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the two N-off and N-on hemilineages (
Lin et al., 2012
). How the same tem-
poral identity information regulates the independent pace of temporal
switching of neuronal fates in the two hemilineages is an interesting question
for the future.
9. CONCLUSIONS AND FUTURE QUESTIONS
Currently, two different temporal sequences have been identified in
NBs of two different systems: the Hb
!
Kr
!
Pdm
!
Cas
!
Grh sequence
in the
Drosophila
VNC, and the Hth
Tll sequence
that patterns medulla NBs. In addition, the INPs of type II NB lineages are
patterned by a TF sequence D
!
Klu
!
Ey
!
Slp
!
D
!
!
!
Ey. These three examples suggest
that TF-sequence-dependent temporal patterning of neural precursors is a
common theme to generate neural diversity. This suggests that more tem-
poral sequences will be identified in other systems. For example, in the
antennal lobe antero-dorsal lineage, Kr specifies one out of 40 temporal
identities in the NB. Further identification of a complete temporal sequence
will rely on candidate gene approaches, and/or screening based on either
gene expression or mutant phenotypes. Does TF-sequence-dependent tem-
poral patterning of neural precursors also function in vertebrate systems?
There is some evidence suggesting that this might be the case. In the ver-
tebrate retina, one ortholog of
hb
,
Ikaros
, specifies early-born cell fates
(
Elliott, Jolicoeur, Ramamurthy, & Cayouette, 2008
). In mammalian cor-
tical neurogenesis, Foxg1, an ortholog of Slp, functions in cortical progen-
itors to suppress early-born cortical cell fates (
Hanashima, Li, Shen, Lai, &
Fishell, 2004
). Although the vertebrate systems are much more complex
than
Drosophila
, studies in flies have provided important concepts that might
be applicable to vertebrates.
Cross-regulations between temporal TFs are important for temporal
transitions: Feedback negative regulation and feedforward positive regula-
tion among the temporal TFs can facilitate the progression of the sequence.
There are many remaining questions to understand the mechanism of tem-
poral transitions. Even in the best-known systems, there can be missing fac-
tors or timing mechanisms in addition to the identified TF sequence. When
all factors involved and their regulatory relationships are characterized by
genetic analysis, theoretical modeling will provide insights into how the
genetic network precisely times the NB temporal progression.
Loss of NB competence is related to epigenetic changes, such as chro-
matin modifications and chromosome architecture. More studies are needed
Grh
