The role of transcription factors in the developing Drosophila eye

~EVIEWS 6 Hiilskamp, M., Pfeifle, C. and Tautz, D. (1990) Nature 346, 577-580 7 Green, J.B.A. (1990) BioEssalJs 12, 437--439 8 Eichele, G. (1989) Trends Genet. 5, 246-251 9 Brockes, J. (199l) Nature 350, 15 10 Ruiz i Altaba, A. and Melton, D.A. (1990) Trends Genet. 6, 57-64 11 Nieuwkoop, P.D. (1973) Adv. Morphol. 10, 1-39 12 New, H.V. and Smith, J.C. (1990) Curr. Opin. Cell Biol. 2, 969-974 13 Smith, J.C. (1989) Development 105,665-677 14 Asashima, M. et al. (1990) Wilhelm Roux's Arch. Dev. Biol. 198, 330-335 15 Smith, J.C., Price, B.M,, Van, N.K. and Huy[ebroeck, D. (1990) Nature 345, 732-734 16 van den Eijnden Van Raaij, A.J. et al. (1990) Nature 345, 819-822 1 7 Thomsen, G. et al. (1990) Cell 63, 485-493 18 Wolpert, L. (1989) in The Molecular Basis of Positional Signalling (Development Supplement) (Kay, R. and Smith, J., eds), pp. 3-12, Company of Biologists 19 Green, J.B.A. etal. (1990) Development 108, 229-238 20 Green, J.B.A. and Smith, J.C. (1990) Nature 347, 391-394 21 Struhl, G., Struhl, K. and Macdonald, P.M. (1989) Cell 57, 1259-1273 22 Driever, W., Thoma, G. and Niisslein-Volhard, C, (1989) Nature 340, 363-367 23 Lewis, J.H., Slack, J.M.W. and Wolpert, L. (1977),L Theor. Biol. 65, 579-590 24 Goldbeter, A. and Wolpert, L. (1990)J. Theor. Biol. 142, 243-250 25 Gurdon, J.B. (1988) Nature 336, 772-774 26 Rosa, F.M. (1989) Cell 57, 965-974 27 Slack, J.M.W. (1991) Nature 349, 17-18

T h e differentiation of distinct cell types is the result of changes in the patterns of gene expression in precursor cells. These patterns are largely regulated by changes in the activities of the pool of transcription factor proteins, and different cell types may be maintained by the expression of different sets of such factors. However, developmental changes of cell type must begin with the modulation of the activities of pre-existing transcription factors. Thus to understand development we must understand how inductive signals from outside a target cell modulate the transcription factor activities within it. The Drosophila adult compound eye consists of about 800 similar facets, or ommatidia, each comprising eight neuronal photoreceptor cells [six outer cells (R1-6) and two different inner cells (R7 and R8)] and 12 accessory cells 1--~ (Fig. 1). The eye develops from an unpatterned monolayer epithelium (the eye imaginal disc), beginning in the third larval instar when a transverse indentation, the morphogenetic furrow, begins to move across the eye field from posterior to anterior 1-5 (Fig. 1). Behind the furrow, the cells of the developing ommatidia are thought to be assembled by the sequential induction of multipotent precursor cells through inductive signals from their predecessors, beginning with the-photoreceptor cells. There are three categories of evidence for the sequential induction of the ommatidial cells; considered as a

28 Ellinger-Ziegelbauer, H. and Dreyer, C. (1991) Genes Dev. 5, 94--104 29 Musci, T.J., Amaya, E. and Kirschner, M.W. (1990) Proc. Natl Acad. Sci. USA 87, 8365-8369 30 Gillespie, L.L., Paterno, G.D. and Slack, J.M. (1989) Development 106, 37-46 31 Stern, C.D. and Canning, D.R. (1990) Nature 343,

273--275 32 Turner, A., Snape, A.M., Wylie, C.C. and Heasman, ~l. (1989) J. Exp. Zool. 251,245-252 33 Cabrera, C.V. (1990) Development 109, 733-742 34 Jones, E.A. and Woodland, H.R. (1987) Development 101,

557-563 35 Weeks, D.L., Rebagliati, M.R,, Harvey, R.P. and Melton, D.A. (1985) Cold Spring Harbor Syrup. Quant. Biol. 50,

21-30 36 Weeks, D.L. and Melton, D.A. (1987) Proc. NatlAcad. Sci. USA 84, 2798--2802 37 Dale, L., Matthews, G., Tabe, L. and Colman, A. (1989) EMBOJ. 8, 1057-1065 38 Tannahill, D. and Melton, D.A. (1989) Development 106, 775-785 39 Mitrani, E. et al. (1990) Cell 63, 495-501 40 Dale, L. and Slack, J.M. (1987) Development 99, 527-551 41 Crick, F.H.C. (1970) Nature 225, 420--422 42 Cooke, J. (1983) J. Embryol. Exp. Morpbol. 76, 95-104 43 Green, J.B.A. and Cooke, J. Semin. Dev. Biol. (in press) 44 Smith, J.C. (1981)J. Embryol. Exp, Morphol. 65 (Suppl.), 187-207 .].B.A. GREENANDJ.C. SMITHAREIN THENATIONALINSTITUTE FORMEDICALRESEARCH,THERIDGEWAY,MILLHILL, LONDON NW7 IAA, UK.

The r01e of transcription factors in the developing Drosophilaeye KEVIN MOSES In the developing Drosophila compound eye, multipotent precursor cells are induced to develop into particular cell types through sequential induction. In the target cells, transcription factors may be modulated by the inductive signals to execute their instructions. Four recentlF isolated genes may encode such developmentally modulated transcription factors. whole, the evidence is compelling. (1) An alternative, lineage-directed model 6 has been discounted by genetic mosaic experiments 1,7,~. (2) A precise and reproducible sequence of specific contacts is established and maintained between the developing ommatidial cells 2.9. The morphology of these cell contacts suggests they may be involved in a process of inductive signalling. (3) The discovery of cell homeotic genes (e.g. sevenless and bride ofsevenless) for which both genetic mosaic data 1°-12 and molecular data 13Ai' support functions as cell surface receptors and ligands.

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FSGH Development and structure of the Drosophila compound eye. A larval eye antennal imaginal disc is shown at the end of the third larval instar, with anterior (A) down and posterior (P) tip. The antenna1 (ANT) precursor lies anterior to the eye (EYE) field, which is connected to the brain by the optic stalk (OS). The morphogenetic furrow (MF, see text) has progressed about half way across the eye field, moving from posterior to anterior. One area has been expanded to show, some of the sequential events of the first ~30 h of ommatidial assembly. Six of -18 columns (or stages) are shown; the intermediate columns have been omitted for clarity. Tile youngest ommatidium is closest to the furrow (-2 h post-furrow), at which stage only one cell (the R8 precursor, sho\~n :is a filled ellipse) is a neuron (by antigenic and morphological criteria). At the second stage shown (-6 h post-furrow) three cells ha~u become neurons (the RS, R2 and R5 precursors). At the third stage shown (~12 h post-furrow) five cells have become neurons Ithu R8, R2, R5, R3 and R4 precursors). At the fourth stage shown (~18 h post-furrow) seven cells have become neurons (the RS. R2, R~,, R3, R4, R1 and R6 precursors). At the fifth stage shown (~26 h post-furrow) all eight photoreceptor cell precursors have become: neurons (R8, R2, RS, R3, R4, R1, R6 and R7) and the first two accessory cone cells are developing (shown as open ovals). At the last stage shown (-36 h post-furrow) ~ ' o more cone cells have joined. Development proceeds into pupal life as tlne other a(cessorv cells are added, and cell morphogenesis continues. The ommatidial cells do not arise from an immediate single precursor cell. hut rather by means of sequential induction (for reviews of these events see Refs 1-5). An adult ommatidium is shown in kmgitudinat section (right) and cross-section (left). In the longitudinal section the apical (AP) surface of the retina is tip, and the basal (BA) stir face is down. At the top right :ire the two cells of the hair-nerve group (hng). The lens (le) and crystalline cone (co) form the optical elements of the ommatidium. They are secreted by the cone cells (c) and focus light onto the photoreceptor cells (pc). The photoreceptor cells are long neuronal cells containing rhabdomeres (rh, rod-shaped photosensitive organelles) and they elaborate axons basally. The sheathing pigment cells :ire not drawn individually (for clarity) and are represented :is the shaded arca. An apical and basal cross-section are shown: the orientation is the same as tbr the developing larval ommatidia shown to the left. The rhabdomeres lie in a characteristic trapezoidal arrangement. There :ire six outer photoreceptor cells (RI%) with large rhabd. that extend the full depth of the retina, and two central photoreceptor cells (R8 below R7) with smaller rhabdomeres. The f o u n d i n g cell of the d e v e l o p i n g o m m a t i d i u m is the p r e c u r s o r cell to p h o t o r e c e p t o r R8 (Fig. 1). Next, the six o u t e r p h o t o r e c e p t o r cell precursors join in a pairwise fashion: R2 a n d R5, then R3 a n d R4, t h e n R1 a n d R6. The last p h o t o r e c e p t o r cell precursor, R7, is followed by the accessory cells, b e g i n n i n g with the four c o n e cells. The entire process takes a b o u t four days 1-s. These events are highly reproducible, a n d individual cell types can be identified b y their position a n d m o r p h o l o g y . After their determination, the develo p i n g photoreceptor cells elaborate axons I and express a series of neural a n d cell type-specific antigens ~.15,1~'. The sequential i n d u c t i o n model p r o p o s e s that as each cell type differentiates it expresses specific cell

surface ligands. The adjacent multipotent precursor calls possess receptors for these ligands, a n d are thus ind u c e d to adopt the next appropriate cell fate'-L The successive steps of ommatidial d e v d o p m c n t arc a n a l o g o u s to the steps in a biochemical pathway, a n d by a similar logic, mutational genetics has b e e n used to identify c o m p o n e n t s required for some of these events. Such mutations can be studied both by their effects on the final adult structure and hy their effects o n cell morphology a n d g e n e expression in the developing eye. Thus far, the best geneticall} characterizcd step is the recruitment of the R-7 cell. This requires expression in R8 of the bride oJ'seveMess product 1", wlnich is a putative ligand i ~, and the expression in the R" cu]l

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cells (R3 and R4) does not proceed normally, and the rough l IW ~ I~il subsequent development of opa homeo, opa domain the ommatidia is abnormal 21. Genetic mosaic experiglass I,, ments 21 (see Fig. 4) showed Zinc fingers that the cells within which rough is required a r e - n o t those visibly affected by the seven-up [ type I mutation (R3 and R4), but the pair that precede them (R2 and R5). It may be that _ _ _ ~ \ \ \ \ \ \ \ \ \ \ \ ' ~ seven-up [ the rough gene product is type II L, I I I only required by R2 and R5 DNA Ilgan¢l binding binding to produce a signal that must be received by R3 and sina ~ ] R4 for their normal developmetal binding ment. Alternatively, rough may have a broader funcFIGli] tion in specifying cell type; Structures of four eye development transcription factors. The deduced sequence structures of in the absence of rough five potential transcription factor proteins that act in compound eye development are drawn to function, the R2 and R5 scale. The positions of known introns are shown as black triangles. The positions of other introns cells may be subject to a (not shown here) are estimated in the seven-up gene from cDNA restriction maps. Sequence homeotic transformation to a domains of interest are shown as shaded boxes. The rough protein is 350 amino acids and different photoreceptor cell contains two Gin/His-rich 'opa' repeats and a 61 residue homeodomain20.2LThe glass protein is type, causing a breakdown 604 amino acids and contains five Cyse-His_,zinc fingers22. The seven-up type I protein 2~is 543 in the normal sequence of amino acids and includes a 66 residue DNA-binding domain with two Cys~-Cys2 zinc fingers, cell inductions. and a 217 residue putative 'ligand-hinding' domain (by homology to the COUP factor~). The seven-up type II protein 23 is 746 amino acids and is collinear with the type I protein to position These two possible ex453, where it diverges after an alternative splice, truncating the 'ligand-binding' domain; the planations have been tested sequence is then shown as a striped box. The alternative splice removes the only intron known by misexpression experiin seven-up, since only cDNA sequence data are available. The sina protein e4 is 315 amino ments. Inappropriate exacids and contains a 38 residue possible metal-binding domain similar to one seen in the pression of embryonic homDicO~osteliumgene DG1 7(Ref. 36). The sina open reading frame is not interrupted by introns. eotic genes often leads to the transformation of cell fates. To test for such effects, the rough gene has been of the sevenless product 1°, a putative receptor 1-~, Some expressed ectopically in two ways26.27: by inducible other genes that may act in intercellular signaling have expression from a heat-shock promoter (hsro) and by been identified and isolated (e.g. Notch iv, scabrousTM and Ellipse lg), and four recently isolated eye development more restricted expression from a sevenless promoter genes may encode transcriptional regulators: rough 2°,21, (sevro). A single induction of hsro results in a scar in the glass 22, seven-up 23 and sina (seven in absentia) 2~. eye, and repeated induction can result in an eyeless phenotype because the ectopic rough protein can stop the progress of the furrow2% While the mechanism by rough which the ectopic expression of rough stops the furrow The rough gene encodes a protein with a homeodomain (Fig. 2) that is most similar to that of the Sex- in hsro is not clear, it may be that the rough protein combs-reduced product 2°,21. The homeodomain was anterior to the furrow disrupts the regulation of genes normally involved in the propagation of the furrow. originally described in several of the embryonic Restricted expression of rough, under the control homeotic and segmentation genes of Drosophila, and of the sevenless promoter (sevro), transforms the R7 cell homeodomain proteins have since been shown to into an outer photoreceptor subtype, and the cone cells regulate transcription in vitro and in vivo 2s. The rough suffer later defects that reduce their ability to deposit gene is first expressed in the developing eye imaginal lens normally 2<27. The transformed R7 cells require disc 26, in the nuclei of a large number of cells in and wild-type sevenless and bride of sevenless function immediately posterior to the morphogenetic furrow. (genes that are normally involved in R7 cell specifiLater, rough expression becomes restricted to a subset of the photoreceptor cells (R2-5, see Fig. 3) 26. Such a cation). Taken together, the normal expression pattern progressive restriction of expression pattern is seen in of rough, the R7 cell transformation caused by ectopic many of the embryonic homeodomain proteins, and is expression of rough in sevro animals, and the bsro data support a broad function for rough in normal developbelieved to result from cross-regulatory and autoregument, probably in specifying an R2/5 cell type. latory interactions between the products and promoters of these genes. In developing rough mutant ommatidia the neural glass The glass gene encodes a protein with five zinc diftE'rentiation of the first three pt~toreceptor cells (R8, finger domains of the Cys2-His e type 22 (Fig. 2). Zinc then R2 and R5) appears to be normal. However the finger domains have been found in a number of neural differentiation of the next two photoreceptor y

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transcription factors and in genes 3 cell symmetrical 2 cone 4 cone that affect d e v e l o p m e n t w h e n mucluster cell cell tated 2~. The glass gene is expressed in all of the cells behind the furrow in the developing eye imaginal disc 29 neural (Fig. 3), and glass mutations 22 result antigens in a specific and cell-autonomous disruption of the development of the photoreceptor cells (Fig. 4). The mutant photoreceptors begin to develop as neurons, but never express photoexpression receptor cell-specific genes (e.g. chaoptic ze,3°) and later die at the m i d - p u p a stage30.' Thus the glass protein is probably a transcription expression factor required for p h o t o r e c e p t o r cell-specific gene expression. The glass protein probably actiO function vates its o w n promoter in many cells 29. The pattern of the activity of the glass promoter (as visualized with an enhancer trap element3~) is (D expression altered by the loss of glass protein function: the initial expression close to the furrow is normal, but later almost all cells shut d o w n the proexpression moter. While glass (like. rough) may not be involved in its o w n initial 0 5 10 15 20 columns induction, it may act later in its own regulation. 0 10 20 30 40 hours The glass protein can bind in vitro to specific DNA sequences 29 FIG~ that are found within two rhodopsin Gene expression and function in the developing eye. Some of the stages of gene enhancers 32 and also upstream ommatidial assembly are shown, proceeding from leti to right. At the top, fore of of the glass promoter. To test their these stages are named. At the bottom, an arrow shows developmental time after lhe function in vivo, glass-binding sites passage of the morphogenetic furrow (on the let],, at 0 on the scale), calibrated both were inserted into constructs in in hours and also in column numbers (colunms, as defined by Tomlinson and Ready ~, are lines of ommatidia of equal age). The cells are only shown as they are recruited which a heterologous promoter and the surrounding pool of uncommitted precursor cells are omitted for clarity. The drives a [3-galactosidase gene. These first cell is photoreceptor R8; at the three cell stage R8, R2 and R5 are shown: at the constructs express [8-galactosidase in five cell stage R8, R2, R5, R3 and R4; at the symmetrical cluster stage RS, R2, R'~, R3, the d e v e l o p i n g visual system under R4, R1 and R6; at the two cone cell stage R8, R2, R5, R3, R4, R l, R6, R7 and the first the genetic control of glass 29. Thus two cone cells; and finally at the four cone cell stage all eight photoreceptors and the glass can function in vivo to activate four cone cells are shown. In the first row, the cells shown in black express neural transcription through binding sites antigens ~ and are elaborating axons. In the second row, the cells that express the defined in vitro. These experiments rough protein are shown in black-'~';additional (unidentified) cells close to the f\H-r~\~ also revealed two phases of glass also express rough. In the third row. those cells that express the gla~,sprotein atc function in the developing comshown in black-'~ (the pool of uncommitted precursor cells also express glass). 1,~ t}w fk)urth row, those cells in which glass activates artificial promoters containing p o u n d eye (Fig. 3). In the first glass-binding sites-'-'are identified: the first (lo,,~-level) phase is shoxvn as shaded, tht. phase, a low level of activity is seen second (high-level) phase as Nack (the pool of uncommitted precursors als~ sh~x in all of the cells in which glass prolow-level glass function). In the fifth row, those cells that express the seve~t ttp tein is present. In the second phase promoter (as detected by an enhancer-trap element) are shown in black -'~. "I'he last there is a high level of activity that row shows those cells that express the sina protein-"; high-lex eI expression i> ~,ho~n is restricted to the p h o t o r e c e p t o r as black and low-level expression as shaded. cells and begins about 16 hours after their recruitment. This modulation of glass activity appears not to involve any sevell-up increase in glass expression, and as it is mediated There are two set'etNq) transcripts e~, types I and through the glass-binding site it probably acts through II. Both e n c o d e proteins with two zinc finger dom:lins the glass protein itself. While there is as yet no eviof the Cys_,-Cys_, type (Fig. 2~. like th()sc f<)und in dence to distinguish among several possible molecular ligand-dependent transcription factors such as the mechanisms for the modulation of glass protein funcsteroid hormone receptors 33. Overall, the type I sexon tion, the modulation of glass activity by inductive up protein is 75% identical to the human C ( ) t P pr(~ signals may be a critical step in the d e v e l o p m e n t of rein ,~' (it is over 90% identical in the I)NA- a n d ligandp h o t o r e c e p t o r cells. binding domains). The type 11 pr<)tcin is c<>llincar with

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normally prevented from developing as photoreceptor cells by signals from R3 and R4. In seven-up mutant clones, when R3 and R4 are transformed into R7 cells, these precursor cells may not receive such signals. Given the similarity of the seven-up type 1 protein to steroid receptor proteins, it is interesting to speculate about what sort of signal a diffusible ligand for sevenup might transmit. While the events of ommatidial assembly have been largely attributed to local positional signals, the ligand for seven-up might be part of a system for longer range signaling, like retinoic acid in the developing chicken limb. Alternatively, there may be no small molecule ligand for the seven-up protein. Unlike other steroid receptor-like transcription factors, COUP binds DNA in the absence of a ligand3~, and in some of these proteins the 'ligand-binding' domain is thought to have other functions 33.

sina The product of the sina (seven-inabsentia) 24 gene includes a possible metalbinding domain (Fig. 2), similar to one seen in the Dictyostelium gene DG1 7 (Ref. 36), which may imply a DNA-binding function. FIGIN The sina protein is expressed in the nuclei Cell requirements for gene function. The cells of the ommatidium are shown of all developing eye cells (Fig. 3) and this diagrammatically. The photoreceptor cells are identified by number, the cone nuclear localization is also consistent with cells by the letter 'c', the outer accessory cells are omitted for clarity and the a DNA-binding function. The expression orientation is the same as in Fig. 1. For each gene, those cells that level of the protein increases in sequence autonomously require that gene's function for their normal development are in each group of photoreceptor cells beshaded. These experiments were conducted by inducing a clonal patch fore they begin to differentiate (except R8, of marked mutant cells in a wild-type eye, and then scoring the genotypes of R2 and R5, see Fig. 3). As for the previously cells in phenotypically wild-type ommatidia which contain both genotypically known mutations sevenless1° and boss 12, wild-type and mutant cells. Thus conclusions can be drawn as to which cells loss of sina function results in transmust have a wild-type copy of a particular gene for their normal development. This indicates within which cells each gene must act. formation of the R7 photoreceptor cell into a non-neuronal cone cell at a stage before it begins to differentiateeL Since sina function is the type I protein at the amino terminus, but diverges autonomously required in the R7 cells for their normal after an alternative splice site that when used truncates the ligand-binding domain. Thus the seven-up type I development (Fig. 4), the sina protein may act in the protein might be a ligand-dependent transcription fac- precursor cell as an ultimate target of the inductive signals transduced by the sevenless protein. Alternatively, tor. The type II protein, lacking the ligand-binding sina could act as a component of a parallel pathway donlain, may have an antagonistic function, like a simof R7 cell induction that does not involve sevenless. ilar product of the thyroid receptor gene35. While sina mutations later affect other cells in addition The seven-up gene 23 was identified as being to the R7 photoreceptors in the eye, the existing data involved in eye development not by its mutant phenoare consistent with the suggestion that the sina protein type but by its pattern of expression when driving functions in the developing R7 photoreceptor cells as [3-galactosidase expression in an enhancer trap expera transcription factor regulated by inductive signals. iment-~L It is expressed in the developing eye (Fig. 3), early in cells R3 and R4, and later in R1, R3, R4 and R6. Homozygous seven-up animals die early in their devel- P e r s ~ v e s The Drosophila c o m p o u n d eye is a large and comopment but homozygous retinal clones show an interesting phenotype: four of the outer photoreceptor cells plex structure, and many more than four transcription are transformed into the R7 cell type, and seven-up factors must act in its development. Some of these function is autonomously required in R1, R3, R4 and R6 may yet be discovered as recessive mutations affecting only the eye, but many transcription factors required for their normal development (Fig. 4). In addition, most in c o m p o u n d eye development may also act elseseven-up mutant ommatidia have one or two extra photoreceptor cells and seven-up function is required in where in the fly. Indeed, seven-up mutations do affect both c o m p o u n d eye and embryonic development 23. Of the R3 and R4 cells to suppress tills phenotype. The extra cells may develop from precursor cells that are the transcription factors originally known for their

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functions in embryonic development,

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(hairy37 and engrailed3U) are expressed in the devel-

oping eye but neither has yet been shown to have any function there. The rough, glass and sina genes also act outside the c o m p o u n d eye. Mutations in glass affect other organs of the visual system 22 and both rough and glass mutations result in a degeneration of the optic lobes of the adult brain; this brain phenotype has been shown to depend on the function of these genes in the retina39. However, there are now data indicating that rough 2o and glass 29 are expressed in a small number of cells in the larval brain, where they may have previously unrecognized functions. In addition to their effects on eye development, sina mutations also affect the development of adult sensory hairs, and sina homozygotes are sterile and subviable. At what stage in cell fate determination do these genes act? The rough, seven-up and sina mutations may be interpreted as having homeotic effects and therefore these genes may act at an early step in cell fate specification. In loss-of-function rough mutants, photoreceptors R2 and R5 may be transformed into other photoreceptor cells, and ectopic expression of rough in the R7 cell precursor transforms that cell also. The primary effect of seven-up in homozygous clones is to transform cells RI., R3, R4 and R6 into R7 cells, and sina transforms the R7 cells into cone cells. Unlike the other three genes, loss of glass function results in the failure of the photoreceptor cells to execute their normal developmental program, without any transformation of cell type. Thus glass may act at a step of cell differentiation after initial specification. Close to the furrow, rough is expressed in many cells, in which it has no apparent effects; similarly, glass and sina are expressed in many cells that do not require their function. It appears likely that the activities of these proteins are modulated in response to inductive signals, like the Drosophila dorsal protein and the related mammalian factor NF-K:B, which are activated by their release from a cytoplasmic inhibitory protein complex and translocation into the nucleus 4°. As no such shift in subcellular localization is seen for the rough, glass or sina proteins, other molecular mechanisms may act, such as covalent modification of the factors or the regulative binding of small molecule cofactors or co-activating proteins. Future biochemical and genetic studies on the modulation of these proteins may lead to a deeper understanding of how inductive signals ultimately result in developmental changes of cell fate.

Acknowledgements I wish to thank Marek Mlodzik, Richard Carthew, Bruce Kimmel, Ulrike Gaul, Rony Tal, Iswar Hariharan, Joe Heilig and Matthew Freeman for their critical reading of the manuscript and for their valuable suggestions. Due to the limitations on the number of references I have cited reviews and a limited number of primary, references, and I apologize to those whose work has not been cited.

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