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in a Drosophila eye possesses 8 photoreceptors and 12
accessory cells. The 8 photoreceptors (R1-R8) are spe-
cialized sensory neurons (Figure. 4.9). Among the
accessory cells, there are cone cells that form the lens
of each ommatidium and pigment cells that surround
the photoreceptors optically shielding one ommatid-
ium from light that enters its neighbors. What is clear
in this system is that lineage is not involved in the
specification of the different cell types. Experimental
results have shown that there is no clear clonal rela-
tionship among the cells of the ommatidia (Ready et
al., 1976). In the developing Drosophila retina, cell-cell
interactions between the postmitotic photoreceptors
and accessory cells are primarily responsible for
specifying cell fate (Banerjee and Zipursky, 1990). Even
after a cell has become postmitotic, it remains tem-
porarily uncommitted to any particular differentiated
fate. The mechanism controlling retinal cell fate diver-
sification depends on the fact that the cells do not dif-
ferentiate all at once, but follow a precise, reproducible
temporal sequence of interactions. Thus, during late
larval life, a wave of differentiation passes over the eye
disc in a posterior to anterior direction (Figure 4.9). The
wave front, or the position at which ommatidial dif-
ferentiation begins, is morphologically visible as a
morphogenetic furrow (MF), a narrow groove formed
by apical constriction of the eye disc cells. As the
furrow advances, cells in its wake aggregate into
“rosettes” that foreshadow the regular ommatidial
pattern. One cell is then singled out in each rosette.
This becomes the R8 photoreceptor. The bHLH
proneural gene atonal is turned on by the signaling
protein Hh (a homolog of vertebrate Shh ), which is
expressed at the posterior tip of the eye disc (Kumar
and Moses, 2000). Initially, atonal comes on in a con-
tinuous band of cells within the morphogenetic
furrow. And this initiates a lateral inhibition mecha-
nism that involves Notch signaling so that atonal
expression becomes restricted to a mosaic of regularly
spaced cells, which subsequently differentiate as R8.
The set of well-spaced R8 cells continues to emit Hh,
which signals across the MF toward the more anterior
cells of the eye disc to induce the next set of R8 cells.
This Hh-mediated feedback mechanism drives the
morphogenetic furrow across the eye disc.
The first cells that join each R8 cell shortly after its
determination become R2 and R5. It is believed that a
signal emanating from R8 instructs the next cells to join
the cluster, R2 and R5. These cells acting in combina-
tion with R8 help give the next cells that join the cluster
R3 and R4 and then R1 and R6 their fates (Ready, 1989).
The last photoreceptor to join the cluster is R7. Thus,
ommatidial clusters incorporate cells in a manner
analogous to how growing crystals incorporate mole-
cules. That is, as new cells are added, they become
neighbors of cells that are already incorporated and
have particular fates. In this way, determination of a
specific fate moves as a wave of crystallization across
the eye disc, and so the developing Drosophila eye has
been called a neurocrystalline array.
Each type of ommatidial cell expresses a unique set
of intrinsic determinants. For example, rough ( ro ) is
expressed in R2, R5, R3; R4, Bar appears in R1 and R6;
Seven-up ( svp ) in R1, R6, R3, R4; Prospero ( pros ) in R7
and cone cells, lozenge ( lz ) in R1, R6, R7, and cone cells,
and Pax2 in cone cells only (Figure 4.9). Each of these
A
B
lens
precluster
rosette
cone cell
primary
pigment cell
secondary
pigment cell
cluster
outer
photoreceptors
8
1
2
cc
R1-6
3
4
R7
inner
photoreceptors
R7,8
morphogenic
furrow
5 6
C
D
PC
Lz
Bar
svp
Morphogenetic
furrow
Rosette
R5
R6
pax2
3
2 ro
4
5
ro
svp
3
4
1
R4
Hh
2
2
Hh
R8
ato
R7
ato
ro
svp
sina
R3
1
3
PC
5
ro
Lz
Bar
svp
2
3
4
pax2
R1
R2
1
2
3
Notch
Signalling
7
8
4
5
6
MF
FIGURE 4.9 A. A schematic longitudinal section through an
ommatidium depicting the different cell types. B. Diagram showing
a surface view of part of the eye disc at a stage when photoreceptor
clusters become assembled. C. A precluster in which the R8 precur-
sor expresses ato in response to a previously generated hedgehog
(Hh) signal. The R8 cell produces its own Hh and by this relay starts
the next R8. R8s spread themselves out through the Notch lateral
inhibition pathway. D. Cascade of photoreceptor determination in a
developing ommatidial cluster. R8, the first cell to be determined
expresses atonal ( ato ) and signals neighboring cells to become R2 and
R5, which then express rough ( ro ). R2 and R5 in combination with R8
then signal the next set of neighboring cells to join the cluster of
“fire” and become R3 and R4, which express seven-up ( svp ). On the
other side, a cluster of seven cells is formed when R1 and R6 are
induced to express Bar , svp , and lozenge ( lz ) by R2, R5, and R8. Finally
R8, in combination with R1 and R6, induce the final photoreceptor
R7 expressing sevenless in absentia ( sina ) and prospero ( pros ) to join.
After the photoreceptors have joined, pigment cells expressing lz
and pax-2 are induced to join the cluster.
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