Snail expression during embryonic development of the coral Acropora: Blurring the diploblast/triploblast divide?

Although corals are nominally diploblastic, the early development of Acropora millepora involves a process that clearly resembles gastrulation in higher metazoans. This similarity at the morphological level led us to search for the Acropora equivalents of genes whose key roles in gastrulation are conserved across the higher Metazoa. We here report the characterisation of one such gene, snail, which in both Drosophila and the mouse is expressed in cells undergoing an epithelial-mesenchyme transition and/or morphogenetic movements. In addition to an N-terminal SNAG domain, the Acropora snail protein contains four zinc fingers with sequences diagnostic for members of the snail protein subfamily. In situ hybridisation reveals expression in epithelial tissue in the central portion of one side of the flattened pre-gastrulation embryo, which continues to express snail as it is engulfed by its opposite layer. Comparison to snail expression during gastrulation in bilaterians such as Drosophila reveals striking similarities and suggests mechanistic, and possibly evolutionary, links between the processes of mesoderm formation in bilaterians and endoderm formation in the Cnidaria.