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Fig. 1. Progenitor temporal transcription factors (TTFs) in
Drosophila. Wild-type (wt) Drosophila neuroblasts (large
circles) express four progenitor TTFs, at different times during
embryogenesis, in the following sequence: Hunchback (Hb) 
Kruppel (Kr)
Pdm Castor (Cas). (A) Each progenitor TTF is associated
with postmitotic progeny (small circles) of a different temporal identity
(blue, red, yellow or green). (a) Loss-of-function of a single progenitor TTF
leads either to the skipping of one temporal identity (shown in grey for
hb-/-, Kr-/- or pdm-/-) or
to stalled temporal series progression, associated with supernumerary early
temporal identities that are Pdm-dependent (cas-/-) or
Hb-dependent (svp-/-). (b) Continuous misexpression of any
of the four progenitor TTFs leads to supernumerary progeny with the
corresponding temporal identity. (B) Known negative cross-regulatory
interactions between progenitor TTFs and the switching factor Svp. Cas is not
only a progenitor TTF but, like Svp, also a switching factor (red). Note that
other known progenitor TTFs (black), such as Hb, do not fulfil this definition
because although misexpression blocks progenitor TTF progression,
loss-of-function does not (Grosskortenhaus
et al., 2005; Isshiki et al.,
2001). (C) Progenitor TTFs are also expressed during
postembryonic (larval and pupal) stages. Most, if not all, neuroblasts first
generate Chinmo+ (blue) neurons during embryonic and early larval
stages. They then switch to producing Broad-Complex+ (pink) neurons
during late larval and pupal stages. Neuroblasts fail to undergo the
Chinmo+ Br-C+ switch if the postembryonic
progression of progenitor TTFs is blocked by the removal of the postembryonic
(PE) pulse of Cas (PE cas-/-) or of Svp (PE
svp-/-), or by misexpressing Cas (PE + cas).
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