Biology Reference
In-Depth Information
axis have slightly different lineages, although they also share significant sim-
ilarities in their lineages and gene expression patterns. Hox genes are critical
players in determining these differences. For example, in
Section 6
we dis-
cussed how the
abdA
Hox gene, together with temporal factors, determines
the length of lineages and the end of neurogenesis. Hox genes also work
with temporal genes to control the generation of specific cell types only
found in certain segments. For example, NB5-6 generates the Ap cluster
neurons only in thoracic segments. Late temporal genes
cas
and
grh
, and
the thoracic Hox gene
Antennapedia
(
Antp
), are required to specify the
Ap cluster neurons (
Karlsson, Baumgardt, & Thor, 2010
). One of the target
genes for integration of temporal and spatial information is Collier, which
plays important roles in the feedforward loops that specify the Ap cluster
neurons as discussed in
Section 5
. It will be interesting to further characterize
the exact molecular mechanism of the integration (
Karlsson et al., 2010
).
Spatial identity also modulates the progression of the temporal sequence
of TFs in NBs. For example, young NB3-3 never expresses Hb, but sequen-
tially expresses Kr, Pdm, Cas, Cas/Grh, and Svp/Grh (
Tsuji et al., 2008
).
The absence of Hb in the sequence is common to both thoracic and abdom-
inal NB3-3, suggesting that this is modulated by the intrasegmental spatial
patterning genes common to NB3-3 in each segment. Although the tempo-
ral sequence is the same in both thoracic and abdominal NB3-3, the speed of
transitions between TFs is faster in abdominal NB3-3s than in thoracic
NB3-3s. Particularly, the switch from Cas to Svp occurs in late embryo
in abdominal NB3-3, while it occurs in the larval stage for thoracic
NB3-3. This spatial difference in the timing of switching to Svp is also true
for other NB lineages and is regulated by Hox genes. Loss of
Antp
or mis-
expression of
abd-A
in thoracic NB3-3 causes precocious Svp expression
during embryogenesis (
Tsuji et al., 2008
). How the Hox genes modulate
the speed of the temporal progression and how the spatial-temporal infor-
mation is integrated to regulate the NB lineage will be interesting questions
for the future.
8. INTEGRATION OF TEMPORAL IDENTITY WITH BINARY
FATE CHOICE
In embryonic NB lineages, GMCs usually divide asymmetrically to
give rise to two progeny with different fates (either two different neuron
types, one neuron and one glia, or one neuron and the other undergoing
programmed cell death, etc.), and this depends on Notch signaling between
