Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1174 Chapter 21: Development of Multicellular Organisms
APICAL
ectoderm
after 4 more rounds
of neuroblast division
BASAL
neuroblast
ganglion mother cell
neuron
glial cell
smaller, becomes specialized as a ganglion mother cell. Each ganglion mother cell
will divide only once, giving a pair of neurons, or a neuron plus a glial cell, or a
pair of glial cells, with Notch-mediated interactions helping to drive the daughters
along different paths. The neuroblast itself becomes smaller at each division, as it
parcels out its substance into one ganglion mother cell after another. Eventually,
typically after about 12 cycles, the process halts, presumably because the neuroblast
becomes too small to pass the cell-size checkpoint in the cell-division cycle.
Later, in the larva, neuroblast divisions resume, but now they are accompanied
by cell growth, permitting the process to continue indefinitely and to generate the
much larger numbers of neurons and glial cells required in the adult fly.
Differences in Regulatory DNA Explain Morphological Differences
In the preceding sections, we have seen that animals contain the same essential
cell types, have a similar collection of genes, and share many of the molecular
mechanisms of pattern formation. But how can we square this with the radical
differences that we see in the body
MBoC6
structures
m22.66/22.36
of animals as diverse as a worm, a
fly, a frog, and a mouse? We asserted earlier, in a general way, that these differences
usually seem to reflect differences in the regulatory DNA that calls into play
the components of the conserved basic kit of parts. We must now examine the
evidence a little more closely.
When we compare animal species with similar basic body plans—different
vertebrates, for example, such as fish, birds, and mammals—we find that corresponding
genes usually have similar sets of regulatory elements: the regulatory
DNA sequences have been well conserved and are recognizably homologous in
the different animals. The same is true if we compare different species of nematode
worms or insects. But, when we compare vertebrate regulatory regions with
those of worms or flies, it is hard to see any such resemblance. The protein-coding
sequences are unmistakably similar, but the corresponding regulatory DNA
sequences appear mostly very different, suggesting that the differences in body
plans mainly reflect differences in regulatory DNA. Although variations in the
proteins themselves also contribute, differences in regulatory DNA would be
enough to generate radically different tissues and body structures even if the proteins
were the same.
It is not yet possible to trace the genetic steps that have led to all the spectacular
diversity of animals. Their lineages have diverged over hundreds of millions of
years, and in most cases too many changes have occurred for us to be able to say
that this or that feature results from this or that mutation. The picture is clearer,
however, for more recent evolutionary events. Studies of both closely related animal
populations and plant populations whose members have different morphologies
have revealed that dramatic developmental effects can result from subtle
changes in regulatory DNA.
A well-studied example is the morphological diversity found in stickleback
fish. After the last ice age ended about 10,000 years ago, marine sticklebacks colonized
many newly formed freshwater streams and lakes. Marine sticklebacks
extend sharp spines from their pelvic skeleton. These spines are thought to help
protect the fish from soft-mouthed fish predators. In contrast, several populations
of freshwater sticklebacks have lost these spines, usually in lakes that lack
such predators. The different morphologies reflect differences in control of the
expression of a transcription regulator called Pitx1. Whereas marine sticklebacks
express the Pitx1 gene in the pelvic bone precursor cells that will form the spikes,
Figure 21–36 Neuroblasts and
asymmetric cell division in the central
nervous system of a fly embryo. The
neuroblast originates as a specialized
ectodermal cell. It is singled out by lateral
inhibition and emerges from the basal
(internal) face of the ectoderm. It then
goes through repeated division cycles,
dividing asymmetrically to generate a series
of ganglion mother cells. Each ganglion
mother cell divides just once to give a
pair of differentiated daughters (typically a
neuron plus a glial cell).