Temporal Patterning of Neural Progenitors in Drosophila - Current Topics in Developmental Biology - page 72

Therefore, NBs of different ages can be observed together in one snapshot
( Egger, Boone, Stevens, Brand, & Doe, 2007; Yasugi, Umetsu, Murakami,
Sato, & Tabata, 2008 ). These NBs generate GMCs, which then produce
medulla neurons that form chains below each NB, with the first-born
neurons located in the deepest position and latest-born neurons in the
most superficial position, near the NB ( Hasegawa et al., 2011; Morante,
Erclik, & Desplan, 2011 ). Recently, the developing medulla has emerged
as a new powerful system for studying temporal patterning of NBs ( Li
et al., 2013; Suzuki, Kaido, Takayama, & Sato, 2013 ).

1.4. Type II NBs in the central brain
In contrast to the regular type I NBs that divide asymmetrically to generate a
series of GMCs that each divides once to produce two postmitotic cells,
type II NBs divide asymmetrically to self-renew and to generate a series of
transit amplifying GMCs called intermediate neural progenitors (INPs), with
each INP dividing asymmetrically to generate several GMCs and eventually
giving rise to 6-12 neurons and/or glia ( Bayraktar, Boone, Drummond, &
Doe, 2010; Boone & Doe, 2008; Viktorin, Riebli, Popkova, Giangrande, &
Reichert, 2011; Weng, Golden, & Lee, 2010; Zhu, Barshow, Wildonger,
Jan, & Jan, 2011 ). Type II lineages allow more neurons than type I lineages
to be quickly generated.

2. SEQUENTIALLY EXPRESSED TRANSCRIPTION
FACTORS IN NBs CONTROL BIRTH-
ORDER-DEPENDENT NEURON FATES
2.1. Temporal sequence of transcription factors
in embryonic VNC NBs

How does a NB generate a stereotypical order of different neuron types
as it ages? Extensive studies in the Drosophila embryonic VNC have led to
the identification of a molecular mechanism of temporal specification,
where a series of transcription factors (TFs) are sequentially and transiently
expressed in NBs as they age. Postmitotic progeny born during each time
window maintains the expression of the TF. This temporal sequence
of TFs includes Hunchback (Hb), Kr¨ppel (Kr), Pdm1/Pdm2 (Pdm), Castor
(Cas), and Grainyhead (Grh) ( Baumgardt, Karlsson, Terriente, Diaz-
Benjumea, & Thor, 2009; Brody & Odenwald, 2000; Cui & Doe, 1992;
Grosskortenhaus, Pearson, Marusich, & Doe, 2005; Isshiki et al., 2001;
Kambadur et al., 1998; Mellerick, Kassis, Zhang, & Odenwald, 1992;

Drosophila