Do cnidarians have a ParaHox cluster? Analysis of synteny around a Nematostella homeobox gene cluster.

The Hox gene cluster is renowned for its role in developmental patterning of embryogenesis along the anterior-posterior axis of bilaterians. Its supposed evolutionary sister or paralog, the ParaHox cluster, is composed of Gsx, Xlox, and Cdx, and also has important roles in anterior-posterior development. There is a debate as to whether the cnidarians, as an outgroup to bilaterians, contain true Hox and ParaHox genes, or instead the Hox-like gene complement of cnidarians arose from independent duplications to those that generated the genes of the bilaterian Hox and ParaHox clusters. A recent whole genome analysis of the cnidarian Nematostella vectensis found conserved synteny between this cnidarian and vertebrates, including a region of synteny between the putative Hox cluster of N. vectensis and the Hox clusters of vertebrates. No syntenic region was identified around a potential cnidarian ParaHox cluster. Here we use different approaches to identify a genomic region in N. vectensis that is syntenic with the bilaterian ParaHox cluster. This proves that the duplication that gave rise to the Hox and ParaHox regions of bilaterians occurred before the origin of cnidarians, and the cnidarian N. vectensis has bona fide Hox and ParaHox loci.