Biology Reference
In-Depth Information
Figure 3.4 Cross-regulations between TFs in the three temporal sequences that pattern
NBs or INPs. Each TF activates the next TF and represses the previous TF (with some
exceptions).
Hb, Kr, or Pdm simply causes the corresponding temporal identity to be
skipped, rather than a blockage of the temporal progression, suggesting that
the cross-regulations are not essential and that other factors are involved in
the switches. However, loss of Cas does lead to a block in the temporal pro-
gression and to persistent Pdm expression (
Brody & Odenwald, 2000;
Grosskortenhaus et al., 2006; Isshiki et al., 2001; Kambadur et al., 1998;
Maurange et al., 2008; Tran & Doe, 2008
).
In the Hth
Tll sequence that patterns medulla
NBs, loss of Ey, Slp, or D does disrupt essential cross-regulations, and the
NBs keep dividing but do not progress to the next TF stage. Slp and
D are also required to repress the previous TF (
Li et al., 2013; Suzuki
et al., 2013
). Thus, Ey, Slp, and D are required for the sequential TF
switching, and, in this regard, they are similar to Cas in the embryonic
sequence. However, there are a few exceptions to these modes of regulation:
No cross-regulation is observed among Hth, Klu, and Ey; and Tll is not
required, but it is sufficient to repress D (
Fig. 3.4
). This suggests that there
are more TFs temporally expressed in medulla NBs that work together with
the Hth
!
Klu
!
Ey
!
Slp
!
D
!
Tll sequence to ensure temporal transi-
tions (
Li et al., 2013; Suzuki et al., 2013
).
In the D
!
Klu
!
Ey
!
Slp
!
D
!
Ey sequence that patterns INPs of type II NB lineages,
D is necessary but not sufficient to activate the next TF, Grh. In turn, Grh
is required to repress D and activate Ey. Ey is required and sufficient to
!
Grh
!
