Historical Constraints and the Evolution of Development 1151
are secondarily simplified bilaterians, and not the sole survivors of the vaunted
intermediary group between protists and metazoans. Moreover, further study of DoxC
affirms its orthology with the lophotrochozoan LoxS, rather than with a middle Hox
gene of the ecdysozoan clade. Therefore, not only are the mesozoans revealed as
simplified metazoan parasites, but we may also place the ancestry of these formerly
enigmatic forms after the major split in protostome phylogeny, and into one of the
two great groups as a relative of platyhelminths, brachiopods, and annelids, rather
than arthropods or onychophores.
These revolutionary discoveries have inspired a growing literature on the
hypothetical phenotype, or at least the shared developmental architecture, of a stem
bilaterian, or even a stem animal. Slack et al. (1993) tried to define a "zootype" as the
"defining character, or synapomorphy" of the kingdom Animalia (p. 491), with
maximal expression in ontogeny at a "phylotopic stage ... at which all major body
parts are represented in their final positions as undifferentiated cell condensations ...
or the stage at which all members of the phylum show the maximum degree of
similarity" (loc. cit.). They base this concept on common possession of "a system of
gene expression patterns, comprising the Hox cluster type genes and some others
[encoding] relative position in all animals" (loc. cit.). As discussed just below, such a
concept may apply to all triploblasts, but probably not to diploblasts (whose Hox
homologs show some common properties of individual action, but not the integrated
spatial and temporal colinearity found in most triploblasts studied so far).
The less ambitious attempt to define the phenotype and organization of a
common bilaterian ancestor (named Urbilateria by de Robertis and Sasai, 1996; see
also Kimmel, 1996; de Robertis, 1997; and Pennisi and Roush, 1997) may be more
tractable, but specific arguments about whether this common ancestor had already
developed recognizably modern versions of segments (Kimmel, 1996), antennae,
photoreceptors, or a heart (see Fig. 10-25 for a cartoon of alternative possibilities
from Pennisi and Roush, 1997)— with obvious implications for views about the
homology of adult phenotypes, beyond the already established homology of
underlying generators—are, in my opinion, premature.
For example, in a challenging proposal, Arendt et al., 2001, propose a
homologous origin for tube-shaped guts of primary larvae in both protostomes and
deuterostromes—structures long granted an independent origin in conventional
evolutionary thinking. They base their argument upon "the shared, and very specific,
expression of brachyury in ventral developing foreguts of the starfish bipinnaria,
echinoid pluteus, enteropneust tornaria and polychaete trochophore larva," suggesting
"common ancestry (homology) of larval foreguts in Protostomia and Deuterostomia,
despite the different developmental origin of these structures" (p. 84). The authors
then claim additional support from equally specific actions of two other
developmental genes—a common expression of goosecoid in the foreguts of various
bilaterians, and of otx along pre- and postoral ciliary bands. As an obvious point of
contention,