Hox genes in Cnidarians : The story of Nematostella and Eleutheria
Prior to study of Nematostella genome,it was believed that No True Hox genes were found in Cnidarians and Hox code is found only in Bilaterians.Cnidarians happens to be one of the most diverse phyla holding their place at the root of metazoan tree, provide unique insights into early Hox evolution since they constitute an outgroup to the Bilateria.The members of this phyla exhibit wide array of body forms and Hox genes known for axial patterning in all bilaterians, were identified in Cnidarians some time back.It remained a matter of debate in the field whether Hox like genes identified in some species are bona fide Hox genes till recently.
We take a look into two landmark papers(published in PLOS ONE 2007) which establishes beyond doubt that Hox genes are present in Cnidarians which makes them most primitive animals to have Hox genes (as we have already discussed in previous posts that Ctenophores and sponges lack Hox genes .This also indicates common ancestor of Cnidarians-Bilaterians had Hox genes).
Pylogenetic analyses of 18 Hox-related genes from Nematostella led to the identification of putative Hox1,Hox2 (orthologs of anterior Hox genes of biletarians), and Hox9 (Posterior /AbdB class) genes.Hox genes are Homeo domain containing proteins and possess some important properities:
Found in clusters: Throughout the animal Kingdom ,when present Hox genes are found in clusters.The relative genomic organization of orthologous Hox genes is well conserved among select Ecdysozoa such as Anopheles, Schistocerca, and Tribolium.However, in various bilaterian taxa (for example, Ciona intestinalis, Caenorhabditis elegans,Drosophila melanogaster, D. pseudoobscura, D. repleta, D. virilis, Oikopleura dioica, Schistosoma mansoni, and Strongylocentrotus purpuratus), the Hox cluster have undergone extensive rearrangements,and in some cases they are extensively lost (eg:Ciona) which makes the identification of cluster or lack of it a very difficult task.But in spite of so much of degenration of Hox clusters they dont lose the Hox code,meaning they specify the same axial territory.One possible explanation for the presence of Hox genes in clusters could be that the proper regulation might depend on close physical linkage.
Study carried out by Ryan et al, to address the origin and early evolution of Hox genes and the Hox code using Nematostella vectensis genome indicated 18 Hox related genes in which 7 appear to come from HOX1 ,HOX2 and HOX 9+ orthologs.They are antho 6 ,antho 6a (HOX 1 / labial ortholog),antho 7,antho 8,antho 8a(Hox 2/ Proboscipedia) and antho 1 ,antho 1a (HOX 9+/AbdB).
Image Credit:
Ryan JF, Mazza ME, Pang K, Matus DQ, Baxevanis AD, et al (2007) Pre-Bilaterian Origins of the Hox Cluster and the Hox Code: Evidence from
the Sea Anemone, Nematostella vectensis. PLoS ONE 2(1): e153. doi:10.1371/journal.pone.0000153
This phylogentic analysis provides evidence that Cnidarians possess both anterior and Posterior HOx classes but lack central Hox genes.It is a matter of debate whether Cnidarians never had central Hox genes or lost during evolution. Phylogentic study carried out by Ryan and co workers indicate that ancestors of Cnidarians had central Hox genes but present day members of phyla lost them during course of evolution.
During larval development, the putative Hox1, Hox2, and Hox9+ homologs are expressed in a number of distinct spatial domains that collectively account for practically the entire primary body axis, from the aboral to the oral extremity. Genome annotation data provide evidence for clustering of some Hox genes and expression patterns further strengthen the fact that rudimentary Hox code was pretty much a part of Cnidarian-Bilaterian ancestor for patterning the primary body axis.The authors are also of opinion that strong stabilizing selection has been operating on this Hox code that has maintained certain core characteristics despite being deployed in a bewildering array of animal forms for over half a billion years.
For more interesting details on Nematostella Hox genes (annotation studies,phylogeny data and expression patterns ) i strongly encourage you to read the article (which can be freely downloaded thanks to PLOS ONE).and I can assure you that it will be an wonderful read.
The second article from Jacob and Schierwater demonstrates that loss of function of Hox genes result in significant alterations of Body plan in Cnidarians.By providing an excellent protocol of RNAi knockdown of gene function in Marine invertebrates (which was missing till then) and demonstrated that knock downs of Hox-like genes(both by RNAI and Morpholinos) in hydrozoan Eleutheria dichotoma create substantial bauplan alterations, including the formation of multiple oral poles (‘‘heads’’) by Cnox-2 and Cnox-3 inhibition, deformation of the main body axis by Cnox-5 inhibition and duplication of tentacles by Cnox-1 inhibition.
Image Credit:
Jakob W, Schierwater B (2007) Changing Hydrozoan Bauplans by Silencing Hox-Like Genes. PLoS ONE 2(8): e694. doi:10.1371/
journal.pone.0000694
The RNAi transfection protocol developed here will be a boon which will open efficient avenues to functional evolutionary genomics not only in Cnidaria, but to basal marine invertebrates in general and probably also to vertebrate larvae.
The reference details of the two articles in Plos one are given below:
Ryan JF, Mazza ME, Pang K, Matus DQ, Baxevanis AD, et al (2007) Pre-Bilaterian Origins of the Hox Cluster and the Hox Code: Evidence from
the Sea Anemone, Nematostella vectensis. PLoS ONE 2(1): e153. doi:10.1371/journal.pone.0000153
Jakob W, Schierwater B (2007) Changing Hydrozoan Bauplans by Silencing Hox-Like Genes. PLoS ONE 2(8): e694. doi:10.1371/journal.pone.0000694
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