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).