Historical Constraints and the Evolution of Development 1107
Moore and Willmer (1997, p. 34) although writing after most of the genetic
discoveries discussed in this section, affirmed the independent evolution of
segmentation as virtually beyond dispute, and therefore an exemplar and "type case"
for good pedagogy in phyletic inference: "As an object lesson to begin with, it is
evident... that the character we score as 'segmentation' has to have arisen at least
twice, since it occurs in the protostome annelid/arthropod grouping and again in the
very distant deuterostome chordates, but not in any of their possible common
ancestors." (Their confidence, presumably, would only be increased by the
subsequent discovery of a fundamental split among the protostome phyla, with
arthropods on one branch and annelids on the other—thus implying a third
independent origin of segmentation.)
But now, at a dawning millennium in human calendrics, two sequential sets of
discoveries have provoked a rethinking even of this most "settled" issue, and some
genuine segmental homology between arthropods and vertebrates now seems almost
inescapable. No simple one-for-one correspondence of somite with metamere can be
specified down the A-P axes of these phyla, and no archetypal form like Geoffroy's
"vertebra" can be reconstructed as an ancestral prototype for all segments. Moreover,
vertebrate somites do not seem to be constructed by the arthropod cascade of gap,
pair rule, segment polarity genes, etc.—see p. 1110 for more detail on these
differences. But anatomical homologies between these two-segmented phyla on
maximally divergent boughs of the bilaterian tree extend well beyond mere
positioning and pattern of A-P differentiation, and also include important aspects of
segmentation as well. If the common ancestor of arthropods and vertebrates did not
already possess a segmented body, this "urbilaterian" (in the terminology of De
Robertis, 1997) had probably established the fundamental genetic pathways behind
segmentation and the differentiation and specialization of segments— a system
maintained ever since in both phyla, and based in large part on the Hox sequences
and their colinearity.
- REDISCOVERING THE VERTEBRATE RHOMBOMERES. Initial data
on the mode of action of vertebrate Hox genes seemed, at first, to support the
traditional conclusion that no segmental homology existed between the two phyla.
The primary sites of Hox action generally correlate with the anterior expression
boundary of each gene—and these boundaries extended past the developing vertebral
column into anterior regions of the embryo. Some enterprising geneticists then
rediscovered an important fact, established in the 19th century by the great German
school of descriptive anatomists, and then forgotten by several subsequent
generations who dismissed such work as the dullest form of cataloguing done at the
least causally relevant scale by the most hidebound methodology of holistic
observation. For these 19th century anatomists had discovered that the vertebrate
hindbrain eventually develops into a unitary structure, but begins as a linear series of
7 or 8 segments called rhombomeres. Moreover, specific rhombomeres seem to
control (or at least correlate with) the development of important aspects of anterior
anatomy, including the deployment of the cranial nerves. Finally, as the spur