1110 THE STRUCTURE OF EVOLUTIONARY THEORY
they do not code for the actual structures built within each segment. The Hox genes
turn on after the segments have been generated by other systems. They then act to
regulate the appropriate (and different) downstream cascades that actually build the
specialized structures of each segment. Thus, we may also search for homologies
between vertebrates and arthropods in the prior systems that specify numbers and
positions of segments before Hox genes begin their work in regulating specific fates.
Although the long germ-band style of segmentation in Drosophila (all segments
forming simultaneously as divisions of an embryo with a fully established A-P axis)
represents a highly derived condition with respect to the plesiomorphic state of most
insects (short germ-band development, with new segments added in a temporal
sequence, one by one at the posterior end), we almost inevitably turn to Drosophila
as an arthropod model of segmentation because our knowledge of this fly so exceeds
our understanding of any other arthropod's development. The identities and
differentiation of Drosophila's segments occur in a programmed cascade of linked
and ever-finer specifications that always draws my mind to the basic model of
Genesis I (by which I intend no statement about creation, needless to say, but refer
only to the geometric style of building complexity by successive division and
differentiation out of primal homogeneity, rather than by addition). In this primal tale
of Western culture, the cosmos begins "without form and void," and its products then
originate by compartmentalization and increasing specification of units: light from
darkness on day one; earthly from heavenly waters on day two; earthly land from
earthly water on day three; and division of heavenly light into sun and moon on day
four.
Drosophila's first specification even begins in a prior generation, for protein
products of maternal genes like bicoid and nanos appear in the egg cytoplasm to
designate the anterior and posterior embryonic poles. These maternal genes activate
gap genes like hunchback that specify broad regions along the A-P axis. Gap genes
then regulate the expression of pair-rule genes, whose bands of activity establish the
embryo's parasegment boundaries. These pair-rule genes express themselves in every
other segment, but also regulate the next level of differentiation in the genetic
cascade: segment-polarity genes like engrailed and wingless. The action of segment-
polarity genes finally establishes the anterior and posterior domains of each segment.
Now that segment boundaries have been set, and the spatial domains of each segment
determined, Hox genes can finally establish segment identities by regulating
downstream cascades of appropriate architects.
Interestingly, although evidence remains limited to a few taxa and effects as I
write this section in January 2000, some apparent vertebrate homologs of these
segmental cascades have been detected with reasonable confidence.
(1) Pair-rule genes and somite formation in zebra fish and chicks. Miiller et al.
(1996) studied the expression of herl, a homolog of the Drosophila pair-rule gene
hairy, in the zebra fish Danio rerio. Expression of herl occurs in transient stipes
within the presomitic mesoderm. Although more than 10