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temporal patterning of NBs. Recent studies identified a different series of
TFs expressed in medulla NBs: Homothorax (Hth), Klumpfuss (Klu), Eye-
less (Ey), Sloppy paired (Slp), Dichaete (D), and Tailless (Tll) are sequentially
expressed in NBs of increasing ages, with Hth expressed in newly differen-
tiated NBs, Klu, Ey, Slp, and D expressed in increasingly older NBs, and
Tll in the oldest NBs. Hth, Ey, and Slp were shown to control the gener-
ation of specific neuron types born during each time window (
Li et al., 2013;
Suzuki et al., 2013
;
Fig. 3.2
). This identification of a second TF sequence
that is different from the one in the embryonic VNC suggests that TF
sequence-dependent temporal patterning of NBs is likely to be broadly uti-
lized, and that different TF sequences can be recruited in different systems.
2.4. Combinatorial temporal patterning of INPs
and type II NBs
A recent elegant paper showed that, in type II NB lineages, both NBs and
INPs are temporally patterned by different temporal series of TFs
(
Bayraktar & Doe, 2013
). Type II NBs sequentially express D/Cas and
Svp as well as other not yet identified members of a TF sequence. Along
the second temporal axis, INPs sequentially express D, Grh, and Ey as they
age: These TFs are required for the sequential production of distinct neural
subtypes. Thus, TF sequences on the two temporal axes act combinatorially
to generate larger neural diversity (
Bayraktar & Doe, 2013
;
Fig. 3.3
).
Figure 3.2 Schematic model showing the sequential expression of Hth, Klu, Ey, Slp, D,
and Tll in medulla NBs. These TFs control the sequential generation of different neuronal
types marked by specific combinations of neuronal markers. Ap is expressed in the N-on
daughters of ganglion mother cells. Tll
รพ
NBs generate glia.
