Anteroposterior molecular registries in ectoderm of the echinus rudiment

Background: Echinoderms and hemichordates are sister taxa that both have larvae with tripartite coeloms. Hemichordates inherit the coelom plan and ectoderm from larvae, whereas echinoderms form the adult rudiment comprising rearranged coeloms and a vestibule that then develops into adult oral ectoderm. Molecular networks that control patterns of the ectoderm and the central nervous system along the anteroposterior (AP) axis are highly conserved between hemichordates and chordates, respectively. In echinoderms, however, little is known about the AP registry in the ectoderm. Results: We isolated ectodermal AP map genes from the sand dollar Peronella japonica and examined their expression. Comparative expression analyses showed that (1) P. japonica orthologs of hemichordate anterior markers are expressed in the larval apical plate, which degenerates during metamorphosis; (2) P. japonica orthologs of the medial markers are expressed in the ambulacral ectoderm of the rudiment; and (3) few P. japonica orthologs of the posterior markers are expressed in ectoderm. Conclusions: We suggest that echinoids only inherit the ambulacral ectoderm from a common ambulacrarian ancestor, which largely corresponds to the collar ectoderm in hemichordates. The ectodermal AP registry provides insights into the AP axis and evolutionary processes of echinoderms from a common ambulacrarian ancestor. Developmental Dynamics 247:1297–1307, 2018. © 2018 Wiley Periodicals, Inc. Key Findings Echinoid orthologs of hemichordate ectodermal anterior markers are expressed in the larval apical plate, which degenerates during metamorphosis. Echinoid orthologs of hemichordate medial markers are expressed in adult ambulacral ectoderm, whereas few posterior marker orthologs are expressed in ectoderm. Ectodermal anteroposterior registries indicate that echinoids only inherit the ambulacral ectoderm from a common ambulacrarian ancestor, which largely corresponds to the collar ectoderm in hemichordates. We suggest that echinoderm rays are not duplicated AP axes of a common ambulacrarian ancestor but are outgrowths of appendages around the adult mouth.