Temporal Patterning of Neural Progenitors in Drosophila - Current Topics in Developmental Biology - page 86

brain NBs exhibit a burst of expression of nuclear Pros at 120 h. pros mutant
clones contain multiple NB-like cells that do not die at 120 h, and even con-
tinue dividing in the adult brain ( Maurange et al., 2008 ).
Pros-dependent cell-cycle exit of thoracic and central brain NBs also
requires inputs from both embryonic and larval temporal sequences:
Sustained Grh expression in pNBs induced by embryonic Cas is required
to prevent premature cell-cycle exit of NBs; while completion of the larval
temporal sequence is required for the NBs to undergo Pros-dependent cell-
cycle exit at 120 h. Remarkably, stalled temporal sequence caused by either
loss of postembryonic Svp expression or persistent Cas expression prevents
cell-cycle exit and NBs continue to divide even in 7-day-old adults
( Maurange et al., 2008 ; Fig. 3.6 ).
6.3. Studies in other systems about the end of neurogenesis
In contrast to VNC and the majority of central brain NBs, mushroom body
NBs that are born at embryonic stages do not have a quiescent stage and con-
tinue proliferating until the end of pupal stage. Tll is required for the
uninterrupted and prolonged proliferation of mushroom body NBs: Loss
of tll causes premature loss of mushroom body NBs in early pupal stage
( Kurusu et al., 2009 ). Surprisingly, in the medulla NB temporal sequence,
Tll appears to have the opposite function: Tll is the last TF that is expressed
in the oldest NBs. These Tll þ NBs show nuclear localization of Pros, indi-
cating that they undergo Pros-dependent cell-cycle exit at the end of their
life, similar to the thoracic and central brain NBs ( Li et al., 2013 ). Whether
Tll is required or sufficient for ending medulla neurogenesis is currently not
known. If so, it will be interesting to understand how Tll plays completely
opposite roles in the mushroom body and in medulla NBs.
7. INTEGRATION OF TEMPORAL AND SPATIAL
INFORMATION DETERMINES LINEAGES
Although almost all NBs in the embryonic VNC follow the same
temporal sequence, they generate different lineages depending on their
spatial identity. There are approximately 30 NBs in each hemisegment,
and each NB has an individual fate based on its position and the expression
of specific molecular markers. Intrasegmental specification is achieved by
superimposed activities of segment polarity and dorso-ventral patterning
genes ( Technau et al., 2006 ). In addition to the intrasegmental spatial pat-
terning, homologous NBs in different segments along the antero-posterior