Cnidarians Reveal Intermediate Stages in the Evolution of Hox Clusters and Axial Complexity

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Abstract
Across major phylogenetic comparisons, the evolution of Hox clusters generally parallels the evolution of axial complexity. Sponges lack a fixed primary body axis and regional axial differentiation. Correspondingly, sponges appear to lack a Hox cluster. Bilaterian animals are characterized, at least primitively, by the presence of an anterior-posterior axis. In many bilaterians, the anterior-posterior axis is finely subdivided into morphologically distinct regions; e.g., consider the many distinct vertebrae of the human vertebral column or the many distinct body segments of the fruitfly. This axial complexity is encoded in part, by the genes of the Hox cluster. Bilaterians possess from seven to upwards of forty Hox genes which sort into four monophyletic classes (anterior, group-3, central, and posterior). Cnidarians (e.g., sea anemones) display an intermediate stage of axial complexity. Unlike sponges, they possess a fixed primary body axis, known as the oral-aboral axis, with a distinct head, body column, and foot. However, the primary axis of cnidarians lacks the degree of axial differentiation found in vertebrates or insects. Cnidarians possess distinct anterior and posterior Hox genes. Cnidarians appear to lack group-3 or central Hox genes. Southern mapping experiments in the sea anemone, Nematostella indicate linkage between an anterior Hox gene, an even-skipped ortholog, and a posterior Hox gene. The linkage of eve to a Hox gene, a condition previously described in a coral, is found in vertebrates but apparently absent in insects. Cnidarians hold the potential to reveal important intermediate stages in the evolution of Hox clusters and axial complexity.