SEA-URCHIN HOX GENES - INSIGHTS INTO THE ANCESTRAL HOX CLUSTER

We describe the Hox cluster in the radially symmetric sea urchin and c ompare our findings to what is known from clusters in bilaterally symm etric animals. Several Hox genes from the direct-developing sea urchin Heliocidaris erythrogramma are described. CHEF gel analysis shows tha t the Hox genes are clustered on a less than or equal to 300 kilobase (kb) fragment of DNA, and only a single cluster is present, as in lowe r chordates and other nonvertebrate metazoans. Phylogenetic analyses o f sea urchin, amphioxus, Drosophila, and selected vertebrate Hox genes confirm that the H. erythrogramma genes, and others previously cloned from other sea urchins, belong to anterior, central, and posterior gr oups. Despite their radial body plan and lack of cephalization, echino derms retain at least one of the anterior group Hox genes, an ortholog ue of Hox3. The structure of the echinoderm Hox cluster suggests that the ancestral deuterostome had a Hox cluster more similar to the curre nt chordate cluster than was expected. Sea urchins have at least three Abd-B type genes, suggesting that Abd-B expansion began before the ra diation of deuterostomes.