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(AP) positional identity genes play important roles
role in determining the identity of the neuroblasts as
illustrated by the loss and/or duplications of particu-
lar sets of neurons. For example, Wingless (wnt) and
Hedgehog proteins activate the expression of a gene
called huckebein in some neuroblasts, and the tran-
scription factors Engrailed and Gooseberry repress
huckebein expression in other neuroblasts, thus estab-
lishing the precise pattern of huckebein protein in spe-
cific neuroblast lineages (McDonald and Doe, 1997).
Another set of genes divides the embryo and the
nervous system along the dorsoventral axis. Three
homeobox genes, vnd , ind , and msh , are expressed in lon-
gitudinal stripes within the neural ectoderm (Cornell
and Ohlen, 2000). vnd is expressed in neuroblasts
closest to the ventral midline, msh is expressed in the
most dorsolateral stripe of the neurectoderm, and ind is
expressed in an intermediate stripe between these two
(Figure 4.5C). As is the case for the AP genes, mutations
in these genes lead to loss of the neuroblast fates that
normally express the mutated gene. The mechanism
responsible for setting up these stripes involves
responses of the promoters of these genes to threshold
levels of the morphogen Dpp, which forms a gradient
of expression from dorsal (high) to ventral (low). Once
set up, the boundaries between the stripes of vnd, ind,
and msh are sharpened by mutual repression.
A neuroblast in any position can thus be uniquely
identified by expression of these spatial coordinate
markers of latitude and longitude (Figure 4.5). These
genes specifying position information along these two
Cartesian axes collaborate to specify a positional
identity for each central neuroblast in the developing
organism. Once expressed in a neuroblast, the spatial
coordinate genes are inherited by all the progeny of
these cells, and act as intrinsic determinants of
neuronal fate.
Each neuroblast divides asymmetrically to produce
a copy of itself and a ganglion mother cell (GMC). The
neuroblast divides several more times giving rise in
ordered succession to a set of GMCs (see Chapter 1).
Each GMC can thus be identified not only by the posi-
tion of the neuroblast from which it arises but also
from the order of its generation (e.g. whether it is the
first, second, or third GMC to arise from a particular
neuroblast). The first GMCs of a neuroblast lineage
tend to lie deeper in the CNS and generate neurons
with long axons, whereas the later arising GMCs stay
closer to the edge of the CNS and generate neurons
with short axons. In generating GMCs, neuroblasts go
through a temporally conserved program of transcrip-
tion factor expression (Figure 4.5 D and E). In the stages
when the first GMCs are generated, most neuroblasts
The CNS of an insect develops from a set of indi-
vidual neuroblasts that enlarge within the epithelium
of the neurogenic region of the blastoderm and then
delaminate to the inside, forming a neuroblast layer.
All these neuroblasts, we learned in Chapter 1, express
proneural transcription factors of the Achaete-Scute
family that give them a common “neural” specifica-
tion. However, each neuroblast is an individual and
through successive divisions reproducibly gives rise to
a unique set of neurons (Figure 4.4). How do these neu-
roblasts get their specific identities? They are arranged
in reproducible columns and rows and so can be iden-
tified by their position. In Chapter 2, we discussed
how the Drosophila embryo is finely subdivided in the
anterior to posterior axis into stripes of expression of
particular combinations of gap genes, pair-rule genes,
Hox genes, and segment polarity genes. These genes
provide neuroblasts with intrinsic positional informa-
tion that reflects their location along the antero-poste-
rior axis (Figure 4.5). Hox genes are expressed in the
middle and posterior portions of the neural pri-
mordium, and the “head gap” genes are expressed
more anteriorly in nested domains and provide posi-
tional information to the neuroblasts that give rise to
particular brain regions or segments. Segment polarity
genes control positional information within each indi-
vidual segment (Bhat, 1999). These anterior-posterior
FIGURE 4.4 Neuroblasts of the Drosophila embryo. A. Shows the
rows of neuroblasts labeled with an antibody to a late neuroblast-
specific protein called Snail. B. Shows neuroblasts labeled with three
different antibodies to the different neuroblast-specific proteins
Hunchback, Eagle, and Castor. (Photos courtesy of Skeath and Doe)
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