Hox genes are homeodomain containing transcription factors playing a crucial role in patterning the antero-posterior ( AP) axis in bilaterians.The mechanism of patterning by Hox genes is remarkably conserved across the entire bilateria even though huge diversification of body plan exists. The fact that orthologous genes display similar genomic organisation and expression patterns with comparable spatial and temporal characteristics in distantly related species has provided clues for understanding the evolution of the body plan. It was with discovery of homeodomain and later with similarity of Hox clusters in Mice and Drosophila patterning the AP axis, led to the beginning of new field called evolutionary developmental biology (evo-devo).
Hox genes from distant related species are expressed in colinear manner ( spatial and temporal ) provided us with the clues for understanding the evolution of body plan. Any morphological changes during animal body plan evolution has been associated with variation in Hox gene expression ,which ultimately affects the developmental regulatory networks.
Each phyla is characterized by a particular Hox pattern responsible for establishment of its specific body plan. This particular pattern establishes a ‘‘Hox code’’ consisting in a combinatorial information
of position along the antero-posterior axis. One of goals of evo devo is to decode the ground plan of the last common ancestor of bilaterians which is called” Urbilateria”. Hox genes and its hypothetical evolutionary sisterparahox genes are considered to be to be part of this ancestral developmental tool-kit ,patterning the ectoderm and endoderm respectively. So according to this hypothesis body plan evolution would be closely linked to the genomic organisation and expression of the Hox/ParaHox gene family.
As mentioned above the role of Hox genes in patterning AP axis among bilaterians is hugely conserved but situation in non bilaterian group is rather complex. Cnidarians are the sister group of bilaterians and by studying Hox and parahox gene expression in these organisms will give important information about evolution of Hox / Parahox genes and importantly the origin of the Hox code patterning system can be better understood.
Cindarians share quite a unique body plan with single polarity axis (oral-aboral) but exhibit different life cycles ,a pelagic (polyp) or a benthic form (medusa) or both alternating. The Cnidaria encompass five main taxa : the Anthozoa (corals, sea anemone), Staurozoa, Cubozoa, Scyphozoa and Hydrozoa.
Anthozoans are the sister group to the remaining cnidarians. Hox and parahox genes are known from many cnidarians species and also expression pattern of few are worked out. But there are conflicting conclusions about early evolution of hox-parahox genes and their functions in relation to axial polarity.

So to answer these vital questions Roxane Chiori et al studied Hox, ParaHox and Hox-related genes by phylogenetic analysis and in situ hybridisation in Clytia hemisphaerica, an hydrozoan species with medusa and polyp stages alternating in the life cycle. One of the important finding of the phylogenetic analysis is that it dooesnot support the popular theory of origin of ParaHox and Hox genes by duplication of an ancestral ProtoHox cluster,but rather supports an origin of ParaHox genes by tandem duplications.
Sixteen ANTP homeodomain sequences have been retrieved by tBLASTn search from our Clytia EST collection. Among them, 8 belong to the Hox-extended family, which includes Hox (HOX1 ‘‘anterior’’ Hox group, named CheHox1, three in the ‘‘posterior’’ Hox HOX9-14 group named CheHox9-14A, CheHox9-14B and Che-Hox9-14C), ParaHox (CheGsx and Checdx), Mox, HlxB9, Rough and Eve genes.Phylogenetic analysis also reveals a diversification of the cnidarian HOX9-14 genes into three groups called A, B, C. Authors studied the expression patterns of Hox and parahox genes by insitu hybridisation and shown that only those belonging to the HOX9-14 and the CDX groups exhibit a restricted expression along the oralaboral axis during development and in the planula larva, while the others are expressed in very specialised areas at the medusa stage.The Hox gene expression data in Clytia and other cnidarians do not support the conservation of a ‘‘Hox code’’ and authors are of the opinion that that the Hox code, collinearity and conservative role along the antero-posterior axis are bilaterian innovations and are not a part of cnidarians. The expression data presented in the latest plos article definitely show that ancestral function of hox genes may be not be that of axis patterning ,but to say this with certainty one needs to have some functional data in these organisms.
Reference and image credit :
Citation: Chiori R, Jager M, Denker E, Wincker P, Da Silva C, et al. (2009) Are Hox Genes Ancestrally Involved in Axial Patterning? Evidence from the Hydrozoan
Clytia hemisphaerica (Cnidaria). PLoS ONE 4(1): e4231. doi:10.1371/journal.pone.0004231

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