The A/P axis in echinoderm ontogeny and evolution: evidence from fossils and molecules

Even though echinoderms are members of the Bilateria, the location of their anterior/posterior axis has remained enigmatic. Here we propose a novel so lution to the problem employing three lines of evidence: the expression of a posterior class Hox gene in the coeloms of the nascent adult body plan wi thin the larva; the anatomy of certain early fossil echinoderms; and finall y the relation between endoskeletal plate morphology and the associated coe lomic tissues. All three lines of evidence converge on the same answer, nam ely that the location of the adult mouth is anterior, and the anterior/post erior axis runs from the mouth through the adult coelomic compartments. Thi s axis then orients the animal such that there is but a single plane of sym metry dividing the animal into left and right halves. We tentatively hypoth esize that this plane of symmetry is positioned along the dorsal/ventral ax is. These axis identifications lead to the conclusion that the five ambulac ra are not primary body axes, but instead are outgrowths from the central a nterior/posterior axis. These identifications also shed insight into severa l other evolutionary mysteries of various echinoderm clades such as the ind ependent evolution of bilateral symmetry in irregular echinoids, but do not elucidate the underlying mechanisms of the adult coelomic architecture.