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basal
Specified SOP's
apical
SP I
Numb
SP IIb
SP IIa
Notch
Notch
FIGURE 4.7 Frames from a time-lapse visualization of an SOP
going through an asymmetrical division. The green label follows
Numb, and the red label follows the chromosomes. In this sequence
(courtesy of F. Schweisguth), it is easy to see Numb localized to one
pole of the SOP and then inherited by a single daughter, spIIb.
Socket
Shaft
Notch
Support
cell
Neuron
1. Each specified SOP undergoes an invariant pattern of
cell divisions. This division pattern has been investi-
gated in detail for the external mechanosensilla (Guo et
al., 1996; Schweisguth et al., 1996). The primary SOP
(spI) for each macrochaete divides into an anterior
daughter called spIIb and a posterior daughter called
spIIa (Figure 4.8). SpIIa produces the outer two acces-
sory cells: the socket cell and the shaft cell. The anterior
daughter SpIIb divides into a neuron and a support
cell, after first giving rise to a glial cell. These different
fates arise through the reuse of the Notch signaling
system during each of the cell divisions. When the SOP
divides into spIIa and spIIb, these two cells interact
with each other via Notch. The SpIIb fate is dominant,
which is shown by the fact that if spIIb is ablated, spIIa
will transform into spIIb. In Notch mutants, both cells
become spIIb, and the result is no bristles or sockets,
and when Notch is experimentally activated in both
cells, they both turn into spIIa and there are no neurons
or glia. Several intrinsic determinants, including the
asymmetrically inherited Numb (see above), control
the fate of spIIa versus spIIb (Figure 4.8). In this case,
Numb is distributed to spIIb upon cell division. In the
absence of Numb, the Notch pathway is active in spIIb,
and it is transformed into spIIa; neither neurons nor
support cells appear, but the sensilla form instead with
double sockets and shafts. Numb mutants are thus
insensitive to touch because the sensory bristles are
uninnervated, hence the origin of the name.
Epi
Socket
Shaft
Support cell
Neuron
FIGURE 4.8 Lineage of a Drosophila external mechanosensory
organ. From top to bottom: A Sensory organ precursor (SOP)
enlarges and delaminates from the epithelium. It divides asymmet-
rically into spIIb, which inherits Numb, and spIIa, which does not.
SpIIa divides again asymmetrically, as does spIIb slightly later.
Notch signaling between daughters is involved in all these asym-
metric divisions so that four daughter cells of the SOP have four dif-
ferent fates: support cell, sensory neuron, socket cell, and shaft cell.
GENERATING COMPLEXITY THROUGH
CELLULAR INTERACTIONS
The compound eye of an insect is composed of a
large number of identical unit eyes, called ommatidia,
each with its own lens and array of cell types. This
system provides a rather different example of how
specific cells get their fates. Each of the 800 ommatidia
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