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requires inputs from the temporal sequence. At late stages of embryogenesis,
Cas turns on the expression of Grh, and turns off the expression of D in NBs.
Thus, Grh is expressed in NBs that enter quiescence and is subsequently
maintained in pNBs through larval stages. The state of Grh
รพ
D
is required
for the abdominal pNBs to undergo apoptosis in response to the burst of
Abdominal-A (
Almeida & Bray, 2005; Cenci & Gould, 2005; Maurange
et al., 2008
). Furthermore, completion of the larval temporal sequence is
required for the pNBs to undergo apoptosis: Either loss of postembryonic
Svp or persistent expression of Cas can block the larval temporal sequence
progression, and these NBs do not die in spite of normal Abdominal-A
expression (
Maurange et al., 2008
;
Fig. 3.6
).
6.2. Pros-dependent cell-cycle exit of thoracic and central
brain pNBs
Most thoracic and central brain NBs (with the exception of mushroom body
NBs) stop dividing at 120 h after larva hatching (
1 day into pupation). In
contrast to the abdominal pNBs that undergo apoptosis, thoracic and central
brain NBs end neurogenesis through cell-cycle exit that is dependent on a
nuclear burst of expression of the homeodomain protein Prospero (Pros)
(
Maurange et al., 2008
). Pros is tethered to cell membrane in NBs, and is
asymmetrically localized to GMCs during asymmetric cell division. In
GMCs, Pros is localized to nuclei where it promotes cell-cycle exit after
one cell division (reviewed in
Yu, Kuo, & Jan, 2006
). Thoracic and central
Figure 3.6 The temporal sequence in embryonic and larval VNC NBs is required for end-
ing neurogenesis. At late stages of embryogenesis, Cas turns on Grh and turns off D in
NBs. Grh, as well as two members of the larval temporal sequence, Cas and Svp, are
required for the NBs to end neurogenesis at the correct time. Thoracic NB lineages
are longer, and undergo pros-dependent cell-cycle exit. Abdominal NB lineages are
shorter, and undergo apoptosis that depends on Reaper, Grim, and Hid.
