Sea urchin Hox genes: insights into the ancestral Hox cluster

We describe the Hox cluster in the radially symmetric sea urchin andcompare our findings to what is known from clusters in bilaterallysymmetric animals. Several Hox genes from the direct-developing sea urchinHeliocidaris erythrogramma are described. CHEF gel analysis shows that theHox genes are clustered on a < or = 300 kilobase (kb) fragment of DNA,and only a single cluster is present, as in lower chordates and othernonvertebrate metazoans. Phylogenetic analyses of 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 groups. Despite their radialbody plan and lack of cephalization, echinoderms retain at least one of theanterior group Hox genes, an orthologue of Hox3. The structure of theechinoderm Hox cluster suggests that the ancestral deuterostome had a Hoxcluster more similar to the current chordate cluster than was expected Seaurchins have at least three Abd-B type genes, suggesting that Abd-Bexpansion began before the radiation of deuterostomes.