Meeting Abstract

61.2  Tuesday, Jan. 6  Axes of evol: Anterior-posterior body patterning is congruent with a disordered Hox cluster in echinoderms MOOI, R.*; DAVID, B.; California Academy of Sciences; Centre National de la Recherche Scientifique, Dijon rmooi@calacademy.org

Recent advances in echinoderm evo-devo include acquisition of the genome sequence for Strongylocentrotus purpuratus. The echinoid Hox cluster is disordered. Genes of the anterior class (Hox1 to Hox3) are translocated in reverse order to the 5′ end of the cluster (Hox4 is missing). Prior to such discoveries, the Extraxial-Axial Theory (EAT) was introduced. The EAT is a reappraisal of body homologies based on embryological and anatomical criteria in all echinoderms, recognizing two main body wall components. The extraxial part is related to somatocoels and develops in the non-rudiment region of the larva. The axial part is associated with the hydrocoel and is derived from a rudiment that develops on the left side of the larva. The merger of EAT and evo-devo datasets permits comprehension of echinoderms in a single framework. Genes of the central (Hox7 and Hox8) and posterior classes (Hox9/10 to Hox11/13b) express first along a somatocoelar "hox vector" in the extraxial domain. Hox5 expresses later and anteriorly near hydrocoelar derivatives in the axial domain. Hox3 expresses still later. Expression of Hox2 and Hox1 remains unknown. Temporal colinearity is strongly upheld, and spatial colinearity is split into divergent vectors congruent with the axial-extraxial distinction of the EAT. Disorder of the Hox cluster does NOT mean incomprehensible disarray. It represents innovations allowing dominant adult expression of the most anterior domain (axial) over the posterior (extraxial). Hox "disorder" is correlated with the 2-part developmental trajectory of echinoderms leading to prevalence of rudiment derivatives (axial elements), thereby superimposing radial symmetry on the anterior-posterior axis.