Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad, 500 007, India
* Author for correspondence (e-mail: shashi{at}ccmb.res.in)
Accepted 8 January 2003
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Drosophila, Haltere, Ultrabithorax, Armadillo, DV signaling
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Growth and patterning during fly wing development are mediated by signaling
from the dorsoventral (DV) organizer. Interactions between dorsal and ventral
cells of the wing pouch set up the organizer by activating Notch (N) at the DV
boundary (Diaz-Benjumea and Cohen,
1993; Diaz-Benjumea and Cohen,
1995
; Williams et al.,
1994
; Irvine and Wieschaus,
1994
; Kim et al.,
1995
; de Celis et al.,
1996b
). N, in turn, activates Wingless (Wg), Cut (Ct) and
Vestigial (Vg) at the DV boundary (Couso et
al., 1995
; Kim et al.,
1995
; Rulifson and Blair,
1995
; Kim et al.,
1996
; Neumann and Cohen,
1996
). Wg is known to diffuse to non-DV cells from the DV boundary
to act as a morphogen (Zecca et al.,
1996
; Neumann and Cohen,
1997
). High levels of Wg are required for activating Achaete (Ac),
whereas moderate levels are sufficient to activate Distal-less (Dll) and low
levels to activate Vg (Neumann and Cohen,
1997
). Thus, Vg is expressed in both DV and non-DV cells. It has
been shown that two different promoters regulate Vg expression in DV and
non-DV cells (Kim et al.,
1996
). They are vg-boundary enhancer (vg-BE) and
vg-quadrant enhancer (vg-QE).
Previously, we have shown that Ubx downregulates DV signaling to
specify haltere fate (Shashidhara et al.,
1999). In haltere imaginal discs, Wg and Ct are expressed only in
the anterior compartment (Weatherbee et
al., 1998
; Shashidhara et al.,
1999
). However, none of the three targets of Wg (i.e. Ac, Dll and
vg-QE) is expressed in the haltere disc
(Gorfinkiel et al., 1997
;
Weatherbee et al., 1998
;
Shashidhara et al., 1999
). As
expression of Wg itself is robust in the anterior DV boundary of haltere
discs, downregulation of its targets, in this compartment at least, could be
due to the repression of event(s) downstream of Wg, such as transduction of Wg
signaling from the DV boundary. Consistently, although overexpression of Ubx
in the wing disc DV boundary results in loss of Wg only in DV boundary cells
of the posterior compartment, it causes loss of vg-QE in non-DV cells
of both the anterior and posterior compartments
(Shashidhara et al., 1999
). We
show that Ubx functions at multiple levels to repress Vg in non-DV cells,
including enhanced degradation of Arm in the haltere pouch. Repression of Vg
at multiple levels appears to be crucial for Ubx-mediated specification of the
haltere fate. Overexpression of Vg in haltere discs overrides Ubx function and
thereby induces haltere-to-wing homeotic transformations.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Histology
X-gal and immunohistochemical staining was performed essentially as
described by Ghysen and O'Kane (Ghysen and
O'Kane, 1989) and Patel et al.
(Patel et al., 1989
),
respectively. The primary antibodies used were, monoclonal anti-Arm
(Riggleman et al., 1990
),
anti-Ct (Blochlinger et al.,
1993
), anti-Salm (de Celis et al., 19666a), anti-Wg
(Brook and Cohen, 1996
) and
anti-ß-galactosidase (Sigma) and polyclonal anti-Vg
(Williams et al., 1991
),
anti-Arm (Ruel et al., 1999
)
and anti-ß-galactosidase (Sigma). Monoclonal anti-Arm and anti-Wg
antibodies were obtained from the Development Studies Hybridoma Bank,
University of Iowa, USA. Confocal microscopy was carried out on Meridian
Ultima. The adult appendages were processed for microscopy as described
previously (Shashidhara et al.,
1999
).
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Wg is required for the maintenance of Vg expression in the DV
boundary
A mutant version of TCF/pan protein, which lacks the N-terminal Arm
interaction domain, functions as a dominant negative for both TCF/pan and Arm
(van der Wetering et al., 1997). We overexpressed DN-TCF/pan using
vg-GAL4 to downregulate Wg signaling in the DV boundary. We
observed loss of Vg in both DV and non-DV cells when Wg signaling is
downregulated (monitored by anti-Vg antibody, vg-BE and
vg-QE staining; Fig.
1D-F). This is contrary to the earlier reports that Wg activity is
not required for the expression of Vg at the DV boundary
(Neumann and Cohen, 1996). We
further tested the cell-autonomy of this phenomenon by generating mitotic
clones of arm. As loss-of-function clones of null alleles of
arm are lethal, we used armH8.6, a
temperature-sensitive hypomorphic allele
(Neumann and Cohen, 1997
). We
monitored Vg expression in small armH8.6 clones, survival
of which were confirmed by DAPI staining. Clonal loss of Arm at the DV
boundary resulted in cell-autonomous loss of Vg expression
(Fig. 1G), confirming a role
for Wg in the maintenance of Vg expression in DV cells. It has been reported
previously that ectopic expression of Vg or Scalloped (Sd; a co-factor of Vg
in the nucleus) causes ectopic Wg expression
(Go et al., 1998
;
Klein and Martinez-Arias,
1998
; Klein and Martinez-Arias
1999
; Liu et al.,
2000
). Thus, Wg and Vg may interact to maintain each other's
expression in the wing disc DV boundary.
|
|
Wg signaling is required, but is not sufficient, to activate
vg-QE
Although Vg is capable of activating vg-QE in both wild-type and
vg1 backgrounds (Fig.
2G), ectopic expression of Wg or activated Arm does not induce
ectopic vg-QE expression (Nagaraj
et al., 1999). This is contrary to the non-cell autonomous loss of
both Vg and vg-QE by the ectopic expression of DN-TCF/pan at
the DV boundary (Fig. 1F), and
cell-autonomous loss of Vg in arm mitotic clones
generated in non-DV cells (Neumann and
Cohen, 1997
) (Fig.
1G).
Ectopic expression of activated N using dpp-GAL4 resulted
in non-cell autonomous activation of Vg in the wing pouch
(Fig. 3C). As N specifies DV
boundary activity and vg-QE expression is inhibited in N-expressing
cells (Klein and Martinez-Arias,
1999), cell-autonomous activation of Vg might correspond to the
activation of vg-BE and non-cell autonomous component might
correspond to vg-QE. As, ectopic Wg also causes non cell-autonomous
activation of Vg (Neumann and Cohen,
1997
), ectopic N might first cell-autonomously activate Wg, which
in turn would activate Vg in neighboring cells. Consistent with this, ectopic
Nintra-induced cell-autonomous activation of Wg
(Fig. 3D) and activated Arm
resulted in cell-autonomous activation of Vg in wing discs
(Fig. 3E).
|
Regulation of Wg and Vg expression in haltere discs
With the new insights into the mechanism of Wg and Vg expression in wing
discs, we studied the mechanism by which Ubx represses their expression in
haltere discs. Wing and haltere discs employ similar genetic pathways for
pattern formation along the A/P and DV axes
(Williams et al., 1993;
Williams et al., 1994
).
However, although Wg is expressed at the anterior DV boundary, Vg is not
expressed in non-DV cells of haltere discs
(Weatherbee et al., 1998
;
Shashidhara et al., 1999
).
Thus, Ubx may repress event(s) downstream of Wg to inhibit Vg expression in
non-DV cells.
Ubx inhibits stabilization of Arm
Stabilization of cytoplasmic Arm is a key step in the transduction of Wg
signaling. Although Arm is present in all cells, cytoplasmic levels of Arm,
which transduces Wg signaling, are higher only in cells in which Wg signaling
is active (Peifer et al.,
1994). For example, cells immediately adjacent to the wing disc DV
boundary show higher levels of Arm than do non-DV cells
(Fig. 4A,B). In the absence of
Wg signaling, cytoplasmic Arm is subjected to Ubiquitin-mediated
degradation.
|
Enhanced degradation of Arm in haltere discs
To further test if Arm degradation is enhanced in haltere discs, we used
Myc-tagged degradation-resistant and degradation-sensitive forms of Arm
[ArmS10 and ArmS2
(Pai et al., 1997)].
ArmS10 has an internal deletion of 43-87 residues at the N
terminus. This deletion removes residues that are normally phosphorylated by
Sgg, thus making it degradation resistant. ArmS2 expresses normal
protein and is susceptible to the degradation machinery. arm-mutant
embryos rescued by ArmS2, secrete normal denticle belts and also
have normally patterned naked cuticle (Pai
et al., 1997
). Thus, similar to endogenous Arm, ArmS2
is stabilized only in Wg signaling cells. Thus, relative levels of
ArmS2 at the DV boundary of wing and haltere discs can be used as
an estimate of the relative efficiency of the Arm-degradation machinery. We
expressed ArmS10 and ArmS2 using the
omb-GAL4 driver, and stained wing and haltere discs with
anti-Myc and anti-Arm antibodies. We observed uniform levels of the
degradation-resistant form of Arm in both wing
(Fig. 4F) and haltere discs
(Fig. 4G). However,
degradation-sensitive ArmS2 accumulated in the DV boundary of wing
discs (Fig. 4H) but not of
haltere discs (Fig. 4I). This
suggests that Arm is degraded more efficiently at the DV boundary of haltere
discs than at the DV boundary of wing discs.
Ubx functions downstream to Sgg to enhance the degradation
of Arm
In DV cells of the wing disc, in which Wg signaling is active, Arm
degradation is inhibited owing to inhibition of the degradation machinery. Dsh
functions immediately downstream of Wg, and inhibits Sgg activity and thereby
stabilizes Arm. Overexpression of Dsh at the haltere DV boundary did not
induce the stabilization of Arm (Fig.
5A) suggesting that the Ubx-mediated inhibition is downstream of
Dsh function. One possibility is that Ubx interferes with Dsh-mediated
inhibition of Sgg activity, thus keeping Sgg active and causing degradation of
Arm. To test this hypothesis, we overexpressed the human colon cancer gene
APC in wing and haltere discs. In both Drosophila and
mammalian cells, it has been shown that APC binds to Arm/ß-catenin even
when Wg/Wnt is active (Papkoff et al.,
1996; Bhandari and Shashidhara,
2001
). In those cells, APC sequesters Arm/ß-catenin, rather
than recruiting it to the degradation machinery. For example, overexpression
of APC in wing discs sequesters Arm only in DV cells
(Bhandari and Shashidhara,
2001
) (Fig. 5B). In
other cells, overexpressed APC participates in the Arm-degradation machinery
and hence no change in Arm expression was observed. Thus, the amount of Arm
sequestered by overexpressed APC could be an assay for the level of Wg/Wnt
activity. As only unphosphorylated Arm is sequestered and not the
phosphorylated form (Munemitsu et al.,
1996
), such an assay could also be used to obtain a relative
estimate of Sgg activity. When we overexpressed human APC at the haltere DV
boundary using vg-GAL4, we observed increased levels of Arm
(owing to sequestration) in the anterior compartment
(Fig. 5C) but not in the
posterior compartment, indicating that Sgg is inactive in the anterior
compartment and active in the posterior compartment. Thus, Ubx-mediated
inhibition of Arm stabilization in the anterior compartment is downstream of
Sgg.
|
Wg is not autoregulated at the haltere disc DV boundary
Although levels of Arm were much lower in haltere discs than in wing discs,
it is possible that available amounts of Arm are sufficient to transduce Wg
signaling. We used Wg-autoregulation as a test for Arm function at the DV
boundary of haltere discs. It has been shown that Wg is autoregulated and Arm
is necessary for this process (Hooper,
1994; Yoffe et al.,
1995
). For example, ectopic activation of Arm function in leg
discs induces ectopic Wg expression
(Bhandari and Shashidhara,
2001
). We observed repression of Wg at the DV boundary when we
overexpressed DN-TCF/pan in wing discs using vg-GAL4
(Fig. 6A). However, we did not
observe any such loss of Wg at the haltere DV boundary
(Fig. 6B), nor was there any
change in the size of haltere pouch. These results suggest that Arm function
is indeed downregulated at the haltere DV boundary.
|
Vg expression at the haltere DV boundary is not dependent on Wg
In haltere discs, in which Wg is not expressed in the posterior
compartment, Vg is still expressed all along the DV boundary, suggesting that
Vg is independent of Wg function in the posterior compartment. In the anterior
compartment also, Vg might not be dependent on Wg as Arm function is
downregulated by Ubx. Indeed, expression of DN-TCF/pan at the haltere DV
boundary did not affect Vg expression in haltere discs
(Fig. 6E). The possibility that
DN-TCF/pan did not downregulate Vg in haltere discs owing to its late
expression (we used vg-GAL4) is ruled out because in wing
discs it downregulated Vg in both DV and non-DV cells
(Fig. 1D-F). This suggests that
Vg expression at the DV boundary of haltere discs is independent of Wg
function.
Ubx-mediated repression of Vg in non-DV cells is downstream
of Arm and upstream of Vg-autoregulation
Overexpression of N, Wg or activated Arm (both Flu-Arm and
ArmS10) at the haltere DV boundary using the
vg-GAL4 driver did not induce activation of Vg in non-DV
cells (monitored by both anti-Vg antibody and vg-QE staining) in
haltere discs, nor did they induce any adult haltere phenotypes (data not
shown). This suggests that Ubx inhibits additional events downstream of DV
signaling.
We then examined the events in non-DV cells that might contribute to the suppression of Vg in the haltere pouch. We observed cell-autonomous activation of Vg in non-DV cells when we expressed activated N using dpp-GAL4 (Fig. 7A). However, unlike in wing discs (Fig. 3D), ectopic N expression failed to activate Wg in haltere discs (Fig. 7B). Furthermore, overexpression of Wg, Dsh or activated Arm directly in non-DV cells using dpp-GAL4, omb-GAL4 or N23-GAL4 drivers did not activate Vg (monitored by both anti-Vg antibody and vg-QE staining; data shown only for Dpp-GAL4/UAS-activated Arm; Fig. 7C). These results further suggest that in addition to repressing DV signaling, Ubx downregulates event(s) downstream of Arm in both anterior and posterior non-DV cells.
|
Haltere-to-wing homeotic transformation by ectopic Vg
vg is a pro-wing gene: ectopic expression of Vg induces ectopic
wing development (e.g. ectopic Vg induces ectopic wing tissue on T2 legs)
(Kim et al., 1996).
Interestingly, ectopic Vg in T3 leg discs induces ectopic haltere development
(Weatherbee et al., 1998
).
Activation of vg-QE (Fig.
7G) by ectopic Vg in non-DV cells of haltere discs results in
homeotic transformation, albeit only partial. Ubx regulates haltere
development by modifying wing-patterning events at multiple levels
(Weatherbee et al., 1998
;
Shashidhara et al., 1999
). As
haltere discs express several other wing-patterning genes (including
vg at the DV boundary), ectopic expression of Vg might override Ubx
function in non-DV cells of haltere discs but not in T3 leg discs. We
therefore expressed Vg in haltere discs using several GAL4 lines. We observed
a high degree of haltere-to-wing homeotic transformations when Vg was
expressed using omb-GAL4
(Fig. 8B). In addition, we
observed enhanced homeotic transformations when Vg was expressed in a
Ubx-heterozygous background (Fig.
8D). Ectopic expression or overexpression of Wg, Dsh or activated
Arm in haltere discs did not induce homeotic transformation (data not shown).
This is consistent with the inability of Wg, Dsh and activated Arm to activate
vg-QE or Vg protein expression in the haltere pouch.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Absolute requirement for Vg in non-DV cells for its quadrant enhancer
activation
We designed experiments to test the current model of Wg and Vg regulation
(which is essentially based on studies on wing imaginal discs) in haltere
discs. In wing discs, both Wg and Vg are subjected to an elaborate regulatory
circuit, the understanding of which would help us to unravel crucial events
during wing development. To examine the Wg and Vg interactions further in DV
and non-DV cells, we carried out experiments that are essentially
complementary to those reported previously.
The experiments described in this report further suggest that Wg and Vg interact to maintain each other's expression at the DV boundary. We have shown that Vg-mediated activation of Wg is independent of Arm and TCF/pan function, which suggests that Vg may activate Wg either directly or through the N signaling pathway. We have also shown that Vg is capable of specifying wing development, even in the absence of Wg signaling. Overexpression of Vg in a vg1/vg1 background (in which no Wg or Vg is expressed) was sufficient to rescue wing phenotypes. This is particularly significant because we expressed Vg in this experiment only in non-DV cells. Our results also suggest that Vg cell-autonomously regulates its own expression through its quadrant enhancer. Clonal analysis of arm suggested that Wg is required to activate vg-QE and Arm was not able to activate this enhancer in vg1 background. Wg signaling might activate Vg either indirectly or by activating some other enhancer of Vg. Once activated, Vg might maintain its expression by autoregulation, which is mediated through its quadrant enhancer (Fig. 8G). This could ensure the maintenance of Vg expression in non-DV cells, once it is activated by Wg signaling. It might also explain how the Wg gradient is translated into uniformly higher levels of Vg in non-DV cells.
However, the above-mentioned model does not reconcile the observation that
Vg, and not Wg, is capable of activating vg-QE in
Ser background
(Klein and Martinez-Arias,
1999). As the vg gene is intact in
Ser background, ectopic expression of Wg using
dpp-GAL4 should have activated one of the enhancers to
induce Vg expression, which in turn would activate vg-QE. A model
that reconciles all the results would, therefore, include a third component,
which may act either in parallel to or downstream of Wg and Vg at the DV
boundary (Fig. 8G). The
presence of such a signaling molecule downstream of Vg has been previously
predicted (Neumann and Cohen,
1996
). Although there is no direct evidence for the existence of
such a molecule, the fact that N23-GAL4 expression in non-DV cells is
dependent on N function and independent of Vg and Wg function (R.B. and
L.S.S., unpublished observations) suggests such a possibility.
Mechanism of Ubx-mediated downregulation of DV signaling in
haltere discs
We also studied possible mechanisms by which Ubx regulates expression of Wg
and Vg in haltere discs. One important finding was the downregulation of Wg
signaling by Ubx at the level of Arm stabilization. We have further shown that
Ubx inhibits stabilization of Arm by acting on event(s) downstream of Sgg.
Normally, the Arm degradation machinery is very efficient and can degrade even
overexpressed Arm. This is evident from the fact that embryos overexpressing
Arm (from armS2) secrete normal denticle belts
(Pai et al., 1997). If a
downstream component functions with enhanced efficiency (either by direct
enhancement of its expression by Ubx or owing to repression of a positive
component of Wg signaling), residual activity of Sgg may be sufficient to
cause enhanced degradation of Arm. Thus, enhanced degradation of Arm in
haltere discs provided us with a new assay system to identify additional
components of Wg signaling. For example, in microarray experiments to identify
genes that are differentially expressed in wing and haltere discs, we observed
that several transcripts of known (e.g. Casein kinase) and putative (e.g.
Ubiquitin ligase) negative regulators of Wg signaling are upregulated in
haltere discs (M.P. and L.S.S., unpublished).
Our results suggest that Wg and Vg regulation in haltere discs is different
from that of wing discs. We have observed that Wg is not autoregulated in
haltere discs. In addition, Vg expression at the haltere DV boundary is
independent of Wg function. However, in both wing and haltere discs, Wg
expression at the DV boundary is dependent on Vg. Wg expression at the
anterior DV boundary of haltere discs could be redundant because
overexpression of DN-TCF at the haltere DV boundary shows no phenotype.
However, Vg at the DV boundary appears to have an independent function.
vg1 flies exhibit much smaller halteres than do wild-type
flies (Williams et al., 1991).
As Wg function (and expression in the posterior compartment) is already
repressed in haltere discs, reduction in haltere size in
vg1 flies suggests Wg-independent long-range effects of Vg
from the DV boundary. This could be one of the reasons why Ubx does not affect
Vg expression at the DV boundary but represses Vg expression in non-DV cells.
In wing discs too, Vg may have such a function on cells at a distance
(Neumann and Cohen, 1996
).
One way to test the requirement of Ubx in DV and non-DV cells directly is
by removing Ubx only from the haltere DV boundary or from non-DV
cells. We have previously reported that clonal removal of Ubx solely
from the haltere DV boundary does not induce cuticle phenotype in the
capitellum (Shashidhara et al.,
1999). However, we could not ascertain the effect on
vg-QE because of haploinsufficiency,
Ubx-heterozygous haltere discs themselves show
activation of lacZ in the entire haltere pouch (data not shown). The
activation of vg-QE in Ubx/+
haltere discs could be a result of reduced Ubx function at the DV boundary, or
in non-DV cells, or in both. We had previously shown that misexpression of Ubx
at the wing disc DV boundary causes non-cell-autonomous reduction in
vg-QE expression (Shashidhara et
al., 1999
). Our current results suggest that Ubx represses
additional event(s) in non-DV cells to downregulate Vg expression. This is
consistent with the recent report on cell-autonomous repression of
vg-QE by ectopic Ubx in wing discs
(Galant et al., 2002
). We
propose that Ubx inhibits the activation of Vg in non-DV cells at three
different levels (Fig. 8G): (1)
Wg in the posterior compartment; (2) event(s) downstream of Sgg that inhibit
the stabilization of Arm; and (3) additional event(s) downstream of Arm in
non-DV cells. In wing discs, as discussed above, Wg and a hitherto unknown DV
component may function together to activate Vg in non-DV cells. As
Vg-autoregulation is not inhibited in haltere discs, it is possible that Ubx
represses Vg activation in non-DV cells by interfering with the Wg-mediated
activation of Vg and/or by repressing the activity of the unknown DV-signal
molecule in the haltere.
We have also provided evidence that repression of Vg in non-DV cells by Ubx
is crucial for haltere development. Overexpression of Vg in haltere discs
causes haltere-to-wing transformations. This is particularly significant
considering the fact that haltere-to-wing homeotic transformations are always
associated with loss of Ubx, by direct removal of Ubx, by
activation of its repressors (e.g. polycomb proteins) or by suppression of its
activators (e.g. trithorax proteins). Mitotic clones of
Ubx alleles in the haltere capitellum normally
`sort out' and often remain as an undifferentiated mass of cells
(Morata and Garcia-Bellido,
1976; Shashidhara et al.,
1999
). This is attributed to differential cell-adhesion properties
of transformed (Ubx) and non-transformed
(Ubx+) cells. No such sorting out of wing-like trichomes
was observed in halteres overexpressing Vg. This implies that cells
surrounding the wing-like trichomes are also transformed, at least at the
level of cell-adhesion properties. This is consistent with our earlier
observations that removal of Ubx from the DV boundary or over-growth
caused by mutations in the tumor-suppressor gene fat confers
wing-like cell-adhesion properties to capitellum cells
(Shashidhara et al., 1999
). As
DV signaling is closely associated with the activation of Vg in non-DV cells
and Vg is primarily a growth-promoting gene, it is likely that the
cell-sorting behaviour of Ubx clones is linked to
their changed growth properties.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bhandari, P. and Shashidhara, L. S. (2001). Studies on human colon cancer gene apc by targeted expression in Drosophila. Oncogene 20,6871 -6880.[CrossRef][Medline]
Blochlinger, K., Jan, L. Y. and Jan, Y. N.
(1993). Postembryonic patterns of expression of cut, a
locus regulating sensory organ identity in Drosophila.Development 117,441
-450.
Brand, A. and Perrimon, N. (1993). Targeted
expression as a means of altering cell fates and generating dominant
phenotypes. Development
118,401
-415.
Brook, W. J. and Cohen, S. M. (1996). Antagonistic interactions between Wingless and Decapentaplegic responsible for dorsal-ventral pattern in the Drosophila leg. Science 273,1373 -1377.[Abstract]
Cabrera, C. V., Botas, J. and Garcia-Bellido, A. (1985). Distribution of proteins in mutants of bithorax complex genes and its transregulatory genes. Nature 318,569 -571.
Castelli-Gair, J., Greig, S., Micklem, G. and Akam, M.
(1994). Dissecting the temporal requirements for homeotic gene
function. Development
120,1983
-1995.
Couso, J. P., Knust, E. and Martinez Arias, A. (1995). Serrate and wingless cooperate to induce vestigial gene expression and wing formation in Drosophila. Curr. Biol. 5,1437 -1448.[Medline]
de Celis, J. F., Barrio, R. and Kafatos, F. C. (1966a). A gene complex acting downstream of dpp in Drosophila wing morphogenesis. Nature 381,421 -424.[CrossRef]
de Celis, J. F., Garcia-Bellido, A. and Bray, S. J.
(1996b). Activation and function of Notch at the dorsal-ventral
boundary of the wing imaginal disc. Development
122,359
-369.
Diaz-Benjumea, F. J. and Cohen, S. M. (1993). Interaction between dorsal and ventral cells in the imaginal disc directs wing development in Drosophila. Cell 75,741 -752.[Medline]
Diaz-Benjumea, F. J. and Cohen, S. M. (1995).
Serrate signals through Notch to establish a Wingless-dependent organizer at
the dorsal/ventral compartment boundary of the Drosophila wing.
Development 121,4215
-4225.
Fortini, M. E., Rebay, I., Caron, L. A. and Artavanis-Tsakonas, S. (1993). An activated Notch receptor blocks cell-fate commitment in the developing Drosophila eye. Nature 365,555 -557.[CrossRef][Medline]
Galant, R., Walsh, C. M. and Carroll, S. B.
(2002). Hox repression of a target gene:
extradenticle-independent, additive action through multiple monomer binding
sites. Development 129,3115
-3126.
Ghysen, A. and O'Kane, C. (1989). Neural enhancer-like elements as specific cell markers in Drosophila.Development 105,35 -52.[Abstract]
Go, M. J., Eastman, D. S. and Artavanis-Tsakonas, S.
(1998). Cell proliferation control by Notch signaling in
Drosophila development. Development
125,2031
-2040.
Gorfinkiel, N., Morata, G. and Guerrero, I.
(1997). The homeobox gene Distal-less induces ventral
appendage development in Drosophila. Genes Dev.
11,2259
-2271.
Halder, G. and Carroll, S. B. (2001). Binding
of the Vestigial co-factor switches the DNA-target selectivity of the
Scalloped selector protein. Development
128,3295
-3305.
Halder, G., Polaczyk, P., Kraus, M. E., Hudson, A., Kim, J.,
Laughon, A. and Carroll, S. (1998). The Vestigial and
Scalloped proteins act together to directly regulate wing-specific gene
expression in Drosophila. Genes Dev.
12,3900
-3909.
Hooper, J. E. (1994). Distinct pathways for autocrine and paracrine Wingless signalling in Drosophila embryos. Nature 372,461 -464.[CrossRef][Medline]
Irvine, K. D. and Wieschaus, E. (1994). Fringe, a boundary-specific signaling molecule, mediates interactions between dorsal and ventral cells during Drosophila wing development. Cell 79,595 -606.[Medline]
Kim, J., Irvine, K. D. and Carroll, S. B. (1995). Cell recognition, signal induction, and symmetrical gene activation at the dorsal-ventral boundary of the developing Drosophila wing. Cell 2, 795-802.
Kim, J., Sebring, A., Esch, J. J., Kraus, M. E., Vorwerk, K., Magee, J. and Carroll, S. B. (1996). Integration of positional signals and regulation of wing formation and identity by Drosophila vestigial gene. Nature 382,133 -138.[CrossRef][Medline]
Klein, T. and Martinez-Arias, A. (1998). Different spatial and temporal interactions between Notch, wingless, and vestigial specify proximal and distal pattern elements of the wing in Drosophila. Dev. Biol. 194,196 -212.[CrossRef][Medline]
Klein, T. and Martinez-Arias, A. (1999). The
Vestigial gene product provides a molecular context for the interpretation of
signals during the development of the wing in Drosophila.Development 126,913
-925.
Lawrence, P. A., Bodmer, R., Vincent, J. P.
(1995). Segmental patterning of heart precursors in
Drosophila. Development
121,4303
-4308.
Lewis, E. B. (1978). A gene complex controlling segmentation in Drosophila. Nature 276,565 -570.[Medline]
Liu, X., Grammont, M. and Irvine, K. D. (2000). Roles for scalloped and vestigial in regulating cell affinity and interactions between the wing blade and the wing hinge. Dev. Biol. 228,287 -303.[CrossRef][Medline]
Morata, G. and Garcia-Bellido, A. (1976). Developmental analysis of some mutants of the bithorax system of Drosophila. Roux's Arch. Dev. Biol. 179,125 -143.
Morimura, S., Maves, L., Chen, Y. and Hoffmann, F. M. (1996). decapentaplegic overexpression affects Drosophila wing and leg imaginal disc development and wingless expression. Dev. Biol. 177,136 -151.[CrossRef][Medline]
Munemitsu, S., Albert, I., Rubinfeld, B. and Polakis, P. (1996). Deletion of an amino-terminal sequence beta-catenin in vivo and promotes hyperphosporylation of the adenomatous polyposis coli tumor suppressor protein. Mol. Cell. Biol. 16,4088 -4094.[Abstract]
Nagaraj, R., Pickup, A. T., Howes, R., Moses, K., Freeman, M.
and Banerjee, U. (1999). Role of the EGF receptor
pathway in growth and patterning of the Drosophila wing through the regulation
of vestigial. Development
126,975
-985.
Neumann, C. J. and Cohen, S. M. (1996). A
hierarchy of cross-regulation involving Notch, wingless, vestigial
and cut organizes the dorsal/ventral axis of the Drosophila
wing. Development 122,3477
-3485.
Neumann, C. J. and Cohen, S. M. (1997).
Long-range action of Wingless organizes the dorsal-ventral axis of the
Drosophila wing. Development
124,871
-880.
Pai, L. M., Orsulic, S., Bejsovec, A. and Peifer, M.
(1997). Negative regulation of Armadillo, a Wingless effector in
Drosophila. Development
124,2255
-2266.
Papkoff, J., Rubinfeld, B., Schryver, B. and Polakis, P. (1996). Wnt-1 regulates free pools of catenins and stabilizes APC-catenin complexes. Mol. Cell. Biol. 16,2128 -2134.[Abstract]
Patel, N. H., Martin-Blanco, E., Coleman, K. G., Poole, S. J., Ellis, M. C., Kornberg, T. B. and Goodman, C. S. (1989). Expression of engrailed proteins in arthropods, annelids and chordates. Cell 58,955 -968.[Medline]
Peifer, M., Sweeton, D., Casey, M. and Wieschaus, E.
(1994). Wingless signal and Zeste-white 3 kinase trigger opposing
changes in the intracellular distribution of Armadillo.
Development 120,369
-380.
Riggleman, B., Schedl, P. and Wieschaus, E. (1990). Spatial expression of the Drosophila segment polarity gene armadillo is postranscriptionally regulated by wingless. Cell 63,549 -560.[Medline]
Ruel, L., Stambolic, V., Ali, A., Manoukian, A. S. and Woodgett,
J. R. (1999). Regulation of the protein kinase activity of
Shaggy (Zeste-white3) by components of the wingless pathway in Drosophila
cells and embryos. J. Biol. Chem.
274,21790
-21796.
Rulifson, E. J. and Blair, S. S. (1995). Notch
regulates wingless expression and is not required for reception of
the paracrine wingless signal during wing margin neurogenesis in
Drosophila. Development
121,2813
-2824.
Shashidhara, L. S., Agrawal, N., Bajpai, R., Bharathi, V. and Sinha, P. (1999). Negative regulation of dorsoventral signaling by the homeotic gene Ultrabithorax during haltere development in Drosophila. Dev. Biol. 212,491 -502.[CrossRef][Medline]
Simmonds, A. J., Brook, W. J., Cohen, S. M. and Bell, J. B. (1995). Distinguishable functions for engrailed and invected in anterior-posterior patterning in the Drosophila wing. Nature 37,424 -427.
Simmonds, A. J., Liu, X., Soanes, K. H., Krause, H. M., Irvine, K. D. and Bell, J. B. (1998). Molecular interactions between Vestigial and Scalloped promote wing formation in Drosophila.Genes Dev. 15,3815 -3820.
van de Wetering, M., Cavallo, R., Dooijes, D., van Beest, M., van Es, J., Loureiro, J., Ypma, A., Hursh, D., Jones, T., Bejsovec, A., Peifer, M., Mortin, M. and Clevers, H. (1997). Armadillo coactivates transcription driven by the product of the Drosophila segment polarity gene dTCF. Cell 88,789 -799.[Medline]
Warren, R. W., Nagy, L., Selegue, J., Gates, J. and Carroll, S. (1994). Evolution of homeotic gene regulation and function in flies and butterflies. Nature 372,458 -461.[CrossRef][Medline]
Weatherbee, S. D., Halder, G., Kim, J., Hudson, A. and Carroll,
S. (1998). Ultrabithorax regulates genes at several
levels of the wing-patterning hierarchy to shape the development of the
Drosophila haltere. Genes Dev.
12,1474
-1482.
Weatherbee, S. D., Nijhout, H. F., Grunert, L. W., Halder, G., Galant, R., Selegue, J. and Carroll, S. B. (1999). Ultrabithorax function in butterfly wings and the evolution of insect wing patterns. Curr. Biol. 11,109 -115.[CrossRef]
White, R. A. H. and Akam, M. (1985). Contrabithorax mutations cause inappropriate expression of Ultrabithorax in Drosophila. Nature 318,567 -569.
Williams, J. A., Bell, J. B. and Carroll, S. B. (1991). Control of Drosophila wing and haltere development by the nuclear vestigial gene product. Genes Dev. 5,2481 -2495.[Abstract]
Williams, J. A., Paddock, S. W. and Carroll, S. B.
(1993). Pattern formation in a secondary field: a hierarchy of
regulatory genes subdivides the developing Drosophila wing disc into
discrete sub-regions. Development
117,571
-584.
Williams, J. A., Paddock, S. W., Vorwerk, K. and Carroll, S. B. (1994). Organization of wing formation and induction of a wing-patterning gene at the dorsal/ventral compartment boundary. Nature 368,299 -305.[CrossRef][Medline]
Xu, T. and Rubin, G. M. (1993). Analysis of
genetic mosaics in developing and adult Drosophila tissues.
Development 117,1223
-1237.
Yoffe, K. B., Manoukian, A. S., Wilder, E. L., Brand, A. H. and Perrimon, N. (1995). Evidence for engrailed-independent wingless autoregulation in Drosophila. Dev. Biol. 170,636 -650.[CrossRef][Medline]
Zecca, M., Basler, K. and Struhl, G. (1996). Direct and long-range action of a Wingless morphogen gradient. Cell 87,833 -844.[Medline]