OVER ONE-HALF BILLION YEARS OF HEAD CONSERVATION - EXPRESSION OF AN EMS CLASS GENE IN HYDRACTINIA-SYMBIOLONGICARPUS (CNIDARIA, HYDROZOA)

We report the isolation of an empty spiracles class homeodomain-contai ning gene, Cn-ems, from the hydrozoan Hydractinia symbiolongicarpus, t he first gene of this class characterized in a lower metazoan, Cn-ems was found to be expressed in the head of gastrozooids, specifically in endodermal epithelial cells of the taeniolae of the hypostome, Cn-ems is not expressed in gonozooids, which lack taeniolae, Experimental co nversion of the posterior region of the planula larva into head struct ures up-regulates expression of the gene, These findings establish tha t the association of ems-class genes with head structures preceded the evolution of bilateral symmetry.