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interneuron and the GW motor neuron; GMC-2 is Kr
þ
, and generates the
EW2 interneuron and a sibling that undergoes apoptosis; GMC-3 is Pdm
þ
,
and differentiates directly into the EW3 interneuron (
Isshiki et al., 2001;
Karcavich & Doe, 2005; Lundell & Hirsh, 1998
). In this case, Hb is necessary
and sufficient for the first-born neuron fates (EW1/GW). Loss of
hb
results in
a lack of the EW1/GW, while EW2 and EW3 are still present. Prolonged
expression of Hb in the NB leads to additional neurons with EW1/GW fates
at the expenseof EW2 andEW3 (
Isshiki et al., 2001;Novotny et al., 2002
).Kr is
necessary and sufficient for the second-born neuron fate EW2 (
Isshiki
et al., 2001
).
There are variations in the function of the temporal TFs: In the NB3-1
lineage, Pdm is not required for specification of the third temporal identity,
but instead to close the preceding Kr
þ
temporal identity window (
Tran &
Doe, 2008
). The first four GMCs of NB3-1 lineage produce HB9
þ
,Islet
þ
RP motor neurons with a birth order of RP1
RP5
(and their non-RP siblings): Hb specifies RP1 and RP4 (high Hb: RP1;
low Hb: RP4); Kr specifies RP3; while Pdm is not required for specifying
RP5, because there is only a modest expansion of Kr expression in NBs in
pdm
mutants, with a few extra RP3 neurons produced, but RP5 neurons are
still specified. Similarly, Cas is required for closing the third (RP5) temporal
identity window: In
cas
mutants, there are ectopic RP5 neurons (
Tran &
Doe, 2008
).
Thus, a series of TFs sequentially expressed in NBs control the sequential
generation of different neural types in multiple NB lineages. Since different
NBs generate different lineages, these TFs do not specify a certain neuron
type, but control the birth-order-dependent neuronal identity. The birth-
order-dependent temporal identity is integrated with the spatial identity
of the NB within each segment or between different segments, and is trans-
lated into specific cell types. For example,
hb
controls the first-born cell fates
in multiple lineages, which can be motor neurons, interneurons or glial cells,
depending on the NB lineage (
Isshiki et al., 2001
).
!
RP4
!
RP3
!
2.2. Similar or different TF sequence in other systems?
Since birth-order-dependent neuronal specification has been widely
observed in various systems, is the same or a similar temporal TF sequence
utilized to pattern neural stem cells of other systems?
In the mushroom body, which is generated by four NBs, each NB
sequentially generates at least three types of neurons. Thus far, no temporal
sequence of TFs that controls the fate of these neurons has been identified in
