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the two GMC daughter cells ( Buescher et al., 1998; Karcavich &Doe, 2005;
Lundell, Lee, Perez, & Chadwell, 2003; Novotny et al., 2002; Schuldt &
Brand, 1999; Skeath & Doe, 1998; Spana & Doe, 1996; Udolph,
Rath, & Chia, 2001 ). During the asymmetric division of GMCs, Numb,
a repressor of Notch signaling, is asymmetrically localized to one daughter
cell, and this cell adopts a Notch-off (N-off ) fate, while the other cell adopts
a Notch-on (N-on) fate ( Buescher et al., 1998; Spana & Doe, 1996 ). All the
N-on (or N-off ) cells within a lineage are collectively called a hemilineage.
In postembryonic NB lineages, Notch/Numb also function in binary-fate
choices of GMC progeny ( Kumar, Bello, & Reichert, 2009; Li et al.,
2013; Lin et al., 2010; Truman, Moats, Altman, Marin, & Williams, 2010 ).
Notch signaling only differentiates between two alternative fates, but the
actual fates depend on the spatial and temporal identity of the NB. For exam-
ple, Notch has distinct lineage-specific effects in the three antennal lobe
NB lineages: antero-dorsal lineage (N-off: PN fates, N-on: apoptosis); ven-
tral lineage (N-off: apoptosis, N-on: PN fates); and lateral lineage (N-off:
PN fates, N-on: local interneuron fates) ( Lin et al., 2010 ).
How does the Notch-dependent binary fate choice integrate with the
temporal identity of NBs? In the developing medulla, the Notch pathway
regulates the maintenance of the temporal TFs in postmitotic neurons
and the expression of Ap. In the Ey and Slp NB stages, Ey or Slp is only
maintained in the N-off daughter, while the N-on daughter turns on Ap.
At the DNB stage, D is only maintained in the N-on daughter together with
Ap. Although most if not all N-on daughters of medulla GMCs express Ap,
they express different combinations of other TFs and adopt different fates
depending on which NB stage they are born from. For example, the
N-on progeny of Ey þ GMCs express Ap and Drifter (Dfr), and this is depen-
dent on both Notch signaling and the expression of Ey in NBs. The N-on
progeny of Slp þ GMCs express Ap and twin of eyeless (Toy), and similarly,
this depends on both Notch signaling and Slp expression in NBs. Thus, the
temporal TFs and the Notch pathway together control the expression of
downstream TFs like Drifter and Toy to control neuron fates ( Li et al.,
2013 ; Fig. 3.2 ).
Studies in the antennal lobe lateral lineage illustrate another interesting
question. In this lineage, PNs and local interneurons are produced as siblings
of each GMC division (N-off: PN, N-on: local interneuron). However,
as there is more diversity of PNs than of local interneurons, the same
local interneuron can be the sibling of different PNs. Thus, it appears that
the tempo of birth-order-dependent fate changes is different between
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