Posterior expression of nanos orthologs during embryonic and larval development of the anthozoan Nematostella vectensis.

Cnidarians are primitive animals located in a basal position in the phylogenetic tree of the Animal Kingdom, as an outgroup of the Bilaterians. Therefore, studies on cnidarian developmental biology may illustrate how fundamental developmental processes have originated and changed during animal evolution. A particular example of this is the establishment of polarity along the body axes, which is under the control of a number of developmental genes, most of them conserved in evolution and playing similar roles in diverged species. Concerning the anterior-posterior axis, genetic and molecular studies on Drosophila have shown that the nanos gene plays an essential role in defining posterior structures during early embryonic development. Here we report the isolation of two nanos orthologs in the anthozoan Nematostella vectensis. We show that nanos mRNA is asymmetrically distributed in the fertilized egg and this asymmetry is maintained during embryonic development. At gastrula and planula larva stages, nanos expression is permanently associated with posterior body regions. These results, together with our previous analysis in the hydrozoan Podocoryne carnea, indicate that posterior nanos expression during development is a conserved feature among cnidarians. Therefore, the potential role of cnidarian nanos in defining axial polarity as a posterior determinant would represent an ancestral trait in the Animal Kingdom.