Where is my head? The evolution of body plan in echinoderms.

 The origin of the pentaradial body plan of echinoderms from a bilateral ancestor is one of the most enduring zoological puzzles. Since echinoderms are defined by morphological novelty, even the most basic axial comparisons with their bilaterian relatives are problematic. To revisit this classical question, we used conserved antero-posterior (AP) axial molecular markers to determine whether the highly derived adult body plan of echinoderms masks underlying patterning similarities with other deuterostomes. We investigated the expression of a suite of conserved transcription factors with well-established roles in the establishment of AP polarity in deuterostomes and other bilaterians using RNA tomography and in situ hybridization in several echinoderm species. Our data show a surprising level of conservation of AP patterning program across echinoderms, but there are also major differences with their bilateral relatives. Together, these findings illustrate how anatomical diversity can arise from molecular conservation, and help to understand the evolution of one of the most bizarre animal phyla.