Historical Constraints and the Evolution of Development 1153
genes among various Cnidaria, corresponding to one medial group precursor, the
anterior Drosophila genes labial and proboscipedia, and the posterior Abdominal-B
(see also Miller and Miles, 1993). But even though these cnidarian genes follow the
same 3' to 5' order as their bilaterian homologs, no evidence exists for colinearity of
action in the development of any body structure (particularly the oralaboral axis).
Moreover, cnidarians apparently lack several key bilaterian Hox elements. Martinez
et al. (1998, p. 748) write that "the genes in the middle of the [bilaterian] Hox clusters
form a monophyletic group that includes no cnidarian genes. This is most readily
explained by derivation of these genes through duplication of a single precursor after
the origin of Cnidaria."
A fascinating study by Cartwright et al. (1999), however, does affirm some
general similarity of Hox action in cnidarians and bilaterians by demonstrating a
formative role for cnidarian Hox in specification of the oral-aboral axis in two
distantly related hydrozoans, Hydra itself and Hydractinia symbiolongicarpus. (This
single cnidarian Hox gene, Cnox-2, specifies full differentiation along the oral-aboral
axis of polyps, whereas the sequential colinear activation of the full bilaterian Hox
suite specifies differentiation along the bilaterian AP axis—thus illustrating once
again that the primary novelty of bilaterian origins resides in the spatial sequence of
Hox genes and the evolution of their coordinated action. We have, in any case, no
reason to view the cnidarian oral-aboral axis as homologous to the AP axis of
bilaterians.)
The study of Cartwright et al. gains strength from the multiple possibilities for
natural and laboratory experiments inherent in the fourfold polymorphism of
Hydractinia polyps, and in the ease of experimental transformation of one type into
another. The "normal" feeding polyp of Hydractinia, the gastrozooid, corresponds to
that of Hydra, and shows full oral-aboral differentiation from the distal mouth and
hypostome to the body column and foot at the proximal end. In both Hydra and
Hydractinia, Cnox- 2 is expressed at high levels in the foot and body column and at
successively lower levels up the axis towards the head, which shows very weak
Cnox- 2 expression.
But Hydractinia symbiolongicarpus also develops three polymorphic variants;
clearly interpretable as intensifications of either the oral or aboral ends of the main
polypary axis (see Fig. 10-26). Gonozooids and dactylozooids are specialized,
respectively, for sexual reproduction and for capturing eggs of the colony's hermit
crab host. Both lack a hypostome and tentacles and seem to represent "an expansion
of the body column to the exclusion of oral regions" (Cartwright et al., 1999, p.
2183). The authors found "no detectable difference in Cnox- 2 expression along the
aboral-oral axis in either the gonozooid or dactylozooid" (p. 2185), and general levels
of expression equaled those found at the base of the gastrozooid—thus affirming the
anatomical inference that both polymorphs develop by extending the specialized
aboral end of the axis to the full length of the polyp, and suppressing the head region
entirely.
In satisfying contrast, a fourth polymorph, the tentaculozooid that plays a role in
defending the colony, resembles a single gastrozooid tentacle, and