Ciona intestinalis ParaHox genes: evolution of Hox/ParaHox cluster integrity, developmental mode, and temporal colinearity.

The Hox gene cluster, and its evolutionary sister the ParaHox gene cluster, pattern the anterior-posterior axis of animals. The spatial and temporal regulation of the genes seems to be intimately linked to the gene order within the clusters. In some animals the tight organisation of the clusters has disintegrated. We note that these animals develop in a derived fashion relative to the norm of their respective lineages. Here we present the genomic organisation of the ParaHox genes of Ciona intestinalis, and note that tight clustering has been lost in evolution. We present a hypothesis that the Hox and ParaHox clusters are constrained as ordered clusters by the mechanisms producing temporal colinearity; when temporal colinearity is no longer needed or used during development, the clusters can fall apart. This disintegration may be mediated by the invasion of transposable elements into the clusters, and subsequent genomic rearrangements.