The radial expression of dorsal-ventral patterning genes in placozoans, Trichoplax adhaerens, argues for an oral-aboral axis

Abstract

The placozoans are a morphologically simplistic group of marine animals found globally in tropical and subtropical environments. They consist of a single named species, Trichoplax adhaerens and have roughly six morphologically distinct cell types. With a sequenced genome, a limited number of cell-types and a simple flattened morphology, Trichoplax is an ideal model organism to understand cellular dynamics and tissue patterning in the first animals. Using new approaches for identification of gene expression patterns this research looks at the relationship of Chordin/TgfB signaling and the axial patterning system of Placozoa. Our results suggest that placozoans have an oral-aboral axis similar to cnidarians and that the parahoxozoan ancestor (common ancestor of Placozoa and Cnidaria) was likely radially symmetric.