Biology Reference
In-Depth Information
the NB. However, a novel bric-a-brac,tramtrack, broad (BTB)-zinc finger
protein, named Chinmo (chronologically inappropriate morphogenesis),
acts in GMCs or young neurons to control the temporal identity of mush-
room body neurons (
Zhu et al., 2006
). Although Chinmo mRNA is equally
expressed throughout the entire NB lineage, Chinmo protein shows a tem-
poral gradient in the neuronal progeny. It is absent in NBs and is expressed at
its highest levels in the early-born neurons, lower in the next-born neurons,
and undetectable in the latest-born neurons. Reducing or increasing
Chinmo levels causes transformation of neurons toward later or earlier fates,
respectively (
Zhu et al., 2006
). Recently, microRNAs of the Let-7-com-
plex, the heterochronic miRNAs originally identified in
Caenorhabditis
elegans
, were found to target
chinmo
to regulate the temporal identity of
Dro-
sophila
mushroom body neurons (
Wu, Chen, Mercer, & Sokol, 2012
). Since
Chinmo protein or Let-7 miRNAs are not detected in NBs, how the
Chinmo gradient in neurons is regulated as the NB ages is not understood.
Postembryonic neuroblasts (pNBs) in the VNC generate 90% of neurons
that constitute the adult CNS. Although a complete temporal TF sequence
has not been identified, recent studies identified two members of the post-
embryonic TF sequence, Cas and Seven-up (Svp) (
Maurange, Cheng, &
Gould, 2008; Tsuji, Hasegawa, & Isshiki, 2008
). They are required for a
temporal switch of pNBs from generating small Chinmo
þ
neurons to
instead producing large Br-C
þ
(Broad Complex) neurons in many different
lineages. Since the switch from Chinmo
þ
to Br-C
þ
happens later than the
transient expression of Cas or Svp, the switch must be directly controlled by
an unknown member of the TF sequence whose expression is promoted
by
cas
and
svp
by feedforward regulation (
Maurange et al., 2008
). Cas and
Svp are also required for the NBs to eventually end neurogenesis. This is
discussed in detail below.
In the anterodorsal lineage of the antennal lobe, Kr was shown to act in
the NB to define one out of 40 temporal fates of PNs (
Kao, Yu, He, Kao, &
Lee, 2012
). Loss of Kr from the NB causes a single PN fate to be skipped.
However, loss of Hb, Pdm or Cas does not produce detectable phenotypes
(
Kao et al., 2012
). It will be interesting to identify more TFs that are tem-
porally expressed in antennal lobe NBs and control the temporal specifica-
tion of a large number of distinct PNs and interneurons.
2.3. Different TF sequence in medulla NBs
In the developing medulla, NBs of different ages can be visualized in one
snapshot, and thus the medulla provides another powerful system to study
