The Questions of Developmental Biology

Neural competence: The achaete-scute genes.
In Drosophila, the genes activated in the neural ectoderm that enable a cell to become a
neuroblast are called the proneural genes. These genes encode the transcription factors Achaete
and Scute, among others. In mammals, the MASH1 gene
(mammalian achaete-scute homologue) is expressed in
subsets of neurons, and may influence neuronal
differentiation in olfactory (smell) receptor cells as well as
other cells of the central nervous system (Johnson et al.
1990; Skeath and Carroll 1992; Cau et al. 1997; Shou et al.
1999).
Singling out neuronal precursors: The Notch-Delta
interaction.
Not every ectodermal cell expressing the proneural
genes becomes a neuron. Many more become glial or skin
cells. As we saw in Chapter 6, the Delta protein (a ligand)
and the Notch protein (a receptor) interact such that Delta
activates signal transduction in the adjacent cell through
that cell's Notch protein. The cell with slightly more Delta
protein on its cell surface will inhibit its neighboring cells
from becoming neurons. In flies, frogs, and chicks, Delta is
found in those cells that will become neurons, while Notch
is elevated in those cells that become the glial cells (Figure
12.26; Chitnis et al. 1995). The down-regulation of Notch
in new neurons (to sustain neural differentiation) also
appears to be the same in chicks and flies (Wakamatsu et
al. 1999).
Differentiation of specific neurons.
In mice, the Nkx2.2 gene is activated by the Sonic
hedgehog signal and encodes a transcription factor that
specifies the identities of ventral nerve cells. Its Drosophila
homologue, vnd, controls dorsal-ventral neuronal identities
in the fly central nervous system in a similar manner
(Skeath et al. 1994; Briscoe et al. 1999).
Therefore, while the nervous system of the adult
fly and the adult vertebrate are remarkably different, the
genes and interactions that instruct the formation of these
nervous systems are remarkably similar. As we shall see in
the example of eye development below, this similarity in
instructions unifies many aspects of the central nervous
system across phyla.