Centro de Biología Molecular Severo Ochoa-C.S.I.C., Facultad de Ciencias-CV, Universidad Autónoma-Cantoblanco, 28049 Madrid, Spain
*Author for correspondence (e-mail: diazbenjumea{at}cbm.uam.es)
Accepted 16 January 2002
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Drosophila, Leg, Bract, EGFr, spitz, poxn
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
One of the main objectives of the analysis of cell signalling is to understand how the reiterative activation of a receptor in different tissues and at different times can elicit different cellular responses. The experimental evidence does not suggest a simple answer, and rather indicates a high complexity in the mechanisms involved. In C. elegans, the LET-23 tyrosine kinase receptor is required in several tissues and it has recently been revealed to signal through the RAS/MAPK in some tissues, but via inositol triphosphate in the germline (Cladinin et al., 1998). In PC12 cells, stimulation by EGF results in a transient activation of MAPK, but stimulation by NGF results in prolonged activation and triggers different responses (Marshall, 1995
). In Drosophila, different thresholds of MAPK activation in the embryo (Greenwood and Struhl, 1997
) and in the developing eye (Halfar et al., 2001
) also trigger different cellular responses. The integration of signals from several pathways provides another mechanism for conferring specificity. The Delta/Notch and RAS/MAPK pathways act in combination to specify R7 fate in the Drosophila eye (Tomlinson and Struhl, 2001
), but antagonistically to specify bristles in the notum (Culí et al., 2001
). EGFr signalling also acts antagonistically to Wg signalling in larval epidermis (OKeefe et al., 1997
), and to Dpp signalling to establish distinct cell fates in tracheal placodes (Wappner et al., 1997
). Finally, the expression of tissue-restricted transcription factors provides developmental contexts that confer specific responses. This is the case for LIN-31 in C. elegans, which is specifically involved in vulva development (Tan et al., 1998
), and for the Drosophila gene yan, an inhibitory ETS transcription factor required in the eye disc but not in other imaginal discs (Lai and Rubin, 1992
).
In this report, we describe a new model of cell fate determination in development involving the RAS/MAPK pathway: the specification of bract cells in the Drosophila epidermis. The cuticle of the adult fly is covered by an array of different kinds of sensory organs. Most of these are large or small bristles that are arranged in very precise patterns. They can also be classified according to their sensitivity to external stimuli as mechanosensory (MB) or chemosensory (ChB) bristles. In the legs and in the proximal costa of the wing, MB occur in association with a cell called bract (Hannah-Alava, 1958). Bracts are clearly distinguishable from the trichomes that the rest of the epidermal cells differentiate in adult cuticle. Detailed analysis of the fine structure of the bract reveals that it is a single epidermal cell (Reed et al., 1975
; Walt and Tobler, 1978
). Early experiments, in which imaginal disc cells are dissociated and re-aggregated, showed that after aggregation bract cells always appear associated to certain bristles. This suggests that the acquisition of bract fate requires the presence of a bristle nearby, and an inductive action carried out by the bristle over a neighbouring epidermal cell has been proposed (García-Bellido, 1966
; Tobler, 1966
).
We have analysed the mechanism of the specification of bract fate and the involvement of neighbouring bristles. Our results indicate that the inductive signal that activates bract fate is mediated by the RAS/MAPK signalling pathway. We propose a model by which, in early pupal development, the expression of the rhomboid gene in sensory organ precursors (SOPs) activates the TGF- homologue encoded by the gene spitz, which then signals through the EGFr to activate the RAS/MAPK pathway. The activation of the pathway in a neighbouring epidermal cell specifies bract fate. The number of bracts per bristle is controlled by a lateral inhibition mechanism mediated by the argos gene. By expressing constitutively activated forms of the RAS/MAPK pathway, we have found that the competence to acquire bract fate is temporally and spatially restricted, supporting the role of the developmental context in determining response specificity. The absence of bracts in chemosensory bristles is explained by the inhibitory action of the poxn gene on the processing of Spitz protein.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Clonal analysis
Fly strains and strategies for clonal analysis of the RAS/MAPK pathway are described in (Díaz-Benjumea and Hafen, 1994). The number of clones scored in different clonal analyses is 15 spiIIA14, 8 top4A, 10 ras1e2F, 9 rafEA75, 16 SosX122 and 5 rho1de11. For cell lineage analysis, y FLP122 P{Act5C>CD2>GAL4} / FM7; UAS-y+ females were crossed with wild-type males, and F1 larvae at 72±6 hours after puparium formation (APF) were heat shocked for 7 minutes at 36°C. Adult males from the progeny were mounted in araldite and their legs scored for the presence of y+ clones. Several clones per leg were found in all the scored flies.
Heat shock experiments
For overexpression experiments, larvae were grown at 18°C, and when they started to pupate, pupae were collected and classified into the following stages: 0-4 hours, 4-8 hours, 8-12 hours and >12 hours APF (García-Bellido, 1971). Pupae were then transferred to new tubes and heat shocked for 30 minutes at 37°C in a water bath. After heat shock they were kept at 18°C, and adults with the appropriate genotype were mounted for cuticle analysis under the compound microscope. No major effects were found in wild-type flies heat shocked under the same experimental conditions as a control (Held, 1990
).
Preparation of adult cuticle
Flies were dissected in glycerol/ethanol, fat was removed in 10% KOH, and the cuticle was dehydrated in ethanol, transferred to acetone and mounted in araldite.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
The RAS/MAPK pathway mediates the induction of bract fate
It has been proposed that the determination of bracts might be related to an inductive action by its corresponding bristle. We analysed the role of the RAS/MAPK pathway as a mediator of this cell signalling. We studied the pattern of bracts in flies carrying viable mutant alleles of several genes of the RAS/MAPK pathway. Viable combinations of loss-of-function alleles of the rolled gene (MAPK) show a complete lack of bracts in wings and legs (Fig. 2A). The gap1 gene encodes a putative Ras GTPase activating protein that acts as a negative regulator of the MAPK pathway (Gaul et al., 1992). Flies homozygous for the loss-of-function allele gap1B2 have extra bracts on legs and wings (Fig. 2B). These extra bracts were always associated with MB and occurred adjacent to the wild-type bract. Both results indicate that the RAS/MAPK pathway mediates the activation of bract fate.
|
Spatial competence in the activation of bract fate
Although there are MBs at many places on the adult fly cuticle, bristles with bracts are only found in specific sites on wings and legs. This suggests a spatial restriction that limits the competence to acquire bract fate. This might occur for two reasons. First, away from these areas, bristles might not activate the signal; and second, cells outside these areas might not be competent to adopt the bract fate. To identify the spatial field of competence for activation of bract fate we used the UAS/GAL4 system (Brand and Perrimon, 1993) to express an activated form of the Raf protein ubiquitously (Raf*) (Martín-Blanco et al., 1999
). This provides a signal-independent way to activate the RAS/MAPK pathway. Ectopic expression of Raf*, under the control of several GAL4 drivers, produces ambiguous results as the pattern of leg bristles is affected, and bracts are either missing or supernumerary in the same experiments. This may be caused by the fact that the argos gene, a EGFr ligand that acts as a competitive inhibitor of other ligands, is itself a target of the pathway (Freeman et al., 1992
). To avoid this problem, we induced pulses of UAS-Raf* expression with a hsp70-GAL4 construct (Brand et al., 1994
). Early pupae were collected and heat shocked for 30 minutes at 37°C. After this heat shock pupae complete development but the individuals die as pharate adults. In these flies, large territories of trichomes were transformed into bracts. In legs, from the proximal femur to the claw, all trichomes were transformed into bracts (Fig. 3A). In the notum, the region between the dorsocentral bristles also differentiated multiple bracts (Fig. 3C). In wings, only the cells of the dorsal and ventral proximal costa differentiated as bracts (Fig. 3B); the rest of wing cells were not affected, apart from a small area in the dorsal hinge that also differentiated a patch of bracts. In the halteres, multiple bracts covered the dorsal and ventral anterior pedicelum (Fig. 3D). [It is noteworthy that this area, which normally lacks bristles, develops bristles with bracts in homeotic transformations caused by mutation of the Ubx gene (Morata and García-Bellido, 1976
).] The prothorax also differentiated a small patch of bracts. In the head, bracts were found at the base of the arista, and in the gena and the supraorbital region. The size and shape of bracts was constant in different parts of the fly. We conclude that in these areas epidermal cells are competent to differentiate as bracts if the RAS/MAPK pathway is activated at the appropriate time.
|
|
Only mechanosensory bristles induce bract fate
ChBs of the leg have no bracts. We consider two models to explain this: (1) ChBs are specified before the time of competence for bract induction, or (2) ChBs prevent bract specification by inhibiting Spi signalling.
The time for specification of ChBs in the leg has been mapped at 4 hours before puparium formation (BPF) (Rodríguez et al., 1990). This time is outside the period of competence for bract induction, which would support the first hypothesis as although chemosensory precursors were able to activate the pathway, epidermal cells would be unable to adopt the bract fate.
The gene pox-neuro (poxn) encodes a transcriptional regulator that acts as a selector gene for the specification of chemosensory fate (Bopp et al., 1989; Dambly-Chaudière et al., 1992
). In poxn mutant flies, ChBs are either lost or transformed into MBs (Awasaki and Kimura, 2001
) and the overexpression of poxn transforms MBs into ChBs (Nottebohm et al., 1992
) (Fig. 5A). We heat shocked pupae carrying hsp70-poxn and hsp70-GAL4/UAS-Raf*. In these flies MBs are transformed in ChBs, and all epidermal cells are transformed into bracts (Fig. 5B). These results suggest that poxn may operate in the SOP cell, and the RAS/MAPK pathway in the bract cell. We have looked at the expression of poxn in leg and wing discs of wandering larvae and early pupae and we found that poxn is expressed in single cells that probably correspond to chemosensory precursors (Fig. 5C, E). poxn is not expressed in In(1)sc101 flies, which lack most of the leg bristles (Fig. 5D). Together, these results would support the second hypothesis as it seems poxn expression in SOP cells is sufficient to repress Spitz signalling.
|
The overexpression of both Poxn and sSpi (hsp70-poxn, hsp70-GAL4/UAS-sSpi) shows the same phenotype as seen with overexpressed Poxn and Raf* (Fig. 5B); MBs are transformed in ChBs and trichomes are transformed into bracts. This confirms that Poxn must act in SOP cells by inhibiting Spi processing.
Spi cleavage requires two additional transmembrane proteins encoded by the genes rhomboid1 (rho1) and Star (S) (Bier et al., 1990; Rutledge et al., 1992
). It is thought that EGFr activation is controlled by the highly regulated pattern of rho1 expression. We examined the expression of rho1 and found that, in early pupae, it is expressed in a dynamic pattern ending in single cell expression (Fig. 5G). In In(1)sc101 pupae, this pattern of expression is missing (Fig. 5H). We tested the requirement for rho1 in the induction of bracts by making rho1 mutant clones in the leg. These clones proliferate normally and differentiate bristles, but do not differentiate bracts; indicating that rho1 is required for bract induction (Fig. 5I). To test whether rho1 and S are sufficient for bract induction, we heat shocked flies carrying hsp70-GAL4/UAS-rho1 + UAS-S at the time of bract induction. In this experiment, we did not find bracts far from bristles nor associated to ChBs, and only a few MBs had two or three bracts (Fig. 5J).
The role of the argos gene in defining the number and position of bracts
There are three aspects we want to consider here: the number of bracts per bristle, bract polarity and the position of the bract. In wild-type flies, there is a single bract per bristle. This suggests a possible mechanism of lateral inhibition. One candidate mediator of this inhibition is the Dl/N signalling pathway. Nevertheless, pupae carrying the thermosensitive allele Nts, heat shocked at different times during pupal development, do not have extra bracts (Held, 1990) (D. del Alamo, J. Terriente and F. J. Díaz-Benjumea, unpublished). We believe instead that the argos (aos) gene mediates this lateral inhibition. Flies homozygous for the viable allele aos
15 show two or three bracts per bristle, clustered on its proximal side (Fig. 6A). Overexpression of Aos with scabrous-GAL4, which is expressed in proneural clusters, removes all the bracts (Fig. 6B). These results indicate that aos plays an active role in limiting the number of bracts per bristle.
|
A subject distinct from the polarity is the location of the bract relative to the bristle. Bracts are always located opposite to where the bristle polarity points. In wild-type legs, this location corresponds to the proximal-most side of the bristle. In three different situations in which the pathway is overactivated aos mutants (Fig. 6A), gap1 mutants (Fig. 2B) and rho1 overexpression (Fig. 5J) the extra-bracts appear clustered on the proximal-most side of the bristle. This suggests that the inductive signal is polarised to proximal. Therefore the position of the bract is likely to be determined by a polarised signal from the bristle. Nevertheless, we cannot reject other possibilities, such as a greater competence of cells at the proximal-most side to respond to the inductive signal, or that bracts are induced anywhere close to the bristle and later moved to the final position through differences in cell affinity.
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
This view is similar to that observed in the specification of bristles by the AS-C genes. Although the expression of AS-C genes is spatially and temporally restricted to cell clusters, ubiquitous ac and sc expression provided at different developmental times, in a genetic background that lacks the endogenous genes, results in a pattern of bristles of the correct type, located in the wild-type positions. So, both pattern and type of bristles are defined by the developmental context in which AS-C genes are expressed (Rodríguez et al., 1990).
We also observed that the Dl/N pathway represses bract fate specification. Overexpressing both Raf* and Nintra, we observed a strong reduction in the number of ectopic bracts produced by Raf* (data not shown). This result suggests that N signalling acts downstream of Raf in the inhibition of the RAS/MAPK pathway.
Another level of control occurs in the bristle cell that sends the inductive signal. Spi protein requires the functions of rho1 and S genes to be processed into a soluble, activated form. S and spi are ubiquitously expressed, and rho1 is expressed in SOP cells. The phenotype of rho1 mutant cells in clones indicates that rho1 is required for the induction of bract fate. Nevertheless, rho1 is also expressed in bract-less ChBs, and ubiquitous overexpression of S and rho1 results in a mild phenotype of extra bracts in wild-type positions. Together these results suggest that another component, whose expression must be restricted to the SOP of MBs, is required for bract induction.
The expression of the poxn gene is both necessary and sufficient for the specification of bract-less ChBs. As ChBs are specified before epidermal cells are competent for bract induction this provides an explanation for the bract-less phenotype of ChBs. Nevertheless, Poxn overexpression suppresses bracts, and the result of the combined overexpression of Poxn and activated Raf or sSpi indicates that Poxn acts in the SOP cells to repress Spi signalling. Poxn and Rho1 are co-expressed in the SOP. So these results do not allow us to deduce the molecular mechanisms by which poxn expression represses Spi signalling. Nevertheless, as the result of S and rho overexpression indicates that they are not sufficient to induce bracts, and that at least one other component present in the SOP cell is required, it can be tentatively suggested that poxn may act upstream of this other gene.
Bract number and position
Bracts always appear on the proximal-most side of the bristle, which raises the question of whether the position of the bract is defined by a polarised signal. Several lines of evidence suggest that the SOP cell is polarised (Hartenstein and Posakony, 1989; Huang et al., 1991
). We have found that in all the experiments that result in extra bracts, these appear clustered on the proximal-most side. It seems likely that the polarisation of the SOP cell leads to a polarisation of Spi signalling which results in a constant position of the bract, although we cannot reject other possibilities.
Concerning the number of bracts per bristle, our results indicate that the aos gene plays a role in mediating a lateral inhibition mechanism. Loss of aos led to clusters of bracts, while overexpression removed all bracts. It is reasonable to think, therefore, that as a result of the activation of the RAS/MAPK pathway in the presumptive bract cell, aos expression is being activated in this cell to inhibit the pathway in neighbouring epidermal cells.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Awasaki, T. and Kimura, K. (2001). Multiple function of poxn gene in larval PNS development and in adult appendage formation of Drosophila. Dev. Genes Evol. 211, 20-29.[Medline]
Bate, C. M. (1978). Development of sensory systems in arthropods. In Handbook of Sensory Physiology, Vol. IX (ed. E. M. Jacobson), pp. 1-53. Berlin: Springer-Verlag.
Bier, E., Jan, L. Y. and Jan, Y. N. (1990). rhomboid, a gene required for dorsoventral axis establishment and peripheral nervous system development in Drosophila melanogaster. Genes Dev. 4, 190-203.[Abstract]
Bopp, D., Jamet, E., Baumgartner, S., Burri, M. and Noll, M. (1989). Isolation of two tissue-specific Drosophila paired box genes, Pox meso and Pox neuro. EMBO J. 8, 3447-3457.[Abstract]
Brand, A. H., Manoukian, A. S. and Perrimon, N. (1994). Ectopic expression in Drosophila. In Drosophila melanogaster: Practical Uses in Cell and Molecular Biology (ed. Goldstein and Fyrberg), pp. 635-654.
Brand, A. H. and Perrimon, N. (1993). Targeted gene expression as a means of altering cell fates and generating dominant phenotypes. Development 118, 401-415.
Cladinin, T. R., DeModena, J. A. and Sternberg, P. W. (1998). Inositol triphosphate mediates a Ras-independient response to LET-23 receptor tyrosine kinase activation in C. elegans. Cell 92, 523-533.[Medline]
Culí, J., Martín-Banco, E. and Modolell, J. (2001). The EGF receptor and N signalling pathways act antagonistically in Drosophila mesothorax bristle patterning. Development 128, 299-308.
Dambly-Chaudière, C., Jamet, E., Burri, M., Bopp, D., Basler, K., Hafen, E., Dumont, N., Spielmann, P., Ghysen, A. and Noll, M. (1992). The paired box gene pox neuro: a determinant of poly-innervated sense organ in Drosophila. Cell 69, 159-172.[Medline]
Díaz-Benjumea, F. J. and Hafen, E. (1994). The sevenless signalling cassette mediates Drosophila EGF receptor function during epidermal development. Development 120, 569-578.
Freeman, M. (1994). Misexpression of the Drosophila argos gene, a secreted regulator of cell determination. Development 120, 2297-2304.
Freeman, M., Klämbt, C., Goodman, C. S. and Rubin, G. M. (1992). The argos gene encodes a diffusible factor that regulates cell fate in the Drosophila eye. Cell 69, 963-975.[Medline]
García-Bellido, A. (1966). Pattern reconstruction by dissociated imaginal disk cells of Drosophila melanogater. Dev. Biol. 14, 278-306.[Medline]
García-Bellido, A. (1971). Parameters of the wing imaginal disc development of Drosophila melanogaster. Dev. Biol. 24, 61-87.[Medline]
Gaul, U., Mardon, G. and Rubin, G. M. (1992). A putative Ras GTPasa activating protein acts as a negative regulator of signaling by the sevenless receptor tyrosine kinase. Cell 68, 1007-1019.[Medline]
Golembo, M., Raz, E. and Shilo, B. Z. (1996). The Drosophila embryonic midline is the site of Spitz processing, and induces activation of the EGF receptor in the ventral ectoderm. Development 122, 3363-3370.
Greenwood, S. and Struhl, G. (1997). Different levels of Ras activity can specify distinct transcriptional and morphological consequences in early Drosophila embryos. Development 124, 4879-4886.
Halfar, K., Rommel, C., Stocker, H. and Hafen, E. (2001). Ras controls growth, survival and differentation in the Drosophila eye by different threshols of MAP kinase activity. Development 128, 1687-1696.
Hannah-Alava, A. (1958). Morphology and chaetotaxy of the legs of Drosophila melanogaster. J. Morphol. 103, 281-310.
Hartenstein, V. and Posakony, J. W. (1989). Development of adult sensilla on the wing and notum of Drosophila melanogaster. Development 107, 389-405.[Abstract]
Held, L. I. (1990). Sensitive periods for abnormal patterning on a leg segmant in Drosophila melanogaster. Rouxs Arch. Dev. Biol. 199, 31-47.
Hiliker, A. J. (1976). Genetic analysis of the centromeric heterochromatin of chromosome 2 of Drosophila melanogaster: deficiency mapping of EMS-induced lethal complementation groups. Genetics 83, 765-782.
Huang, F., Dambly-Chaudière, C. and Ghysen, A. (1991). The emergence of sense organs in the wing disc of Drosophila. Development 111, 1087-1095.[Abstract]
Jan, Y. N. and Jan, L. Y. (1993). The peripheral nervous system. In The Development of Drosophila melanogaster, Vol. 2, pp. 1207-1244: New York: Cold Spring Harbor Laboratory Press.
Jhaveri, D., Sen, A., Venugopala Reddy, G. and Rodrigues, V. (2000). Sense organ identity in the Drosophila antenna is specified by the expression of the proneural gene atonal. Mech. Dev. 99, 101-111.[Medline]
Kretzschmar, D., Brunner, A., Wiersdorff, V., Pflugfelder, G. O., Heisenberg, M. and Schneuwly, S. (1992). giant lens, a gene involved in cell deterrmination and axon guidance in the visual system of Drosophila melanogaster. EMBO J. 11, 2531-2539.[Abstract]
Lai, Z.-C. and Rubin, G. M. (1992). Negative control of photoreceptor development in Drosophila by the product of the yan gene, an ETS domain protein. Cell 70, 609-620.[Medline]
Lawrence, P., Struhl, G. and Morata, G. (1979). Bristles patterns and compartment boundaries in the tarsi of Drosophila. J. Embryol. Exp. Morphol. 51, 195-208.[Medline]
Lindsley, D. L. and Zimm, G. G. (1992). The Genome of Drosophila melanogaster. San Diego: Academic Press.
Marshall, C. J. (1995). Specificity of receptor tyrosine kinase signalling: transient versus sustained extracellular signal-regulated kinase activation. Cell 80, 179-185.[Medline]
Martín-Blanco, E., Roch, F., Noll, E., Baonza, A., Duffy, J. B. and Perrimon, N. (1999). A temporal switch in DER signalling controls the specification and differentiation of veins and interveins in the Drosophila wing. Development 126, 5739-5747.
Melnick, M. B., Perkins, L. A., Lee, M., Ambrosio, L. and Perrimon, N. (1993). Developmental and molecular characterization of mutations in the Drosophila-raf serine/threonine protein kinase. Development 118, 127-138.
Moghal, N. and Sternberg, P. W. (1999). Multiple positive and negative regulators of signaling by the EGF-receptor. Curr. Opin. Cell Biol. 11, 190-196.[Medline]
Morata, G. and García-Bellido, A. (1976). Developmental analysis of some mutants of the biothorax system of Drosophila. Wilhelm Rouxs Archiv. 179, 125-143.
Nottebohm, E., Dambly-Chaudière, C. and Ghysen, A. (1992). Connectivity of chemosensory neurons is controlled by the gene poxn in Drosophila. Nature 359, 829-832.[Medline]
OKeefe, L., Dougan, S. T., Gabay, L., Raz, E., Shilo, B. Z. and DiNardo, S. (1997). Spitz and Wingless, emanating from distinct borders, cooperate to stablish cell fate across the Engrailed domain in the Drosophila epidermis. Development 124, 4837-4845.
Perrimon, N. (1994). Signalling pathways initiated by receptor protein tyrosine kinases in Drosophila. Curr. Opin. Cell Biol. 6, 260-266.[Medline]
Price, J. V., Clifford, R. J. and Schüpbach, T. (1989). The maternal ventralizing locus torpedo is allelic to faint little ball, an embrionic lethal, and encodes the Drosophila EGF receptor homolog. Cell 56, 1085-1092.[Medline]
Rebay, I., Fehon, R. G. and Artavanis-Tsakonas, S. (1993). Specific truncations of Drosophila Notch define dominant activated and dominant negative forms of the receptor. Cell 74, 319-329.[Medline]
Reed, C. T., Murphy, C. and Fristrom, D. (1975). The ultrastructure of the differentiating pupal leg of Drosophila melanogaster. Rouxs Arch. Dev. Biol. 178, 285-302.
Rodríguez, I., Hernández, R., Modolell, J. and Ruiz-Gómez, M. (1990). Competence to develop sensory organs is temporally and spatially regulated in Drosophila epidermal primordia. EMBO J. 9, 3583-3592.[Abstract]
Rogge, R. D., Karlovich, C. A. and Banerjee, U. (1991). Genetic dissection of a neurodevelopmental pathway: Son of sevenless functions downstream of the sevenless and EGF receptor tyrosine kinases. Cell 64, 39-48.[Medline]
Rutledge, B. J., Zhang, K., Bier, E., Jan, Y. N. and Perrimon, N. (1992). The Drosophila spitz gene encodes a putative EGF-like growth factor involved in dorsal-ventral axis formation and neurogenesis. Genes Dev. 6, 1503-1517.[Abstract]
Schweitzer, R., Shaharabany, M., Seger, R. and Shilo, B. Z. (1995). Secreted Spitz triggers the DER signaling pathway and is a limiting component in embryonic ventral ectoderm determination. Genes Dev. 9, 1518-1529.[Abstract]
Schweitzer, R. and Shilo, B.-Z. (1997). A thousand and ones roles for the Drosophila EGF receptor. Trends Genet. 13, 191-196.[Medline]
Simon, M. A., Bowtell, D., Dodson, G. S., Laverty, T. R. and Rubin, G. M. (1991). Ras1 and a putative guanine nucleotide exchange factor perform crucial steps in signaling by sevenless protein tyrosine kinase. Cell 67, 701-716.[Medline]
Simpson, P. (1990). Lateral inhibition and the development of sensory bristles of the adult peripheral mervous system of Drosophila. Development 109, 509-519.[Abstract]
Staehling-Hapton, K., Jackson, P. D., Clark, M. J., Brand, A. H. and Hoffmann, F. M. (1994). Specificity of bone morphogenetic protein related factors: cell fate and gene expression changes in Drosophila embryos by decapentaplegic but not 60A. Cell Growth Differ. 5, 585-593.[Abstract]
Struhl, G. and Adachi, A. (1998). Nuclear access and action of Notch in vivo. Cell 93, 649-660.[Medline]
Struhl, G., Fitzgerald, K. and Greenwald, I. (1993). Intrinsic activity of the Lin-12 and Notch intracellular domains in vivo. Cell 74, 331-345.[Medline]
Tan, P. B., Lackner, M. R. and Kim, S. K. (1998). MAP kinase signalling specificity mediated by the LIN-31 WH and LIN-1 ETS transcription factor complex during C. elegans vulva induction. Cell 93, 569-580.[Medline]
Tobler, H. (1966). Zellspezifische Determination und Beziehung zwischen Proliferation und Transdetermination in Bein- und Flüfelprimordien von Drosophila melanogater. J. Embryol. Exp. Morphol. 16, 609-633.[Medline]
Tokunaga, C. (1962). Cell lineage and differentiation on the male foreleg of Drosophila melanogaster. Dev. Biol. 4, 489-516.
Tomlinson, A. and Struhl, G. (2001). Delta/Notch and Boss/Sevenless signals act combinatorially to specify the Drosophila R7 photoreceptor. Mol. Cell 7, 487-495.[Medline]
Walt, H. and Tobler, H. (1978). Ultrastructural analysis of differentiating bristles organs in wild type, shaven-depilate and Mitomycin C-treated larvae of Drosophila melanogaster. Biol. Cell 32, 291-298.
Wappner, P., Gabay, L. and Shilo, B.-Z. (1997). Interactions between the EGF receptor and DPP pathways establish distinct cell fates in the tracheal placodes. Development 124, 4707-4716.