The unusual gene order in the Echinoderm Hox cluster is related to the embryo and larva symmetries

Background: Hox gene collinearity relates the sequential location of Hox genes in the 3’ to 5’ direction on the chromosome with the linear arrangement of the body elements along the anterior-posterior (A/P) axis of bilaterian embryos. This spatial Hox gene collinearity has been almost universally respected in diverse organisms like worms, insects or vertebrates. It is therefore surprising that the above well established collinearity rule is violated in the case of Echinoderms. No explanation of this violation is apparent. Here a hypothesis is put forward which provides a cue to understand the abnormal serial gene location in the sea urchin disorganized Hox cluster. . CC-BY 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/016923 doi: bioRxiv preprint first posted online Mar. 23, 2015;