CARTWRIGHT, Paulyn: Hox genes, Hydroids, and Colonial Integration

A trend toward increased colony-wide integration is a common theme in the evolution of colonial metazoans. This trend is nowhere more apparent than within the Cnidarian class, Hydrozoa, where colonial forms range from a simple array of individual polyps, to a highly integrated, complex organization of polyps. Integrated colonies commonly display two prominent features: a division of labor and a stolonal mat. Division of labor is achieved through morphologically distinct and functional specialized polyp types, called polyp polymorphs. A stolonal mat is a network of branching endodermal canals intercalated between two epithelial layers from which the polyps bud. Morphological differences between simple and integrated colonies possessing polyp polymorphs and a stolonal mat largely reflect changes in the placement of structures along the aboral/oral axis of the polyp. This suggests that the evolution of colonial integration may be, at least in part, a result of changes in the signaling mechanisms demarcating the relative placement of axial structures. This hypothesis does however demand the existence of a genetic system capable of regulating the placement of these structures. Hox genes are plausible candidates for such a role, as they are involved in specifying axial positional information in diverse metazoan taxa. The Cnidarian Hox gene, Cnox-2, was therefore used as a marker to explore whether its expression domains correlate with the differences in axial patterning in the colonial hydrozoan, Hydractinia symbiolongicarpus. Cnox-2 expression patterns were found to be closely associated with the different axial patterns found in polyp polymorphs and the stolonal mat, suggesting that changes in the regulation of developmental genes, such as Hox genes, may have played an important role in the evolution of colonial integration.