- "Activation of Iro-C genes could account for the requirement
for EGFR activity to specify the notum at the end of the second
instar as this correlates with when these genes are first expressed.
However, loss of EGFR signaling at a slightly earlier time (midfirst
instar to mid-second instar, see below), prior to activation of
the Iro-C genes, also results in loss of the notum. A possible
explanation for this comes from the finding that vn expression
is lost in vn mutants (Fig. 2F). This suggests EGFR activity must
be sustained, via a positive feedback loop involving transcriptional
activation of vn, during the second instar, to activate the Iro-C
genes and hence specify notum at the end of this period. Interestingly,
the vn gene is also a target of EGFR signaling in the embryo (Golembo
et al. 1999; Wessells et al. 1999)." (Wang,
2000)
- "Figure 5. Model to account for the role of EGFR signaling
in wing and notum development. The wing and notum are specified
by the action of two secreted signaling molecules, Wg and the
EGFR ligand, Vn, which are expressed in the distal and proximal
regions of the early second instar wing disc, respectively. Wg
represses Vn expression restricting it to the proximal region.
Wg and Vn define three domains in the early disc: (1) distal,
high Wg/no EGFR signaling; (2) central, low Wg/low EGFR signaling;
(3) proximal, high EGFR/no Wg. High EGFR signaling activity defines
the notum by activating notum specific genes; it also functions
to maintain vn expression and can repress the expression of wing
specific genes. Lower EGFR activity is also sufficient to activate
ap expression defining the dorsal (D) compartment. An organizer
for wing patterning is established at the DV boundary; wings fail
to develop in the absence of Vn/EGFR signaling because this organizer
is absent." (Wang,
2000)
- "The results described here suggest that division of the
early wing disc into presumptive wing and body wall regions is
defined by the action of two secreted signaling molecules, Wg
and Vn (Fig. 5). wg, a pro-wing gene, is required to repress vn
expression, which at high levels antagonizes wing development.
Antagonism betweenWg and EGFR signaling has also been demonstrated
in segmental patterning of the embryo (O’Keefe et al. 1997;
Szuts et al. 1997) and in development of the head and third instar
wing pouch (Amin et al. 1999; Wessells et al. 1999), suggesting
such a relationship between these pathways may be a common theme
in a number of cell fate choices. Finding that one of the main
functions of Wg in early wing specification is to repress Vn/EGFR
signaling in the distal region of the early disc raises the question
as to whether this is the only role of Wg in wing specification
and hence if wing-cell fate can be specified in the absence of
both signals. This seems unlikely as nubbin, an early wing cell
marker (Ng et al. 1996), is not misexpressed proximally in a vn
mutant, where cells would lack both signals. Vn/EGFR signaling
promotes development of the notum by maintaining its own activity
through transcriptional activation of vn itself, and also promotes
expression of ap. Thus, both vn and ap appear to be targets of
EGFR signaling, but the domain of ap is clearly wider than that
of vn, indicating that ap can be activated at a lower signaling
threshold than vn. Vn is a secreted molecule and thus could generate
a gradient of EGFR activity. This provides an explanation for
how EGFR signaling can regulate both wing and notum development:
vn autoregulation and notum development requires high EGFR signaling
activity whilst ap expression and subsequent wing development
requires lower signaling activity (Fig. 5). Interestingly, vertebrate
EGFR and its ligands are expressed in the chick limb bud in a
pattern that appears to overlap with the vertebrate ap homolog
Lhx2, and these factors are required for limb outgrowth in the
chick (Dealy et al. 1998; Rodriguez-Esteban et al. 1998). In light
of our present results it will be important to determine whether
EGFR signaling controls Lhx2 expression and thus plays a role
in regulating outgrowth of the vertebrate limb. Our results may
also have implications for the evolution of insect wings. If the
control of body wall development by EGFR signaling is ancestral,
and comparative analysis of other arthropods will be required
to assert this, then one of the first steps towards evolution
of wings could have occurred when EGFR signaling assumed control
of ap." (Wang,
2000)
- In the wild-type third instar wing disc, the notum and the wing
hinge territories are separated by a fold in the epithelium (Figs.
2A and 3A,B; Bryant 1978) (Diez
del Corral, 1999)
- "The well-known organizing borders of the wing disc coincide
with the A/P and D/V cell-lineage restriction boundaries (Blair
1995; Brook et al. 1996). A similar boundary, located between
the notum and the wing hinge, has been inferred after examining
lineage clones in the adult cuticle (Garc?´a-Bellido et
al. 1976). However, at the disc level, we have not found evidence
for the existence of this cell-lineage restriction. If present,
it certainly does not coincide with the border of Iro-C-expressing
cells, as cell-lineage clones generated by the Minute technique
(Morata and Ripoll 1975) and induced as late as 72–96 hr
AEL straddled this border (Fig. 5C). We conclude that upon proliferation,
cells that cross the border of Iro-C expression most likely change
their fate from notum to proximal hinge or vice versa." (Diez
del Corral, 1999)
- "The patterns of expression of Iro and other genes important
for development of the wing disc suggest that this disc is organized,
in the proximal/distal axis, in concentric domains (Fig. 5D).
Iro expression (Fig. 5A; Go´mez-Skarmeta et al. 1996) coincides
with the outermost domain, which gives rise to the notum and meso-thoracic
pleura, whereas the vg quadrant enhancer-mediated expression coincides
with the innermost one (Kim et al. 1996), essentially the wing
blade territory. In between, the expressions of nub (Ng et al.
1995) and l(2)09261 coincide with increasingly larger domains
(Fig. 4), and the rings of late expression of wg (Phillips and
Whittle 1993) mark concentric subdomains of the proximal wing.
The finding that Iro-C promotes the development of notum as opposed
to dorsal hinge structures suggests a specific genetic address
for the hinge domain, for which we do not know of a corresponding
marker. Palaeontological data on the structure of primitive fossil
pterygote hinges suggest that the hinge structures (tergal pteralia)
originated from the tergal lobes (lateral projections of the tergum)
(Kukalova-Peck 1978); that is, phylogenetically, they are probably
body wall structures (Fig. 5E). Accordingly, the Iro-C homeodomain
proteins help establish a subdivision between two tergal territories
and could do so by antagonizing genes specifying hinge fate."
(Diez
del Corral, 1999)
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