, mutant clones of genes that are proposed to actdownstream of fz, such as dsh (but see below), stbm,RhoA and nemo, show cell-autonomous effects in theeye11,19–21. It is important to note, however, that whereasFZ signaling conveys polarity information along theequatorial–polar axis, the ultimate chirality of omma-tidia also requires the passage of the morphogeneticfurrow, which contributes vectorial information alongthe anterior–posterior axis31–33. This dual requirementfor directional signals along the equatorial–polar and ante-rior posterior axes has been referred to in the literatureas the two-vector, or cruciform planar polarity model33,34.

By contrast with the wing polarity system, whereWG appears to play no role, gain-of-function or loss-of-function in WG signaling components can influenceommatidial polarity34. Mutant clones of arrow, zw3,arm and dsh (in rare cases) were shown to disruptommatidial polarity non-autonomously, with a biastowards the equatorial side of the clone, opposite towhat is seen for fz. These data have been interpreted tosuggest that eye planar polarity is specified via twosequential signaling cascades. WG first signals in thepolar to equatorial direction, inducing the expression ofa graded secondary signal, Factor X, which subsequentlyactivates the FZ pathway in the equatorial to polar

orientation34. Consistent with this scenario, wg isexpressed at the two poles of the eye disc, from where itmight diffuse inwards to pattern the dorsal–ventralaxis33,35–37. In future work, it will be important to deter-mine whether the effects of WG signaling perturbationson ommatidial polarity can be explained entirely by dis-ruptions in equator specification and the topography ofmorphogenetic furrow progression. Furthermore, sincethe WG and FZ signaling pathways share certain com-ponents, such as dsh, it is possible that experimentalperturbations of one pathway might cross-activate or sup-press signaling by the other. Indeed, ectopic expressionof wg during polarity specification in the wing might beable to titrate DSH activity from the FZ pathway8.

In strong analogy to the role proposed for WG inthe eye, two recent reports investigating abdominal pat-terning similarly implicate the Hedgehog (HH) signalingpathway with the induction of a secondary, polarizingsignal (again, Factor X)38,39. As described above for thethorax, the Drosophila abdomen is similarly decoratedwith arrays of hairs and bristles that project towards theposterior. Whereas HH acts as a gradient morphogen todirectly pattern positional identity in the anterior ofeach abdominal segment, HH signaling only indirectlyinfluences planar polarity, most probably via a secondarysignaling event. Although tissue polarity mutants doshow abdominal phenotypes25, it remains to be deter-mined whether the FZ pathway responds to Factor X inthis context. Nevertheless, if this hypothesis provestrue, the Drosophila eye and abdomen might exemplifya novel developmental paradigm for linking the patterningof positional information (via WG or HH, respectively),to the control of planar polarity by FZ.

Because the ligand for FZ remains unknown, experi-ments have only indirectly addressed the question ofhow the signal propagates from cell to cell, and over whatdistance it might act. However, given the likelihood that

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TABLE 1. The tissue-polarity genes

Affected tissues Gene (abbreviation) Hairs Bristles Ommatidia Protein identity Refs

frizzled (fz) X X X Seven-transmembrane protein; putative receptor for 5wnt signaling factors

dishevelled (dsh) X X X Novel, predominantly cytoplasmic protein with three 28, 29conserved domains: DIX (axin-like), PDZ (protein interaction), and DEP (implicated in G-protein regulation)

RhoA X ? X Small RAS-like GTPase 11strabismus (stbm) X X X Putative transmembrane protein with PDZ binding 20

domainVan Gogh (Vang) X X X Unknown 56prickle (pk) X X X Novel LIM-domain-containing protein (two isoforms, a

PK and SPLE produced from alternatively spliced RNAS)dachsous (ds) X ? ? Cadherin-like protein 30, 55inturned (in) X X – Novel, putative transmembrane protein 26fuzzy (fy) X X – Novel protein with four putative transmembrane 27

domainsmultiple wing hairs (mwh) X – – Unknownnemo (nmo) – – X Serine/threonine kinase; human homolog localizes to 21, 45

the nucleusroulette (rlt) – – X Unknown 21

aD. Gubb, pers. commun.

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the ligand is a WNT protein, it is tempting to extrapolatefrom the lessons of the WG signaling paradigm. Severalstudies have indicated that WG is likely to behave as amorphogen in a variety of contexts, perhaps supportingthe hypothesis that a gradient of diffusing ligand under-lies planar polarity specification40,41. Furthermore, it hasrecently been demonstrated that WG upregulates theexpression of its own receptor, DFz2, which in turnmodulates WG diffusion42. If this relationship also holdstrue for FZ and its ligand, the non-autonomous polaritydisruptions caused by fz mutant clones might arise fromdiscontinuities in the slope of a diffusing signal gradi-ent. While the mechanism for WNT diffusion remainsunknown, studies of mutant forms of WG are consistentwith the hypothesis that its movement might be regu-lated by a transcytosis mechanism43,44. The answer towhether these mechanisms of signal propagation areconserved for the planar polarity ligand ultimatelyawaits its identification.

Remaining hurdles for a model of polarity signalingTissue specificity

How can a common set of signaling molecules func-tion in a variety of cell types to generate an array of dis-tinct outputs? Explaining signaling specificity continues tobe a major challenge, and the case of Drosophila planar

polarity poses an especially diffi-cult problem. A successful polaritysignaling model must account forat least two properties. First, theunit of polarity, and therefore thenature of the signal responder, isdistinct in each tissue. Whereas inthe wing, a single cell responds topolarizing cues in order to correctlyorient a hair, the polarity of thoracicbristles and eye ommatidiainvolves higher-order, multi-cellclusters. Second, and equally per-plexing, the ultimate cellularresponse to the planar polarity sig-nal is uniquely tissue-specific. Inorder to specify the polarity ofhairs, bristles and ommatidia, FZsignaling must apparently be ableto regulate a wide range of cellularprocesses, including actin polymer-ization, mitotic spindle orientation,photoreceptor differentiation andommatidial rotation. One solutionto this dilemma is that tissue-specific factors lie downstream of a ‘core’ FZ signaling pathway andare responsible for mediating thedistinct signal responses. Thegenes in and fy are specificallyrequired for the morphogenesis ofhair and bristle polarity, and mwhis exclusively required for winghair polarity. Analogously, nemoand roulette selectively function ineye ommatidial rotation21. Neverthe-less, whereas in and fy encodenovel, putative integral membrane

proteins26,27, Nemo is a nuclear kinase21,45, making itmore difficult to envision how a common signaling path-way might impinge upon such distinct target molecules.

Pathway redundancyMysteriously, null mutations in the tissue polarity

genes do not result in random polarity patterns. Instead,mutations cause distinctly aberrant, but reproduciblepolarity phenotypes. In a fz or dsh mutant wing, forexample, selected regions exhibit intricate polarity pat-terns in which adjacent cells project hairs in differentdirections. By contrast, in other regions, cells possess acommon but aberrant, non-distal hair polarity.Significantly, the spatial organization of these defects isnot random, but instead creates highly reproduciblepatterns for all wings of a given null mutant genotype.This peculiar feature of the phenotypes suggests that,while tissue polarity gene products are required for thecorrect implementation of polarity, a partially redun-dant system might still convey some directional infor-mation in their absence. One possible scenario is thatthe cytoskeletal polarity of each epithelial cell is some-how linked to that of adjacent cells, perhaps via adherensjunctions. Alternatively, the apparent redundancy inplanar polarity specification might be explained bymechanical constraints imposed by the geometry of the

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Hairs

Bristles

Ommatidia

Wild type Mutant

Proximal Distal 12

3

(1) fz, dsh, pk (2) in, fy (3) mwh

NumbSpindle Sheath Neuron Anterior

PosteriorSocketpI

ShaftpIIb

pIIa

Equator Posterior

Dorsal pole

Ventral pole

Anterior

FIGURE 3. Drawings of wild-type and mutant planar polarity specification in three tissues.In wings, hair growth is restricted to the distal vertex of each hexagonal wing cell. In fz,

dsh or pk mutants, mispolarized hairs elongate from the center of each cell, whereas in, fyand mwh mutants grow ectopic, mispolarized hairs that are restricted to the cell periphery.

A thoracic bristle is the product of a stereotyped sequence of asymmetric cell divisions.Mutations in fz or dsh randomize the orientation of the pI mitotic spindle, which is aligned

parallel to the anterior–posterior axis in wild type, but do not disrupt the asymmetricsegregation of Numb protein or the orientations of subsequent divisions. Ommatidia

adopt one of two chiral forms that are selectively found in the dorsal or ventral eyehemispheres. In fz, dsh, pk, stbm or RhoA mutants, some ommatidia are misrotated orshow chirality reversals. Defects in photoreceptor cell fate specification can generate

ommatidia that vary from the wild-type, hexagonal shape, and might be achiral.

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developing imaginal epithelia and its constituent cells4.Either of these scenarios would predict that, in theabsence of FZ signaling, epithelial cells obey a defaultprepattern for planar polarity.

However, prepatterning by redundant mechanismscan still not adequately explain why the null mutantphenotype of each tissue polarity gene is distinct25. Ifeach of the gene products functioned in a linear path-way to specify planar polarity, we might predict thatremoval of any individual component would block sig-nal transduction, resulting in a uniform phenotypicresponse. Instead, the defective polarity pattern of fz,dsh or pk null mutants is as unique as a ‘finger print’,instantly betraying their genotype to the experiencedexaminer. This observation, in addition to the lack ofdefinitive epistasis amongst the genes, might indicatethat some of the factors function in a multi-protein com-plex as opposed to occupying discrete molecularniches on a sequential assembly line.

Destination unknownA key goal for understanding planar polarity specifi-

cation in Drosophila is to identify the ultimate molecu-lar targets of FZ signaling. Nevertheless, it has been dif-ficult thus far to determine whether FZ signalingimpinges directly on the cytoskeleton, the nucleus, orperhaps even diverges in both directions during thespecification of planar polarity. Recent experimentsusing depolymerizing drugs have demonstrated that theactin and microtubule cytoskeletons are both requiredfor asymmetric prehair initiation and subsequent hairoutgrowth46. Similarly, regulating the orientation of themitotic spindle and cleavage plane during bristle mor-phogenesis presumably involves cytoskeletal control.Therefore, the specification of hair and bristle polaritymight be mediated by a common cytoskeletal regulat-ory molecule. RHOA, a small GTPase that is known todirectly influence cytoskeletal dynamics and has beenimplicated downstream of FZ, is obviously a prime can-didate11,47. Recently, we have demonstrated that the FZreceptor can recruit DSH to the cell membrane, and thatthe dsh1 allele, which specifically abolishes DSH activityin polarity signaling (but not in WG signaling), encodesa missense mutation in the C-terminal DEP domain8,9.Because DEP domains have been described in severalproteins that regulate small GTPases48, it is tempting tospeculate that physiological FZ signaling might promoteasymmetric recruitment of DSH to the membrane,where it might activate small GTPases such as RHOA,leading directly to a polarized cytoskeletal response. Itwill be interesting to determine whether any of therecently identified RHO effectors with proposed roles incytoskeletal control participate in this pathway47,49.

However, RHO GTPases are also known to bepotent activators of the JNK and MAP kinase signalingcascades that each converge on the cell nucleus49, rais-ing the possibility that activation of RHOA could alterprograms of gene transcription. More recently, mutantalleles of JNK pathway components were shown toinfluence ommatidial planar polarity in a sensitizedgenetic background, and more directly, DSH expressioncould induce phosphorylation of the transcription factor,JUN, in a cell-culture system9,11. Although it is possiblethat a cytoskeletal response might be a secondary, indirect

consequence of changes in gene transcription, it is diffi-cult to imagine how FZ signaling could convey thedirectional information necessary to polarize cells solelyby activating a nuclear pathway. Therefore, it is perhapsmost plausible to postulate that FZ signaling bifurcatesdownstream of RHOA, and that independent signalbranches induce cytoskeletal vs nuclear responses(Fig. 1). Such a model raises the intriguing possibilitythat the diversity of cellular responses to FZ signalingmight reflect differences in the relative strengths of dis-tinct RHO-dependent pathways that impinge on thecytoskeleton or nucleus.

Lessons from worms and frogsTo what extent can we find evidence for a FZ-based

polarity signaling system in other organisms? In thenematode, Caenorhabditis elegans, genes encodingmembers of a WNT signaling pathway are required forthe specification of endodermal (E) cell fate in onedescendent of the EMS blastomere50,51. WNT activity isalso required for the rotation of the EMS and othermitotic spindles, resulting in polarized cell divisions.Whereas induction of the E cell fate was found torequire the homologs of WG, FZ, ARM and Pangolin/Lef1, only the WG and FZ homologs were necessary forEMS spindle polarization. These results suggest a diver-gence in the nematode WNT signaling pathway, down-stream of FZ but upstream of ARM, with at least onebranch of the pathway controlling cell-polarization events.It will be interesting to determine if the molecular basisof this pathway divergence is analagous to the case inDrosophila, and whether the control of EMS spindle ori-entation is mechanistically similar to the proposed role forFZ signaling in polarizing bristle precursor cell divisions17.

Emerging data from studies of the frog, Xenopuslaevis, reveal further evidence for divergent WNT path-ways. Whereas XWnt1 induces embryonic axis duplicationby activating a WG-like signaling pathway, XWnt5a failsto induce axis duplication but, rather, alters morpho-genetic movements during gastrulation52. Interestingly, aheterologous FZ, human Fz5, can serve as an ‘adaptor’that binds XWnt5a but retains the specificity to activatethe XWnt1 signaling pathway, thus allowing XWnt5a toinduce axis duplication53. Therefore, in Xenopus, whileone FZ pathway appears to be similar to the WG path-way and impinges on the nucleus to modulate geneexpression, we speculate that the other might resemblethe FZ-mediated polarity signal, either in transductionmechanism, polarizing function, or both. Lastly, murineWNTs have been divided into multiple classes based ontheir abilities to induce transformation of culturedcells54. While little is known about the signaling mecha-nisms used by these WNTs, one explanation for theirdiffering transforming efficiencies might be the existenceof divergent signaling pathways, one or more of whichcould control cell polarity.

Concluding remarksGenetic investigation into the mechanisms of planar

polarity specification in Drosophila has generated sub-stantial recent progress. Cumulatively, the data suggestthat the FZ receptor and a cassette of core signalingmolecules collaborate with tissue-specific factors to pat-tern planar polarity in multiple tissues. Nevertheless, many

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questions still remain, especially regarding how intra-cellular signal specificity is maintained and the nature ofglobal signaling control. While recent work in the nema-tode and frog is tantalizing, much work lies ahead todetermine whether the control of cell polarity by FZreceptors is conserved. However, judging by the remark-able conservation of WNT and other signal transductionpathways, we anticipate finding numerous roles for FZsignaling in planar polarity control in higher organisms.

AcknowledgementsWe thank our colleagues P.N. Adler, M. Mlodzik and

D. Gubb for communicating data before publication.Special thanks to D. Gubb for generously providing theeye images in Fig. 2, and to D. Gubb, P. Lawrence andothers for stimulating conversation regarding the manu-script. We apologize to authors whose data we wereforced to omit due to space limitations.

Note added in proofIt has recently been demostrated that Van Gogh and

stabismus are allelic56.

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J.M. Shulman is in the Wellcome/CRC Institute, TennisCourt Road, Cambridge, UK CB2 1QR.N. Perrimon is in the Howard Hughes Medical Institute,Department of Genetics, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.J.D. Axelrod is in the Department of Pathology, R226A,Stanford University School of Medicine, 300 Pasteur Drive,Stanford, CA 94305, USA.

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