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Romani et al., 1996; Yang, Yeo, Dick, &Chia, 1993
). Although different NBs
in the VNCdelaminate and start differentiation at different times, most of them
follow the same TF sequence. These temporally expressed TFs have been
shown to be both required and sufficient for birth-order-dependent neuronal
specification in several different NB lineages (
Baumgardt et al., 2009; Benito-
Sipos et al., 2010; Cleary & Doe, 2006; Grosskortenhaus, Robinson, &
Doe, 2006; Isshiki et al., 2001; Kambadur et al., 1998; Novotny, Eiselt, &
Urban, 2002; Pearson & Doe, 2003; Tran & Doe, 2008
). For example, each
of the first five GMCs in the NB7-1 lineage gives rise to a motor neuron
(U1-U5) and a sibling cell. The unique identity of each of the U1-U5 motor
neurons is specified by the sequentially expressed TFs: Hb is necessary and suf-
ficient to specify U1 and U2 fates (high Hb: U1, lowHb: U2); Kr is necessary
and sufficient to specify theU3 fate; Pdmalone specifiesU4;whilePdmandCas
together specify U5 (
Cleary &Doe, 2006; Grosskortenhaus et al., 2005; Isshiki
et al., 2001; Pearson & Doe, 2003
;
Fig. 3.1
). In the NB7-3 lineage, there are
only three GMCs generated: GMC-1 is Hb
þ
,Kr
þ
, and generates the EW1
Figure 3.1 Schematic model showing the sequential expression of Hb, Kr, Pdm, and Cas
in the embryonic VNC NB7-1. These TFs control the sequential generation of different
U motor neurons. The Polycomb repressor complex is required for the progressive loss
of NB competence to generate motor neurons. At the end of the first five divisions, the
hb locus relocates to the nuclear periphery, and the NB completely loses its competence
to generate motor neurons.
